C: Difference between revisions

From 太極
Jump to navigation Jump to search
(Replaced content with "= Socket Programming = * [http://beej.us/guide/bgnet/ Beej's Guide to Network Programming] * [http://alas.matf.bg.ac.rs/manuals/lspe/0789722410 Linux Socket Programming...")
Tag: Replaced
Line 1: Line 1:
== Socket Programming ==  
= Socket Programming =  


* [http://beej.us/guide/bgnet/ Beej's Guide to Network Programming]
* [http://beej.us/guide/bgnet/ Beej's Guide to Network Programming]
Line 22: Line 22:
* http://web.cecs.pdx.edu/~jrb/tcpip/lectures/pdfs/sockets.pdf
* http://web.cecs.pdx.edu/~jrb/tcpip/lectures/pdfs/sockets.pdf


=== Terms ===
== Terms ==


There are two types of '''address domains'''.
There are two types of '''address domains'''.
Line 35: Line 35:


There are two types of '''ports''':
There are two types of '''ports''':
* well known Ports | those that rarely change overtime. For instance, servers that provide mail, file transfer, remote login,
etc.
* '''dynamic ports''' | typically used only for the life of a process. For instance, pipes can be implemented using message passing
Examples of servers:
* mail server
* login server
* file server
* web server
* streaming server
'''TCP''' vs '''UDP'''
* TCP is used for services with a large data capacity, and a persistent connection
*„ UDP is more commonly used for quick lookups, and single use query-reply actions.
* Some common examples of TCP and UDP with their default ports:
** DNS lookup UDP 53
** FTP TCP 21
** HTTP TCP 80
** POP3 TCP 110
** Telnet TCP 23
=== General Idea ===
The steps involved in establishing a socket on the client side are as follows:
#    Create a socket with the '''socket'''() system call
#    Connect the socket to the address of the server using the '''connect'''() system call
#    Send and receive data. There are a number of ways to do this, but the simplest is to use the '''read'''() and '''write'''() system calls.
The steps involved in establishing a socket on the server side are as follows:
#    Create a socket with the '''socket'''() system call
#    Bind the socket to an address using the '''bind'''() system call. For a server socket on the Internet, an address consists of a port number on the host machine.
#    Listen for connections with the '''listen'''() system call
#    Accept a connection with the '''accept'''() system call. This call typically blocks until a client connects with the server.
#    Receive and send data by using '''read'''() and '''write'''().
=== Internet Hearsay ===
* [http://www.ibm.com/developerworks/library/l-sockpit/ Tools for debugging sockets applications] using '''netstat''' and '''tcpdump''' GNU/Linux tools.
<pre>
View all TCP sockets currently active
$ netstat --tcp
View all UDP sockets
$ netstat --udp
View all TCP sockets in the listening state
$ netstat --listening
View the multicast group membership information
$ netstat --groups
Display the list of masqueraded connections
$ netstat --masquerade
View statistics for each protocol
$ netstat --statistics
Display all traffic on the eth0 interface for the local host
$ tcpdump -l -i eth0
Show all traffic on the network coming from or going to host plato
$ tcpdump host plato
Show all HTTP traffic for host camus
$ tcpdump host camus and (port http)
View traffic coming from or going to TCP port 45000 on the local host
$ tcpdump tcp port 45000
</pre>
* [http://www.roman10.net/using-telnet-to-help-debugging-tcp-based-application/ Use telnet to help debugging tcp based app]
* [http://cboard.cprogramming.com/c-programming/136509-tcp-ip-server-client-programm-plusfiles.html bzero is deprecated. Use memset(foo, 0, size) instead]
=== Make http request via telnet ===
* http://blog.tonycode.com/tech-stuff/http-notes/making-http-requests-via-telnet
Below, we only input two lines. One is '''telnet linus.nci.nih.gov 80''' and the other is '''HEAD / HTTP/1.0\n\n'''. Remember the one carriage character and one line feed at the end of request line. We can change the HTTP method in the 2nd input to '''GET /HTTP/1.0\n\n''' to fetch the full page. See the book [http://www.http-guide.com/ HTTP: The Definitive Guide] and [http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol wikipedia].
<pre>
$ telnet linus.nci.nih.gov 80
Trying 137.187.182.124...
Connected to ncias-p942-v-1.nci.nih.gov.
Escape character is '^]'.
HEAD / HTTP/1.0
HTTP/1.1 200 OK
Date: Thu, 21 Mar 2013 14:47:26 GMT
Server: Apache
Last-Modified: Tue, 12 Mar 2013 13:52:32 GMT
ETag: "302a-4d7ba99db0800"
Accept-Ranges: bytes
Content-Length: 12330
Connection: close
Content-Type: text/html
Connection closed by foreign host.
$
</pre>
== Socket Programming Examples using C/C++/Qt ==
=== Example 1 - Linux/Unix ===
http://www.linuxhowtos.org/C_C++/socket.htm. The codes are saved under [http://taichi.selfip.net:81/lang/c/socket/example1/ here].
<pre>
$ gcc server.c -o server
$ gcc client.c -o client
$ ./server 51717
$ ./client 192.168.0.21 51717
Please enter the message: Who are you?
I got your message
</pre>
where 192.168.0.21 is the ip address on the server. The server side will show
<pre>
Here is the message: Who are you?
</pre>
If everything works correctly, the server will display your message on stdout, send an acknowledgement message to the client and terminate. The client will print the acknowledgement message from the server and then terminate.
* We can actually run the client on a different machine (eg server on my Ubuntu and client on my Windows) although we can also run both client and server on the same machine.
* Once we use the port (51717) one time, we can not use the same port to run it again??? The screen shows an error "'''ERROR on binding: Address already in use'''". The problem is we may need to wait until 4 minutes for avoiding this message. See the solution in [http://stackoverflow.com/questions/2147592/bind-error-address-already-in-use here] or [http://www.yolinux.com/TUTORIALS/Sockets.html SO_REUSEADDR option in setsockopt()]. That is, we just need to change server.c to add the following
<pre>
...
int iOption = 1; // Turn on keep-alive, 0 = disables, 1 = enables
...
// Immediately after the declaration of sockfd, we do
if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, (const char *) &iOption,  sizeof(int)) == -1) {
    error("setsockopt");
    exit(1);
}
</pre>
* On Windows, we can use [http://stackoverflow.com/questions/1592055/bind-address-already-in-use TCPView] to see which process is listening on which port, socket status. On Linux, we can use '''netstat -a''' command (it gives a long output)
<pre>
mli@PhenomIIx6:~/Downloads$ sudo netstat -a | grep 51717
tcp        0      0 *:51717                *:*                    LISTEN
</pre>
* We can choose any port number between 2000 and 65535.
* If we use 51717 port for example, the server will open that port. But once the program is finished, the port will be closed immediately. Use linux command '''netstat -lp --inet''' to check which ports are opened.
Similar examples:
* http://www.thegeekstuff.com/2011/12/c-socket-programming/. This example is similar to Example 1. The server continuously runs and sends the date and time as soon as a client connects to it. To temporarily change the time, use <pre>sudo date +%T -s "10:13:13"</pre>.
* http://www.binarytides.com/server-client-example-c-sockets-linux/
* http://www.amparo.net/ce155/socket-ex.html More complicated example
* [http://stackoverflow.com/questions/7483301/how-do-i-print-client-address-while-running-server-code-in-socket Print client IP address]
=== Example 2 (in C)  ===
http://www.prasannatech.net/2008/07/socket-programming-tutorial.html
Server side (The code is [http://taichi.selfip.net:81/lang/c/socket/example2/tcpserver.c here]):
<pre>
$ ./tcpserver
TCPServer Waiting for client on port 5000
I got a connection from (127.0.0.1 , 36123)
SEND (q or Q to quit) : yes
RECIEVED DATA = Got it.
SEND (q or Q to quit) : how are you
RECIEVED DATA = I am fine.
SEND (q or Q to quit) : q
q
^C
$
</pre>
Client side (The code is [http://taichi.selfip.net:81/lang/c/socket/example2/tcpclient.c here]):
<pre>
$ ./tcpclient
Recieved data = yes
SEND (q or Q to quit) : Got it.
Recieved data = how are you
SEND (q or Q to quit) : I am fine.
</pre>
=== Example 3 - Simple HTTP server ===
The example is modified (hear files only) from http://rosettacode.org/wiki/Hello_world/Web_server (This is a wonderful website which include creating hello world web server using different programming languages). PS. the instruction in http://mwaidyanatha.blogspot.com/2011/05/writing-simple-web-server-in-c.html is worth a look but 4 lines of creating HTML standard headlines is not successful and too complicated.
We can test the server by
* opening a browser and type http://localhost:8080. Create the file by '''nano testServer.c'''
* telnet localhost 8080
* curl -i http://localhost:8080
<syntaxhighlight lang='c'>
#include<netinet/in.h>
#include<stdio.h>
#include<stdlib.h>
#include<sys/socket.h>
#include<sys/stat.h>
#include<sys/types.h>
#include<unistd.h>
char response[] = "HTTP/1.1 200 OK\r\n"
"Content-Type: text/html; charset=UTF-8\r\n\r\n"
"<html>\r\n"
"<head><title>Bye-bye baby bye-bye</title>\r\n"
"<style>\r\n"
"  body { background-color: #111 }\r\n"
"  h1 { font-size:4cm; text-align: center; color: black;"
"  text-shadow: 0 0 2mm red} \r\n"
"</style></head>\r\n"
"<body><h1>Goodbye, world!</h1></body></html>\r\n";
int main()
{
int one = 1, client_fd;
struct sockaddr_in svr_addr, cli_addr;
socklen_t sin_len = sizeof(cli_addr);
int sock = socket(AF_INET, SOCK_STREAM, 0);
if (sock < 0)
err(1, "can't open socket");
setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(int));
int port = 8080;
svr_addr.sin_family = AF_INET;
svr_addr.sin_addr.s_addr = INADDR_ANY;
svr_addr.sin_port = htons(port);
if (bind(sock, (struct sockaddr *) &svr_addr, sizeof(svr_addr)) == -1) {
close(sock);
err(1, "Can't bind");
}
listen(sock, 5);
while (1) {
client_fd = accept(sock, (struct sockaddr *) &cli_addr, &sin_len);
printf("got connection\n");
if (client_fd == -1) {
perror("Can't accept");
continue;
}
write(client_fd, response, sizeof(response) - 1); /*-1:'\0'*/
close(client_fd);
}
}
</syntaxhighlight>
Compile and run it by ''gcc testServer.c; ./a.out''.
=== Example 4 - Mimic browser request ===
The code is based on the post.
http://codebase.eu/tutorial/linux-socket-programming-c/. My local copy of [http://taichi.selfip.net:81/lang/c/socket/browser].
This is another similar post. http://www.binarytides.com/receive-full-data-with-recv-socket-function-in-c/ which teaches how to receive full data with recv socket function in C.
==== Testing tcpclient.c ====
The result is a program that connects to google and downloads (the first 1000 bytes of) the google homepage.
<pre>
$ g++ tcpclient.cpp
$ ./a.out
Setting up the structs...
Creating a socket...
Connect()ing...
send()ing message...
Waiting to recieve data...
1000 bytes recieved :
HTTP/1.1 200 OK
Date: Wed, 20 Mar 2013 14:41:54 GMT
Expires: -1
Cache-Control: private, max-age=0
Content-Type: text/html; charset=ISO-8859-1
Set-Cookie: PREF=ID=78ef359985426090:FF=0:TM=1363790513:LM=1363790514:S=UO5PtdM9ETqX6Mm_;
Set-Cookie:
Server: gws
X-XSS-Protection: 1; mode=block
X-Frame-Options: SAMEORIGIN
Transfer-Encoding: chunked
8000
<!doctype html><html itemscope="itemscope" itemtype="http://schema.org/WebPage">
<head><meta content="Search the world's information,
Receiving complete. Closing socket...
$
</pre>
==== Testing tcpserver.c and tcpclient2.c ====
Server side:
<pre>
$ ./tcpserver
Setting up the structs...
Creating a socket...
Binding socket...
Listen()ing for connections...
Connection accepted. Using new socketfd : 4
Waiting to recieve data...
37 bytes recieved :
GET / HTTP/1.1
host: www.google.com
send()ing back a message...
Stopping server...
$
</pre>
Client side (modify tcpclient.c to use IP 127.0.0.1 and port 5556):
<pre>
$ ./tcpclient2
Setting up the structs...
Creating a socket...
Connect()ing...
send()ing message...
Waiting to recieve data...
10 bytes recieved :
thank you.#▒
Receiving complete. Closing socket...
$
</pre>
=== Example 5 - Windows socket (almost implies C++) ===
* http://www.tenouk.com/cnwinsock2tutorials.html
* http://msdn.microsoft.com/en-us/library/windows/desktop/ms738545%28v=vs.85%29.aspx
* http://www.binarytides.com/winsock-socket-programming-tutorial/
* [http://www.win32developer.com/tutorial.shtm Win32 and Winsock networking tutorials]
* http://www.codeproject.com/Articles/412511/Simple-client-server-network-using-Cplusplus-and-W
=== Example 6 Get image using Qt ===
See Chapter 14. Foundation of Qt Development. The code is on https://github.com/arraytools/Qt/tree/master/FQD/Chapter14.
=== Example 7 Trip planner using Qt ===
See Chapter 15. Networking on C++ GUI Programming Qt 4. The code is on http://taichi.selfip.net:81/lang/c/qt-book/chap15/.
== Basic(s) ==
=== C++ standard ===
* https://isocpp.org/ [https://isocpp.org/std/status Status], [https://isocpp.org/std/the-standard Current Standard], Articles, Events & Training.
* [https://en.wikipedia.org/wiki/C%2B%2B C++ Wikipedia]
=== Cheat sheet, Tutorial, Crash course ===
* [https://www.youtube.com/watch?v=2egL4y_VpYg What – if anything – have we learned from C++?] by Bjarne Stroustrup.
* https://github.com/jameshfisher/cc-cheatsheet The cheat sheet is created using LibreOffice Write (odt format) and it fits a A4 paper.
* http://www.pa.msu.edu/~duxbury/courses/phy480/Cpp_refcard.pdf or my [http://taichi.selfip.net:81/lang/c/Cpp_refcard.pdf local copy].
* [http://cs.fit.edu/~mmahoney/cse2050/how2cpp.html How to programming C++] by Matt Mahoney. It gave more explanation like using 'const' pointer to pass large object to avoid overhead of copying.
* [http://developer.kde.org/~wheeler/cpp-pitfalls.html C++ pitfalls]. It covers references, const keyword, Public, Protected and Private Labels, virtual Methods and mutable Class Members.
* [http://www.digilife.be/quickreferences/QRC/STL%20Quick%20Reference%201.29.pdf STL reference] by Yotam Medini.
* [http://en.cppreference.com/w/ cppreference.com]
* [http://www.dev-hq.net/c++/ dev-hq.net] Good explanation.
* [http://www.learncpp.com/ learncpp.com] Quite good w/o advertising, w/ discussions & keep update.
* [http://en.opensuse.org/images/b/b9/C---for-C-programmers.pdf C++ for C programmers]
* [http://www.loria.fr/~rougier/teaching/c++-crash-course/index.html C++ crash course for C programmers]
* [http://www.stat.berkeley.edu/scf/paciorek-cppWorkshop.pdf C++ for Statisticians] with a focus on interfacing from R and R packages.
* [http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list The Definitive C++ Book Guide and List]
* [http://mathbits.com/MathBits/CompSci/compsci.htm Introduction to Programming in C++] which is suitable for high school classes.
* Youtube channels from [https://www.youtube.com/user/BoQianTheProgrammer/featured Bo Qian]. The playlists include advanced C++, modern C++, STL, Boost library, algorithms, etc.
* https://class.coursera.org/cplusplus4c-002/lecture C++ for C Programmers
* [https://www.cs.umd.edu/class/fall2010/cmsc216/Web/index.htm Introduction to Computer Systems] from UMD. [https://www.cs.umd.edu/class/spring2016/cmsc216/ CMSC 216] from 2016.
* http://www.makeuseof.com/tag/best-youtube-programming-tutorials/ The 17 Best YouTube Programming Tutorials (including C++)
* https://blog.tartanllama.xyz/
=== Differences between C and C++ ===
* http://cs-fundamentals.com/tech-interview/c/difference-between-c-and-cpp.php
* http://techwelkin.com/difference-between-c-and-c-plus-plus
* https://www.invensis.net/blog/it/25-key-differencesbetween-c-plus-plus/
=== Books for C++ ===
[http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list The Definitive C++ Book Guide and List] from the stackoverflow post.
* Bjarne Stroustrup.
** [http://www.amazon.com/The-Programming-Language-4th-Edition/dp/0321563840/ The C++ Programming Language, 4th Edition] 1.2k pages (epub)
** [http://www.amazon.com/Programming-Principles-Practice-Using-Edition/dp/0321992784 Programming: Principles and Practice Using C++ (2nd Edition)] and [http://www.stroustrup.com/Programming/ book website] 1.2k pages (epub)
** [http://www.amazon.com/Tour-In-Depth-Series-Bjarne-Stroustrup/dp/0321958314/ref=pd_sim_14_4?ie=UTF8&refRID=13VJ6V7389WTX1SSXCYT A Tour of C++] (less than 200 pages)
* [http://www.amazon.com/exec/obidos/ASIN/0321714113/solarianprogr-20/ C++ Primer (2012)] by Stanley B. Lippman. (epub)
* [http://www.amazon.com/Accelerated-C-Practical-Programming-Example/dp/020170353X/ Accelerated C++: Practical Programming by Example (2000)] by Andrew Koenig (epub)
* [http://cppstdlib.com/ The C++ Standard Library - A Tutorial and Reference] by Nicolai M. Josuttis
* Effective C++ books by [http://www.artima.com/cppsource Scott Meyers]
** [http://www.amazon.com/More-Effective-Improve-Programs-Designs/dp/020163371X/ More Effective C++: 35 New Ways to Improve Your Programs and Designs (1996)]
** [http://www.amazon.com/Effective-Specific-Improve-Programs-Designs/dp/0321334876 Effective C++: 55 Specific Ways to Improve Your Programs and Designs (2005)]
** [http://www.amazon.com/Effective-Modern-Specific-Ways-Improve/dp/1491903996/ref=zg_bs_9045760011_3 Effective Modern C++: 42 Specific Ways to Improve Your Use of C++11 and C++14 (2014)] (epub)
* [http://www.amazon.com/Primer-Plus-Edition-Developers-Library/dp/0321776402/ref=pd_cp_14_3?ie=UTF8&refRID=1FDBNJMAWRQ2M1V7X0SK C++ Primer Plus] by  Stephen Prata
* [http://www.amazon.com/Jumping-into-C-Alex-Allain/dp/0988927802/ref=zg_bs_9045760011_18 Jumping into C++] by Alex Allain
* [http://www.amazon.com/exec/obidos/ASIN/0470932449/solarianprogr-20/ Professional C++] containing C++11 syntax, by by Marc Gregoire, Nicholas A. Solter, Scott J. Kleper
* [http://www.amazon.com/C-Guide-Inc-BarCharts/dp/1423202635/ref=pd_bxgy_14_img_y C++ Guide Pamphlet]
* [https://www.amazon.com/Cookbook-Solutions-Examples-Programmers-Cookbooks/dp/0596007612 C++ Cookbook: Solutions and Examples for C++ Programmers] by D. Ryan Stephens
* [http://www.amazon.com/C-Demystified-Jeff-Kent/dp/0072253703 C++ Demystified: A Self-Teaching Guide] by Jeff Kent (one chapter covers Character, C-String, and C++ String Class Functions) the book does not cover too much.
* C++ Without Fear
* [https://www.amazon.com/Absolute-C-6th-Walter-Savitch/dp/0133970787 Absolute C++] by Walter Savitch & Kenrick Mock. Explanation is very clear. Format is excellent.
* [http://mariusbancila.ro/blog/2017/05/09/my-book-modern-cpp-programming-cookbook-has-been-published/ Modern C++ Programming Cookbook] by Marius Bancila (May 2017) and the source code in [https://github.com/PacktPublishing/Modern-Cpp-Programming-Cookbook github]
* [http://www.amazon.com/gp/bestsellers/books/9045760011/ref=sr_bs_1 Best sellers on C++] from Amazon.
=== Books for C ===
* [https://www.hpe.com/us/en/insights/articles/top-linux-developers-recommended-programming-books-1808.html Top Linux developers' recommended programming books]
* [http://www.ossblog.org/master-c-programming-with-open-source-books/ MASTER C++ PROGRAMMING WITH OPEN-SOURCE BOOKS]
* S Oliveira and D Stewart (2006) [http://www.amazon.com/exec/obidos/ASIN/0521675952/7210-20 Writing scientific software: A guide to good style].  Cambridge University Press.     
* BW Kernighan and DM Ritchie (1988) [http://www.amazon.com/exec/obidos/ASIN/0131103628/7210-20 The C programming language], 2nd edition.  Prentice Hall, Englewood Cliffs, NJ.
* PJ Plauger (1992) [http://www.amazon.com/exec/obidos/ASIN/0131315099/7210-20 The standard C library]. Prentice Hall, Englewood Cliffs, NJ.
* WH Press et al. (1992) [http://www.amazon.com/exec/obidos/ASIN/0521431085/7210-20 Numerical recipes in C: The art of scientific computing], 2nd edition.  Cambridge University Press.
* BW Kernighan and R Pike (1999) [http://www.amazon.com/exec/obidos/ASIN/020161586X/7210-20 The practice of programming]. Addison-Wesley, Reading, MA.
* S Oualline (1992) [http://www.amazon.com/exec/obidos/ASIN/1558512918/7210-20 C Elements of Style].  M&T Books, San Mateo, CA.
* S Oualline (1997) [http://www.amazon.com/exec/obidos/ASIN/1565923065/7210-20 Practical C programming], 3rd edition.  O'Reilly & Associates, Sebastopol, CA.
* K Loudon (1999) [http://www.amazon.com/exec/obidos/ASIN/1565924533/7210-20 Mastering algorithms with C].  O'Reilly & Associates, Sebastopol, CA.
* MK Loukides et al. (1997) [http://www.amazon.com/exec/obidos/ASIN/1565921127/7210-20 Programming with GNU software].  O'Reilly & Associates, Sebastopol, CA.
* Derek M. Jones (2009) [http://www.knosof.co.uk/cbook/ C Language Book/An Economic and Cultural Commentary] (free pdf)
* Kenneth Reek (1998) [https://www.cs.rit.edu/~kar/pointers.on.c/ Pointers on C]
=== C++11 for Ubuntu 12.04 ===
http://askubuntu.com/questions/113291/how-do-i-install-gcc-4-7
<pre>
sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get install gcc-4.7 g++-4.7
# Also, don't forget to update-alternatives, as suggested here
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.6 60 --slave /usr/bin/g++ g++ /usr/bin/g++-4.6
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.7 40 --slave /usr/bin/g++ g++ /usr/bin/g++-4.7
sudo update-alternatives --config gcc
</pre>
When issuing the last command, it will ask what version of gcc to use.
=== First code ===
==== The simplest c/c++ code ====
Mentioned by the above book ''Exploring Beaglebone: Tools and Techniques for Building with Embedded Linux''.
<pre>
main() { }
</pre>
==== Hello World ====
C version  (compiled by gcc helloworld.c)
<pre>
#include <stdio.h>
int main(int argc, char *argv[])
{
    printf("Hello World!\n");
    return 0;
} 
</pre>
C++ version (compiled by g++ <helloworld.cpp>). Note that though gcc is installed by default but '''g++''' is not installed by default in Ubuntu 14.04 and Linux Mint 17.3. We need to install g++ manually.
<pre>
#include <iostream>
#include <string>
using namespace std;
int main()
{
string name;
cout << "Enter your name: ";
cin >> name;
cout << "\nHello, " << name << "! It's nice to meet you!";
return 0;
}
</pre>
Note that following the keyword cout is the '''insertion operator''' (or '''output stream operator''') '<<' and following the keyword cin is the '''extraction operator''' (or '''input stream operator''') '>>'. Both are public member functions in iostream class.
The angular brackets (<>) around the include filename means that it is a standard, rather than a user-defined include.
The header stdio.h is located in /usr/include/ directory and iostream header is in /usr/include/c++/4.X/iostream.
Running '''g++ helloworld.cpp''' actually involved several steps (process cpp to another cpp, created assembly code, compile assembly code, link and create an executable). See the book ''Exploring Beaglebone: Tools and Techniques for Building with Embedded Linux'' Chapter 2.3.
==== #include <iostream> ====
* http://stackoverflow.com/questions/26086842/why-should-i-include-the-header-file-iostream-after-using-the-namespace-std
When you do #include <iostream> it causes a set of classes and other things to be included in your source file. For iostream, and most of the standard library headers, they place these things in a namespace named std.
So the code for #include <iostream> looks something like this:
<pre>
namespace std {
    class cin  { ... };
    class cout { ... };
    class cerr { ... };
    class clog { ... };
    ...
}
</pre>
==== namespace ====
* http://www.cplusplus.com/doc/tutorial/namespaces/
<syntaxhighlight lang='cpp'>
#include <iostream>
using namespace std;
namespace foo
{
  int value() { return 5; }
}
namespace bar
{
  const double pi = 3.1416;
  double value() { return 2*pi; }
}
int main () {
  cout << foo::value() << '\n';
  cout << bar::value() << '\n';
  cout << bar::pi << '\n';
  return 0;
}
</syntaxhighlight>
==== The std namespace --- C++ Standard Library ====
https://en.wikipedia.org/wiki/C%2B%2B_Standard_Library
* The C++ Standard Library is a collection of classes and functions.
* Features of the C++ Standard Library are declared within the '''std''' namespace.
* The C++ Standard Library also incorporates 18 headers of the ISO C90 C standard library ending with ".h", but their use is deprecated.[2] No other headers in the C++ Standard Library end in ".h".
In the above <helloworld.cpp> C++ program, <iostream> is one of standard headers included in the C++ Standard Library. The [https://en.wikipedia.org/wiki/C%2B%2B_Standard_Library wikipedia] page categorizes the standard headers by
# Contains: <array>, <list>, <map>, <vector>, ...
# General: <algorithm>, <utility>, ...
# Localization: <locale>, <codecvt>
# Strings: <string>, <regex>
# Streams and Input/Output: <fstream>, <iostream>, <istream>, <ostream>, <sstream>
# Language support
# Thread support library
# Numerics library
# C standard library
==== int main() return value ====
* http://stackoverflow.com/questions/204476/what-should-main-return-in-c-and-c
* http://stackoverflow.com/questions/13205941/whats-the-meaning-of-the-return-value-of-int-main-and-how-to-display-it
The return value for main should indicate how the program exited. Normal exit is generally represented by a 0 return value from main. Abnormal termination is usually signalled by a non-zero return but there is no standard for how non-zero codes are interpreted.
=== Main function ===
[https://opensource.com/article/19/5/how-write-good-c-main-function How to write a good C main function]
=== Naming convension ===
* https://google.github.io/styleguide/cppguide.html
* For private data members, prefixed with '''m_''' (eg. see [http://www.themagpi.com/issue/issue-23/ TheMagPi-issue 23]).
** [http://stackoverflow.com/questions/13018189/what-does-m-variable-prefix-means What does `m_` variable prefix means?]
** https://en.wikipedia.org/wiki/Hungarian_notation
** For example, the [https://sourceforge.net/projects/qroundprogressbar/ QRoundProgressBar] (Qt Circular Progress Bar Widget).
=== Some Words about Standalone Application vs Web Application ===
* Users can make use of the hardware power on my own machine
* Users don't worry his/her data will be used by 3rd party
* Users don't need to worry about potential network problem
* Users don't need to worry his/her jobs need to be waited in queue
* Maintainer don't need to worry the server can be hacked (purposely or incidentally). The server has to pass some security exam before it can be opened to public.
=== Header/Include guard, Preprocessor ===
'''#ifndef ... #define ... #endif'''. See [http://en.wikipedia.org/wiki/Include_guard wikipedia] and [http://www.cplusplus.com/doc/tutorial/preprocessor/ cplusplus.com].
=== Basic Arithmetic ===
<pre>
  double a = 10, b=3;
  cout << int (a/b) << endl;      // 3
  cout << int (a/b + .5) << endl;  // 3
  a= 11;
  cout << int (a/b) << endl;    // 3
  cout << int (a/b + .5) << endl;  // 4
</pre>
=== bool type ===
* http://en.cppreference.com/w/cpp/language/types (true and false)
* [http://www.cplusplus.com/forum/beginner/71562/ true vs TRUE] and false vs FALSE
=== Data Type Ranges ===
* http://msdn.microsoft.com/en-us/library/s3f49ktz(v=vs.90).aspx
{| class="wikitable"
! type
! bytes
! range
|-
| int
| 4
| –2,147,483,648 to 2,147,483,647
|-
| unsigned int
| 4
| 0 to 4,294,967,295
|-
| long
| 4
| –2,147,483,648 to 2,147,483,647
|-
| unsigned long
| 4
| 0 to 4,294,967,295
|-
| long long
| 8
| –9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
|-
| unsigned long long
| 8
| 0 to 18,446,744,073,709,551,615
|-
| float
| 4
| 3.4E +/- 38 (7 digits)
|-
| double
| 8
| 1.7E +/- 308 (15 digits)
|}
* http://www.cplusplus.com/doc/tutorial/variables/
=== Conditional operation ? : ===
* https://msdn.microsoft.com/en-us/library/exefbdtf.aspx
<syntaxhighlight lang='c'>
int main(int argc, char** argv)
{
  // logical-OR expression  ?  expression  :  conditional-expression
  char* filename = argc >= 2 ? argv[1] : (char*)"input.txt";
}
</syntaxhighlight>
=== Increment ++ operator ===
Note that ++variable is slightly faster than its alternative variable++ since the alternative needs to make a copy of itself before returning the result. (see SeqAn tutorial [http://seqan.readthedocs.org/en/master/Tutorial/FirstStepsInSeqAn.html#tutorial-first-steps-in-seqan here])
=== namespace ===
==== Create Utilities Function ====
Don't put them in a class; just make them non-member functions at namespace scope.
* http://stackoverflow.com/questions/8226489/organising-utility-functions-in-c
<source lang="cpp">
// header file <utility.hpp>
namespace utility
{
  void function1();
  void function2();
  void function3();
  template<typename T>
  void function4()
  {
      //function template definition should be here!
  }
}
// cpp <utility.cpp>
#include "utility.hpp"
namespace utility
{
  void function1()
  {
        //code
  }
  void function2()
  {
        //code
  }
  void function3()
  {
        //code
  }
}
</source>
=== Memory Management ===
* An Introduction to '''Pointers''' for Programmers http://www.makeuseof.com/tag/introduction-pointers-programmers/
* [http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-096-introduction-to-c-january-iap-2011/lecture-notes/ MIP Open Course]
On page 25 of the memory management lecture note, it mentions
* If using int x; the allocation occurs on a region of memory called the stack.
* If using new int; the allocation occurs on a region of memory called the heap.
Heap is outside the scope of a given function and must be explicitly cleaned up with '''free'''. When free is called, the memory is released back to system but the pointer is not set to be null. Therefore if malloc and free are used within a loop the pointer should be set to null afterwards, to allow a test on the pointer after the next malloc call. See also
* Issue 17 of [http://www.themagpi.com/issue/issue-17/ The MagPi].
* [https://stackoverflow.com/a/17444554 What is the default value after malloc in c?]. Consider [http://www.cplusplus.com/reference/cstdlib/calloc/ calloc()] or malloc() + [http://www.cplusplus.com/reference/cstring/memset/ memset()].
Access the memory allocation of variable out of scope of function  will cause compilation warnings and may result in unexpected crashes. The following example appeared in the MagPi magazine. A similar example also appeared in the 10.6 (Three kinds of memory management) of Accelerated C++.
<syntaxhighlight lang='cpp'>
include <stdio.h>
int* fun() {
  int i=0;  /* A solution is to use 'static int i=0;'          */
  return &i; /* Return the memory address of i. Do not do this! */
}
int main() {
  printf("%p\n",fun()); /* Print the memory address of i within fun() */
  return 0;
}
</syntaxhighlight>
Solutions:
1. Use the 'static' keyword as in the comment of the code
2. If a pointer is assigned the address of memory on the heap inside a function, then it can be accessed afterwards. After the function call, it is still necessary to call free to release the dynamically allocated memory.
<syntaxhighlight lang='cpp'>
#include <stdio.h>
#include <stdlib.h>
int* fun() {
  int *i=0;
  i=(int*)malloc(sizeof(int));
  return i;
}
int main() {
  int *p = 0;
  p = fun();
  printf("%p\n",p);
  free(p);
  return 0;
}
</syntaxhighlight>
3. Use standard containers such as vector. See the [https://github.com/arraytools/C/blob/master/accelerated_unix/chapter05/split.cc split function] in Accelerated C++ Chapter 5.
<syntaxhighlight lang='cpp'>
#include<vector>
vector<string> split(const string& s)
{
  vector<string> ret;
  ...
  return ret;
}
</syntaxhighlight>
As mentioned in the [http://www.stroustrup.com/bs_faq2.html Stroustrup C++ Style and Technique FAQ]. ''A vector keeps track of the memory it uses to store its elements. When a vector needs more memory for elements, it allocates more; when a vector goes out of scope, it frees that memory. Therefore, the user need not be concerned with the allocation and deallocation of memory for vector elements.''
==== Heap vs Stack ====
When an object is needed within several different functions calls, it might be helpful to create it on the heap instead.
The difference between creating an object on the stack and the heap is
* Objects (eg '''automatic variables''' including ordinary local variables, local variables declared using '''auto''', and local variables declared using '''register''') on the stack are automatically cleaned up when they go out of scope. '''The advantage of the stack is the speed and book-keeping. The drawback of the stack is the data is lost once the function returns.'''
* Objects (eg pointers) on the heap stay in memory. It has to be explicitly deleted.
<syntaxhighlight lang='cpp'>
// Using stack
#include <iostream>
#include "Square.h"
using namespace std;
int main() {
  Square s; // Using the default constructor
  Square s2(3. 0, ' b' ); // Using the second constructor
  cout << " s.area()=" << s.area() << " , s.colour()=" << s.colour() << endl ;
  cout << " s2.area()=" << s2.area() << " , s2.colour()=" << s2.colour() << endl ;
  return 0;
}
// Using heap
#include <iostream>
#include "Square.h"
using namespace std;
int main() {
  Square *s = new Square(); // Using the default constructor
  Square *s2 = new Square(3. 0, ' b' ); // Using the second constructor
  cout << "s->area()=" << s->area() << " , s->colour()=" << s->colour() << endl ;
  cout << "s2->area()=" << s2->area() << " , s2->colour()=" << s2->colour() << endl ;
  delete s;
  delete s2;
  return 0;
}
</syntaxhighlight>
Some references:
# [https://www.quora.com/What-could-be-the-advantages-of-stack-over-heap-dynamic-memory-allocation-in-C What could be the advantages of stack over heap dynamic memory allocation in C?]
# [https://www.raspberrypi.org/magpi-issues/MagPi23.pdf The MagPi issue 23]
# C++ Primer Plus Chapter 9.
==== The '''auto_ptr''' Class (deprecated as of C++11) ====
* See Chapter 16 of C++ Primer Plus.
* http://www.cplusplus.com/reference/memory/auto_ptr/. This class template is deprecated as of C++11. unique_ptr is a new facility with a similar functionality, but with improved security (no fake copy assignments), added features (deleters) and support for arrays.
=== new, delete operators, pointers and dynamic  memory ===
Use new/delete instead of malloc()/free() which depends on <cstdlib> in C++. For example, the following code was a modification from Listing 4.22 (p169) <delete.cpp> from the book 'C++ Primer Plus'.
The memory created by new operator is called '''heap''' or '''free store'''. Forgetting to use '''delete''' operator will cause a memory leak. This kind of storage is called '''dynamic storage''' which differs from '''automatic storage''' and '''static storage/stack'''.
<syntaxhighlight lang='cpp'>
#include <iostream>
#include <cstring> // strlen()
using namespace std;
int main()
{
  char * pn = new char(strlen("/home") + strlen("/Downloads") + 1);
  strcpy(pn, "/home");
  cout << pn << endl;
  cout << strlen(pn) << endl;
  strcat(pn, "/Downloads");
  cout << pn << endl;
  cout << strlen(pn) << endl;
  delete [] pn;
  return 0;
}
</syntaxhighlight>
* '''new''' allows to allocate a variable length arrays while allocating arrays on the stack stack size must be constant. See p52 of [http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-096-introduction-to-c-january-iap-2011/lecture-notes/ MIT Open Course] or p33 of [http://www.themagpi.com/issue/issue-23/ TheMagPi issue 23].
* '''new''' and '''delete''' should be used together. See p62 to p67 of the above MIT note.
* Pay attention to the scope of variables. If a variable is declared within a parenthesis, it will evaporate once it exists the parenthesis. See p69 of the above MIT note for the problem and p75 for the solution.
* See Chapter 4, 9, 12 of C++ Primer Plus. Or Chapter 14 of C++ Without Fear
* When a program interacts with other programs in a GUI or network environment, it typically passes or receives pointers to objects.
==== scalar ====
<pre>
int *pn = new int;  // dynamic
delete pn;
VS
int higgens;
int *pt = &higgens;  // not dynamic
</pre>
==== Array ====
* [https://www.howtoforge.com/linux-c-programming-tutorial-part-19-pointers-and-arrays/ Linux C Programming Tutorial Part 19: Pointers and Arrays] from HowToForge.
* Every array contains a sequence of one or more objects of the same type. The number of elements in the array must be known at compile time, which requirement implies that arrays cannot grow or shrink dynamically the way library containers do. See 10.1.3 of Accelerated C++.
* Because arrays are not class types, they have no members. In particular, they do not have the ''size_type'' member to name an appropriate type to deal with the size of an array.
* The <cstddef> header defines '''size_t''', which is a more general type. <syntaxhighlight lang='cpp'>
const size_t NDim = 3;
double coords[NDim];
static const double numbers[] = {97, 94, 90, 0 };
static const size_t ngrades = sizeof(numbers) / sizeof(*numbers);
</syntaxhighlight>
* There is a fundamental relationship between arrays and pointers. <syntaxhighlight lang='cpp'>
*coords = 1.5;  // the array's initial element
</syntaxhighlight>
: <syntaxhighlight lang='cpp'>
int *pn = new int[10]; // OR  int *pn; pn = new int[10];
pn[1] =3;
delete[] pn;
int *parr[40]; // array of 40 pointers to int
</syntaxhighlight>
==== function ====
<syntaxhighlight lang='cpp'>
# Listing 4.22.1 <delete.cpp> from C++ Prime Plus (5th ed)
char *name;
char *getname(); // prototype
char *getname() {
  char* out = new char[5];
  ...
  return out;
}
name = getname();
delete [] name;
# 1. It is possible to use 'new' in a function and use 'delete' in the main function.
# 2. This memory is not controlled by scope. It means new & delete gives you more
#    power to control over how a program uses memory.
# 3. See also PPP Chapter 20.1.
#    double* jack_data = get_from_jack(&jack_count);
#    vector<double>* jill_data = get_from_jill();
#    //...
#    delete[] jack_data;
#    delete jill_data;
</syntaxhighlight>
==== use new(delete) in a class constructor(destructor) ====
<syntaxhighlight lang='cpp'>
# Listing 12.4, 12.5 and p591-593 of C++ Primer Plus (5th ed)
String::String (const String & st)
{
  len = st.len;
  str = new char [len + 1];
  std::strcpy(str, st.str);
}
String::~String()
{
  delete [] str;
}
</syntaxhighlight>
==== dereference ====
<syntaxhighlight lang='cpp'>
Fraction *pFrac = new Fraction(1, 2);
(*pFrac).get_num(); // OR pFrac->get_num();
(*pFrac).member;  //  pFrac->member;
</syntaxhighlight>
==== combining dereference and increment in a single expression ====
<syntaxhighlight lang='cpp'>
auto pbeg = v.begin();
// print elements up to the first negative value
while (pbeg != v.end() && *pbeg >= 0)
    cout << *pbeg++ << endl; // print the current value and advance pbeg
</syntaxhighlight>
<span style="color: red"> The precedence of postfix increment is higher than that of the dereference operator, so *pbeg++ is equivalent to *(pbeg++) </span>. The operand of * is the unincremented value of pbeg. Thus, the statement prints the element to which pbeg originally pointed and increments pbeg.
==== variable array size in new ====
<syntaxhighlight lang='cpp'>
int n;
cout << "How many elements?";
cint >> n;
int *p = new int[n];
....
delete [] p;
</syntaxhighlight>
==== null pointer ====
See [http://c-faq.com/null/index.html Null Pointers] in comp.lang.c Frequently Asked Questions.
There are several ways to create a null pointer.
<syntaxhighlight lang='cpp'>
int *p1 = nullptr; // C++11
int *p2 = 0;
int *p3 = NULL; // must #include cstdlib header which defines the preprocessor variable NULL as 0.
</syntaxhighlight>
Modern C++ programs generally should avoid using NULL and use nullptr instead.
To test whether a pointer is valid, we can use either
<pre>
if (p0 != nullptr)  // consider p0 valid
</pre>
OR
<pre>
if (p0)  // consider p0 valid; p0 is not zero.
</pre>
==== dealing with problems with memory allocation ====
If the memory requested is not available, the new operator returns a null pointer.
You can test for this possibility and take the appropriate action.
<syntaxhighlight lang='cpp'>
int *p = new int[1000];
if (!p) {
cout << "Insufficient memory.";
exit(-1);
}
</syntaxhighlight>
==== Memory leak ====
[http://stackoverflow.com/questions/9979776/does-calling-new-twice-on-the-same-pointer-without-calling-delete-in-betwe Does calling '''new''' operator twice on the same pointer without calling '''delete''' operator in between cause a memory leak?]
The answer is Yes.
==== Test memory leak ====
The following C++ code was used to test a memory leak and also the capacity of memory. Note that when 'n' is declared as unsigned int, the maximum value can be 4,294,967,295=2^32-1 which corresponds to 32,767 MB (about 32 GB) in size for a double vector (assume 8 bytes). If I change the type of n to "unsigned long long", its range can go up to 18,446,744,073,709,551,615=2^64-1 or 137438953472 GB in size for a double vector.
Note that the program should be built in x64 instead of win32 version in Visual Studio if we like to test it on non-Windows-XP OS on Windows. Linux OS does not need to worry...
<syntaxhighlight lang='cpp'>
#include <iostream>
using namespace std;
void foo(unsigned long long n)
{
    double *ptr = new (std::nothrow) double[n];
    if (!ptr) {
        cout << "Failed to allocate double[n]" << endl;
    } else {
        ptr[1] = 1.0;
        ptr[n-1] = 2.0;
        cout << "double[n] is allocated successfully!" << endl;
    }
}
int main()
{
    cout << "The program is used to test memory leak. \n";
    cout << "Do not worry. It won't crash your computer.\n";
    cout << "The source code is available on https://gist.github.com/arraytools/6689581 \n" << endl;
    unsigned long long n;
    int testChoice;
    cout << "Enter your choice 1=main, 2=sub: ";
    cin >> testChoice;
    cout << "\nNext enter the array size (<= 18,446,744,073,709,551,615) : ";
    cout << "\nSome common scenarios";
    cout << "\n268,435,456-1 = 2GB";
    cout << "\n536,870,912-1 = 4GB";
    cout << "\n1,073,741,824-1 = 8GB";
    cout << "\n2,147,483,648-1 = 16GB";
    cout << "\n4,294,967,296-1 = 32GB";
    cout << "\n8,589,934,592-1 = 64GB";
    cout << "\n";
    cin >> n;
    if (testChoice == 1)
    {
        double *ptr = new (std::nothrow) double[n];
        if (!ptr)
        {
            cout << "Failed to allocate double[n]" << endl;
        } else {
            ptr[1] = 1.0;
            ptr[n-1] = 2.0;
            cout << "double[n] is allocated successfully!" << endl;
        }
    } else {
        foo(n);
    }
    return 0;
}
</syntaxhighlight>
and my testing result on Windows XP with 2GB physical memory & 2GB virtual memory. The program is compiled into 32-bit console application. It is a different story when tested on 64-bit Windows 7.
{| class="wikitable sortable collapsible" border="1" style="border-collapse:collapse; text-align:center;"
|-
! bgcolor="#00BFFF"|Array Size !! bgcolor="lightgreen"|Main !! bgcolor="#FE6F5E"|Function
|-
| 20,000,000 =150 MB || OK || OK
|-
| 200,000,000=1.5 GB || OK || OK
|-
| 2000,000,000=15 GB || Not OK || Not OK
|-
| 500,000,000 =4 GB || Not OK || Not OK
|-
| 250,000,000=1.91 GB || OK || OK
|-
| 270,000,000=2.06 GB || Not OK || Not OK
|}
=== Enumerations ===
See 2.3.3 of The C++ Programming language or http://en.cppreference.com/w/cpp/language/enum
==== Unscoped enumeration ====
<syntaxhighlight lang='cpp'>
enum Color { RED, GREEN, BLUE };
Color r = RED;
switch(r)
{
    case RED  : std::cout << "red\n";  break;
    case GREEN: std::cout << "green\n"; break;
    case BLUE : std::cout << "blue\n";  break;
}
</syntaxhighlight>
==== Scoped enumerations ====
<syntaxhighlight lang='cpp'>
enum class Color { RED, GREEN = 20, BLUE };
Color r = Color::BLUE;
switch(r)
{
    case Color::RED  : std::cout << "red\n";  break;
    case Color::GREEN: std::cout << "green\n"; break;
    case Color::BLUE : std::cout << "blue\n";  break;
}
// int n = r; // error: no scoped enum to int conversion
int n = static_cast<int>(r); // OK, n = 21
</syntaxhighlight>
=== this keyword/pointer ===
The '''this''' keyword is valid only inside a member function, where it denotes a pointer to the object on which the member function is operating. For example, inside Vec::operator=, the type of this is Vec*, because this is a pointer to the Vec object of which operator= is a member. For a binary operator, such as assignment, this is bound to the left-hand operand. Ordinarily, this is used when we need to refer to the object itself, as we do here both in the initial if test and in the return.
* http://en.cppreference.com/w/cpp/language/this
=== Communication between objects ===
See Pi magazine [http://www.themagpi.com/issue/issue-24/ issue 24]. The secret is to use '''this''' pointer to pass a pointer to the data member object.
In header files,
<pre>
// Child.h
#ifndef CHILD_H
#define CHILD_H
class Parent; // forward declaration to reduce precompile time
class Child {
....
}
#endif
// Parent.h
#ifndef PARENT_H
#define PARENT_H
class Child; // forward declaration to reduce precompile time
class Parent {
...
}
#endif
</pre>
In C++ files,
<pre>
// Child.cpp
Child::Child(Parent *parent):
  m_parent(parent) {
}
void Child::run() {
  cout << m_parent->x() << ", " << m_parent->y() << endl;
}
// Parent.cpp
Parent::Parent(unsigned int x, unsigned int y):
  m_child(0),
  m_x(x),
  m_y(y) {
  if (!m_child) m_child = new Child(this);
  m_child->run();
}
</pre>
=== C++ videos ===
* (73) http://thenewboston.org/list.php?cat=16
* (54) videos http://xoax.net/cpp/crs/console/index.php
=== Type casting ===
by declaiming explicitly like (float)5 or using suffix like 5f.
=== Global variables, header ===
http://stackoverflow.com/questions/9702053/how-to-declare-a-global-variable-in-c
1. Put '''extern int myvar''' in the header file
2. Put '''int myvar''' in the cpp file
<syntaxhighlight lang='cpp'>
// header file <myheader.h>
extern int x
// cpp file
#include "myheader.h"
void foo()
{
  x = 5;
}
</syntaxhighlight>
=== C vs C++ with functions === 
In non-object programming, we use
<pre>
function(x, parameter)
</pre>
In C++ programming, we use
<pre>
x->function(parameter) // if x is a pointer
x.function(parameter)  // if x is not a pointer
</pre>
=== <cstdlib> vs <stdlib.h> ===
The first one is for C++ and the other is for C. See [http://stackoverflow.com/questions/2900785/whats-the-difference-between-cstdlib-and-stdlib-h here].
=== Function Prototypes ===
For example,
<pre>
void cheers(int);    // prototype: no return value
double cube(double x);  // prototype: returns a double
int main()
{
...
}
</pre>
Functon prototyping is often hidden in the '''include''' files.
=== Pass function name in a function ===
See
* Chapter 4 of Accelerated C++. [https://github.com/arraytools/C/blob/master/accelerated_unix/chapter04/Student_info.cc <Student_info.cc>]. Well there is no functions as arguments in this case. Just want to show an opposite of write_analysis() function.
<syntaxhighlight lang='cpp'>
int main() {
    ....
    vector<double> homework;
    read_hw(cin, homework);
    // note that returning the stream allows our caller to write
    // if (read_hw(cin, homework)) { /* ... */ }
    // as an abbreviation for
    // read_hw(cin, homework);
    // if (cin) { /* ... */ }
    ....
}
istream& read_hw(istream& in, vector<double>& hw)
{
    // 'in' is not copied. We will modify it and return it.
    ....
    return in;
}
</syntaxhighlight>
* Chapter 6 of Accelerated C++. The caller [https://github.com/arraytools/C/blob/master/accelerated_unix/chapter06/grade_analysis.cc <grade_analysis.cc>] and function [https://github.com/arraytools/C/blob/master/accelerated_unix/chapter06/analysis.cc <analysis.cc>].
<syntaxhighlight lang='cpp'>
int main() {
  ...
  write_analysis(cout, "median", median_analysis, did, didnt);
  write_analysis(cout, "average", average_analysis, did, didnt);
  ...
}
void write_analysis(ostream& out, const string& name,
                    double analysis(const vector<Student_info>&),
                    const vector<Student_info>& did,
                    const vector<Student_info>& didnt)
{
out << name << ": median(did) = " << analysis(did) <<
              ", median(didnt) = " << analysis(didnt) << endl;
}
</syntaxhighlight>
For the '''ostream &''' parameter, see [[#Write_a_Function_to_Contain_an_Argument_of_Output_Device_.28eg_file.2C_screen.29|Write a Function to Contain an Argument of Output Device]].
=== Argument Passing in Functions ===
References as a function argument seems more popular for numerical values and Pointers as a function argument seems popular for char arrays. It is introduced in C++ Primer Chapter 6.2.2.
The argument can be a scalar or an array. Also it be numerical values or character strings.
==== Pointers as a function argument ====
The following example is coming from http://www.nongnu.org/c-prog-book/online/x641.html. See also [http://www.cplusplus.com/doc/tutorial/pointers/ cplusplus.com] tutorial about pointers.
<syntaxhighlight lang='cpp'>
#include <stdio.h>
int swap_ints(int *first_number, int *second_number);
int main()
{
  int a = 4, b = 7;
  printf("pre-swap values are: a == %d, b == %d\n", a, b);
  swap_ints(&a, &b);
  printf("post-swap values are: a == %d, b == %d\n", a, b);
  return 0;
}
int swap_ints(int *first_number, int *second_number)
{
  int temp;
  temp = *first_number;
  *first_number = *second_number;
  *second_number = temp;
  return 0;
}
</syntaxhighlight>
Another example
<syntaxhighlight lang='cpp'>
#include <stdio.h>
void printArray(int * arr, int size)
{
    int i;
    printf("Array elements are: ");
    for(i = 0; i < size; i++)
    {
        arr[i] += 10;
        printf("%d, ", arr[i]);
    }
}
int main()
{
    int arr[5] = {1, 2, 3, 5, 4};
    printArray(arr+1, 5);    // Pass array directly to function printArray
    return 0;
}
// Output: Array elements are: 12, 13, 15, 14, 32774,
</syntaxhighlight>
==== '''References as a function argument''' ====
The new variable is called '''Reference Variables''' which is a name that acts as an '''alias'''. This is described in C++ Primer Plus Chapter 8. See also
* [http://www.tutorialspoint.com/cplusplus/cpp_references.htm tutorialspoint.com]
* [http://www.thegeekstuff.com/2013/05/cpp-reference-variable/ thegeekstuff.com]
* [http://www.dev-hq.net/c++/12--pointers-and-references dev-hq.net]
Here we create a reference that looks and acts like a standard C++ variable except that it operates on the same data as the variable that it references.
The following example is modified from  [http://developer.kde.org/~wheeler/cpp-pitfalls.html C++ pitalls].
<source lang="cpp">
int foo = 3;    // foo == 3
int &bar = foo;  // foo == 3. bar is of type int &. The ampersand & is NOT the address operator.
bar = 5;        // foo == 5
int * prats = &foo;  // prats is a pointer. *prats and bar can be used interchangeably with foo
                              // and use the expression &bar and prats interchangeably with &foo.                             
</source>
and the same concept of references is used when passing variables.
<source lang="cpp">
#include <iostream>
using namespace std;
void foo( int &i )
{
    i++;
}
int main()
{
    int bar = 5;  // bar == 5
    cout << bar << endl;
    foo( bar );    // bar == 6
    cout << bar << endl;
    foo( bar );    // bar == 7
    cout << bar << endl;
    return 0;
}
</source>
However, reference variable 1. it is necessary to initialize the reference when you declare it 2. a reference is rather like a '''const''' pointer; you have to initialize it when you create it and once a reference pledges its allegiance to a particular variable, it sticks to its pledge.
More examples from [http://www.cplusplus.com/forum/beginner/42244/ this post].
<syntaxhighlight lang='cpp'>
// OK case.
main: int i=6;
        chgInt(&i);
function:  void chgInt(int *p);
                  *p = 10 + *p;
// Not OK case.
main: char *name = "old";
          chgStr(name);
function: void chgStr(char *n);
              n = "new"; // This will make a new string, not changing the original one
// Correction
main: SAME
function: void chgStr(char* &n);
              n = "new";
</syntaxhighlight>
In fact, the way of using '''char* &n''' in function argument is also used by Foundation of Qt Development List 1.1 and 1.2 where string is used instead of char*. Below is the code of List 1.1:
<syntaxhighlight lang='cpp'>
class MyClass
{
public:
    MyClass( const string& text );
    const string& text() const;
    void setText( const string& text );
    int getLengthOfText() const;
private:
    string m_text;
};
</syntaxhighlight>
In PPP book, Stroustrup also uses call by references everywhere, such as:
<syntaxhighlight lang='cpp'>
// 21.9 Container algorithms
void test(vector<int>& v)
{
  sort(v.begin(), v.end());
}
int main()
{
  vector<int> vs;
  test(vs);
}
</syntaxhighlight>
A real power of using references is in the example of PPP book Chapter 20.1 where we can use C' way in C++ to access a value in C++'s vector.
<syntaxhighlight lang='cpp'>
# C array
double* jack_data = get_from_jack(&jack_count);   
#  jack_data[i]  ---- value
#  &jack_data[i] ---- address
# C++ vector
vector<double>* jill_data = get_from_jill();   
# Then instead of using the basic way to access the data
#  (*jill_data)[i]  ---- value
#  &(*jill_data)[i] ---- address
# we can use the reference method
vector<double>& v = *jill_data;
#  v[i]  ---- value
#  &v[i] ---- address
</syntaxhighlight>
==== Comparison of Reference and Pointer ====
{| class="wikitable sortable collapsible" border="1" style="border-collapse:collapse; text-align:center;"
|-
! bgcolor="#00BFFF"|  !! bgcolor="lightgreen"|Reference (pass by reference) !! bgcolor="#FE6F5E"|Pointer (pass by address)
|-
| Main || <pre>
int a;
int & bar = a;
foo(a);
// pass variable (not pass address of the variable)
// it 'can' be passing 'values' of variables;
// determined by prototype of the function def.
</pre> note: no * is used. & is used in the declaration.
||
<pre>
int a;
int * bar = &a;
foo(&a);
// pass address of variables
</pre> note: * is used in the declaration.
|-
| Function ||
<pre>
foo(int &b)
// b is an alias. It can be used directly.
// For example, b = 10;
</pre> note: no * is used. & is used in the declaration.
|| <pre>
foo(int *b)
// b is a pointer to an int
// For example, *b = 10;
</pre> note * is used in the declaration.
|}
=== lvalue and rvalue ===
* Chapter 8.5.6 Pass-vy-value vs pass-by-reference of PPP (an easy to follow book with essential concepts there).
* Chapter 6.4 Objects and Values of The C++ Programming Language.
* Chapter 7.7.1 Lvalue References and 7.7.2 Rvalue References of The C++ Programming Language.
<syntaxhighlight lang='cpp'>
void g(int a, int& r, const int& cr)
{
    ++a;
    ++r;
    int x = cr;
}
int main()
{
    int x = 0;
    int y = 0;
    int z = 0;
    g(x, y, z); // x==0; y==1; z==0
    g(1, 2, 3); // error: reference argument r needs a variable to refer to
    g(1, y, 3); // OK since cr is const we can pass a literal
}
</syntaxhighlight>
=== rvalue reference, && and Move semantics ===
This is used to avoid 'copy' data. The situation happened in copy constructor or assignment. It is important especially for large data.
* See Programming: Principal and Practice (2nd ed) 18.3.4. It is a new feature in C++11.
* Google it.
=== Pass the (address) of the pointer (char array) as function argument ===
http://stackoverflow.com/questions/1217173/how-to-have-a-char-pointer-as-an-out-parameter-for-c-function.
<pre>
#include <string.h>
#include <iostream>
using namespace std;
void SetName( char **pszStr )
{
    char* pTemp = new char[10];
    strcpy(pTemp,"Mark");
    *pszStr = pTemp; // assign the address of the pointer to this char pointer
}
void SetNameBetter(char *& pszStr )
{
    char* pTemp = new char[10];
    strcpy(pTemp,"MarkBetter");
    pszStr = pTemp; // this works because pxzStr *is* the pointer in main
}
void SetNameNotOK( char *pszStr )
{
    char* pTemp = new char[10];
    strcpy(pTemp,"Mark");
    pszStr = pTemp;
}
int main(void){
   
    char* pszName = NULL;
    // SetName( pszName );
    SetName( &pszName ); // pass the address of this pointer so it can change
    cout<<"Name - "<< pszName<<endl; // not *pszName
    SetNameBetter( pszName ); // pass the pointer into the function, using a reference
    cout<<"Name - "<< pszName<<endl; // not *pszName
    delete [] pszName;
    return 0;
}
</pre>
=== Function and Arrays ===
(C++ Primer Ch 6.2.4 Array Parameters) Because arrays are passed as pointers, functions ordinarily don't know the size of the array they are given. There are three techniques used to manage pointer parameters
# using a marker to specify the extent of a character array (null character for C-style strings)
# using the standard library conventions (begin and end pointers) and
# explicitly passing a size parameter.
See Chapter 7 of C++ Primer Plus (5th ed).
<pre>
# Version 1.
int sumarray(int arr[], int n)
{
  int total = 0;
  for(int i=0; i<n; i++) total += arr[i];
  return total;
}
# Version 2.
int sumarray(int * arr, int n)
{
  int total = 0;
  for(int i=0; i<n; i++) total += arr[i];
  // arr[i] is equivalent to *(arr + i)
  return total;
}
# Version 3a.
int sumarray(const int arr[], int n); //protect input array
# Version 3b.
int sumarray(const int *begin, const int *end)
{
  const int *pt;
  int total = 0;
  for(pt=begin; pt != end; pt++) total += *pt;
  return total;
}
</pre>
==== Pointer and const ====
See Chapter 7 of C++ Primer Plus (5th ed).
<pre>
int sloth = 3;
const int * ps = &sloth; 
// a pointer to const int, prevent using the pointer to change the pointed-to value
int * const finger = &sloth; 
// a const pointer to int, prevent from changing where the pointer points.
</pre>
==== Functions and Two-Dimensional Arrays ====
<pre>
int data[3][4] = {{1,2,3,4}, {5,6,7,8}, {9,10,11,12}};
// data is an array with 3 elements.
int total = sum(ar, 3);
// Since the first element of data is an array of 4 int values. so ar should be a pointer to array-of-four-int.
// What is the prototype?
int sum(int (*ar)[4], int size);
// the parentheses are needed because '''int *ar[4]''' should declare
// an array of 4 pointers to int.
OR
int sum(int ar[][4], int size);
int sum(int ar[][4], int size)
{
  int total = 0;
  for(int r=0; r< size; r++)
    for(int c=0; c<4; c++)
      total += ar[r][c];
  return total;
}
</pre>
Note that the parentheses around *ar are necessary:
<pre span style="color: red">
int *matrix[10]:    // array of 10 pointers
int (*matrix)[10]:  // pointer to an array of ten ints
</pre>
==== command line arguments, arguments to main ====
<syntaxhighlight lang='cpp'>
int main(int argc, char *argv[]) { ... } // argv is an array of pointers to C-style character strings.
int main(int argc, char **argv) { ... }  // argv points to a char *.
argv[0] = "prog"; // actually it is 'p', 'r', 'o', 'g', '\0'.
argv[1] = "-i";  // or '-', 'i', '\0'.
argv[2] = "inputfile";
argv[3] = "-o";
argv[4] = "outputfile";
</syntaxhighlight>
==== Varying parameters ====
(C++11) '''initializer_list''' parameters.
<pre>
void error_msg(initializer_list<string> i1)
{
  for (auto beg = i1.begin(); beg != i1.end(); ++beg)  cout << *beg << " ";
  // OR for (const auto &elem : i1)  cout << elem << " ";
  cout << endl;
}
...
if (expected != actual) {
  error_msg({"foo", expected, actual});
else
  error_msg({"foo", "Okay"});
}
</pre>
==== Ellipsis parameters ====
Ellipsis parameters are in C++ to allow programs to interface to C code that uses a C library facility named '''varargs'''.
<pre>
void foo(param_list, ...);
void foo(...);
</pre>
==== Return a pointer to an array ====
See C++ Primer Ch 6.3.3.
A function cannot return an array. It can return a pointer or a reference to an array. However, the syntax used to define functions that return pointers or references to arrays can be intimidating.
(C++11) Trailing return type. Trailing returns can be defined for any function with complicated return types, such as pointers (or references) to arrays. A trailing return type follows the parameter list and is preceded by ->. To signal that the return follows the parameter list, we use '''auto''' where the return type ordinarily appears.
<pre>
// fcn takes an int argument and returns a pointer to an array of 10 ints
auto func(int i) -> int(*)[10];
</pre>
Alternativly, if we know the array(s) to which our function can return a pointer, we can use '''[http://en.cppreference.com/w/cpp/language/decltype decltype]''' to declare the return type. For example, the following function returns a pointer to one of two arrays, depending on the value of its parameter.
<pre>
int odd[] = {1, 3, 5, 7, 9};
int even[] = {2, 4, 6, 8, 10};
// returns a pointer to an array of 5 int elements
decltype(odd) *arrptr(int i)
{
  return(i % 2) ? &odd : &even; // return a pointer to the array
}
</pre>
==== Pointer, vector and element ====
* See PPP 20.1 (Storing and processing data) and 20.1.1 (Working with data).
<pre>
// If jill_data is a pointer from get_from_jill() which returns a pointer to a vector
vector<double>* get_from_jill();
vector<double>* jill_data = get_from_jill();
int i = 1;
cout << *jill_data[i];  // means *(jill_data[i]) which is not we want.
cout << (*jill_data[i]); // good because [ ] binds tigher than *.
delete jill_data;
</pre>
==== pass a vector of strings using reference ====
See how to pass a vector of strings using reference.
<syntaxhighlight lang='cpp'>
#include <iostream>
#include <vector>
#include <string>
using namespace std;
void foo(char* &s) {
    /* Goal: print an array of character s */
    /* approach 1: not working. Only the first character is shown.
      cout << *s << " ";    */
    /* approach 2: simplest solution */
    cout << s << " ";
    /* approach 3: loop. Works fine
      while(*s) printf("%c",*s++);    */
    /* approach 4: non-loop. Works fine.
    /* std::string str(s, s + strlen((const char *)s)); convert char* to string
    cout << str << " ";                                and then print out  */
}
void foo2(vector<string> vs) {
    // for(auto it : vs) cout << it << endl; for C++11
    for (std::vector<string>::iterator it = vs.begin(); it!=vs.end(); ++it)
        cout << ' ' << *it;
}
void foo3(vector<string> &vs) {
    for (std::vector<string>::iterator it = vs.begin(); it!=vs.end(); ++it)
        cout << ' ' << *it;
    cout << endl;
    for (unsigned int i=0; i < vs.size(); ++i)
        cout << ' ' << vs[i];
}
int main()
{
    const int argc=4;
    // C style of an array of pointers to characters
    char** argv = new char *[argc];;
    argv[0] = "This";
    argv[1] = "Is";
    argv[2] = "A";
    argv[3] = "Book";
    cout << "char** type" << endl;
    cout << "Call directly" << endl;
    for(int i=0; i<argc; i++)  cout << argv[i] << " ";
    cout << endl;
    cout << "Call foo()" << endl;
    for(int i=0; i<argc; i++) foo(argv[i]); // only 1 string at a time
    cout << endl << endl;
    // std::vector<std::string> vs = {"This", "Is", "A", "Book"}; C++11
    std::vector<std::string> vs;
    vs.push_back("This");
    vs.push_back("Is");
    vs.push_back("A");
    vs.push_back("Book");
    cout << "string vector type" << endl;
    cout << "Print from main()" << endl;
    for (std::vector<string>::iterator it = vs.begin(); it!=vs.end(); ++it)
        cout << ' ' << *it;
    cout << endl;
    cout << "Call foo2() pass by value" << endl;
    foo2(vs);  // pass whole vector
    cout << endl;
    cout << "Call foo3() pass by reference" << endl;
    foo3(vs);  // pass whole vector
    cout << endl;
    delete[] argv;
    return 0;
}
</syntaxhighlight>
And the output
<pre>
char** type
Call directly
This Is A Book
Call foo()
This Is A Book
string vector type
Print from main()
This Is A Book
Call foo2() pass by value
This Is A Book
Call foo3() pass by reference
This Is A Book
This Is A Book
Press <RETURN> to close this window...
</pre>
A similar example is to create the vector elements in subroutine and again the vector is passed by reference.
<syntaxhighlight lang='cpp'>
#include <iostream>
#include <vector>
#include <string>
using namespace std;
void foo(vector<string> &vs) {
    vs.push_back("This");
    vs.push_back("Is");
    vs.push_back("A");
    vs.push_back("Book");
}
void foo2(vector<string> &vs) {
    for (std::vector<string>::iterator it = vs.begin(); it!=vs.end(); ++it)
        cout << ' ' << *it;
    cout << endl;
}
int main()
{
    std::vector<std::string> vs;
    cout << "string vector type" << endl;
    cout << "Print from main()" << endl;
    for (std::vector<string>::iterator it = vs.begin(); it!=vs.end(); ++it)
        cout << ' ' << *it;
    cout << endl;
    cout << "Pass by reference" << endl;
    foo(vs);  // pass whole vector
    foo2(vs);
    return 0;
}
</syntaxhighlight>
For 2 dimensional matrix of string, see [[#2_dimensional_matrix|the example below]].
=== Container class in C++ vs simple array in C ===
https://isocpp.org/wiki/faq/containers
=== try { ... } catch { ... } for exception handling ===
* http://msdn.microsoft.com/en-us/library/6dekhbbc(v=vs.80).aspx
* http://www.cplusplus.com/doc/tutorial/exceptions/
* http://www.dev-hq.net/c++/22--try-and-catch. It explains why we typically want to separate throw (usually defined in a function) and try-catch (usually defined in a caller function). It also shows we'd better use 'pass the exception by reference' practice. The values thrown from the function can be a numerical number, a string or a custom class.
<syntaxhighlight lang='cpp'>
try
{
    throw 20;
}
catch (int e)
{
    cout << "An exception occurred. Exception Nr. " << e << endl;
}
</syntaxhighlight>
Second example.
<syntaxhighlight lang='cpp'>
  char *buf;
  try {
      buf = new char[512];
      if( buf == 0 )
        throw "Memory allocation failure!";
  }
  catch( char * str ) {
      cout << "Exception raised: " << str << '\n';
  }
</syntaxhighlight>
Third example.
<syntaxhighlight lang='cpp'>
try {
    return grade(s);
} catch (domain_error) {
    return grade(s.midterm, s.final, 0);
}
</syntaxhighlight>
Fourth example,
<syntaxhighlight lang='cpp'>
#include <iostream>
using namespace std;
int divide_numbers(int a, int b)
{
    if(b==0)
        throw 1;
    return a/b;
}
int main()
{
    int a, b;
    cout << "One: ";
    cin >> a;
    cout << "Two: ";
    cin >> b;
    try
    {
        cout << divide_numbers(a, b);
    }
    catch(int& code)
    {
        cout << "ERROR CODE: " << code;
    }
    catch(...)
    {
        cout << "An unknown error has occurred.";
    }
    //Continue doing whatever afterwards like normal
    return 0;
}
</syntaxhighlight>
Fifth example,
<syntaxhighlight lang='cpp'>
#include <iostream>
using namespace std;
int main()
{
    try
    {
        int* myarray = new int[100000000000000];
        delete [] myarray;
    }
    catch(exception& e) //Takes a reference to an 'exception' object
    {
        cout << "Error allocating memory: " << e.what() << endl;
    }
}
</syntaxhighlight>
Sixth example,
<syntaxhighlight lang='cpp'>
// http://www.tenouk.com/cpluscodesnippet/domainerrortypeid.html
// http://www.cplusplus.com/reference/exception/exception/?kw=exception
#include <iostream>
#include <exception>  // operator exception
#include <typeinfo>  // operator typeid
using namespace std;
int main(void)
{
try
{
throw domain_error("Some error with your domain!");
}
catch (std::exception &err)
{
cerr<<"Caught: "<<err.what()<<endl;
cerr<<"Type: "<<typeid(err).name()<<endl;
};
}
</syntaxhighlight>
==== Exception classes ====
http://www.cplusplus.com/reference/exception/exception/
* logic_error
* domain_error
* invalid_argument
* length_error
* out_of_range
* runtime_error
* range_error
* overflow_error
* underflow_error
=== '''Class''' ===
The fundamental ideas behind classes are '''data abstraction''' and '''encapsulation'''.
Data abstraction separate '''interface''' and '''implementation'''. The interface of a class consists of the operations that users of the class can execute. The implementation includes the class's data members, the bodies of the functions.
Encapsulation enforces the separation of a class' interface and implementation.
==== Class vs struct ====
The only difference between '''struct''' and '''class''' is the default access level.
Members in a class are private by default while members defined in '''struct''' are public.
==== A simple class (C vs Qt) ====
Note that a constructor is not needed.
{| class="wikitable"
!
! C
! Qt
|-
| myclass.h
| <syntaxhighlight lang='cpp'>
#include <string>
using namespace std;
class MyClass
{
public:
//  MyClass();
    string file;
    string createCmd(const string & cmd);
};
</syntaxhighlight>
| <syntaxhighlight lang='cpp'>
#include <QtCore>
class MyClass
{
public:
//  MyClass();
    QString file;
    QString createCmd(const QString & cmd);
};
</syntaxhighlight>
|-
| myclass.cpp
| <syntaxhighlight lang='cpp'>
#include "myclass.h"
//MyClass::MyClass()
//{
//}
string MyClass::createCmd(const string & file)
{
    string cmd;
    cmd = file + file;
    return cmd
}
</syntaxhighlight>
| <syntaxhighlight lang='cpp'>
#include "myclass.h"
//MyClass::MyClass()
//{
//}
QString MyClass::createCmd(const QString & file)
{
    QString cmd;
    cmd = file + file;
    return cmd;
}
</syntaxhighlight>
|-
| main.cpp
| <syntaxhighlight lang='cpp'>
#include <iostream>
#include <string>
#include "myclass.h"
using namespace std;
int main(int argc, char *argv[])
{
    MyClass myclass;
    cout << myclass.createCmd("ABCD ") endl;
}
</syntaxhighlight>
| <syntaxhighlight lang='cpp'>
#include <QCoreApplication>
#include <QtDebug>
#include "myclass.h"
int main(int argc, char *argv[])
{
    MyClass myclass;
    qDebug() << myclass.createCmd("ABCD ");
}
</syntaxhighlight>
|}
We can build the Cpp codes by '''g++ myclass.cpp main.cpp'''.
==== Single colon and double colons ====
===== Single colon =====
* Single colon ":" was used in [http://www.cplusplus.com/doc/tutorial/inheritance/ inheritance]. For instance,
<syntaxhighlight lang='cpp'>
class fourwheeler {}
class car: public fourwheeler {}
</syntaxhighlight>
* Single colon can be used in the class (constructor) member initializer list. See [[C#Defining_class_member_functions_and_member_initializer_list|Defining class member functions and member initializer list]] or (may not be that clear example) pp49-50 of The C++ Programming Language (4th ed, Stroustrup).
<syntaxhighlight lang='cpp'>
class Vector {
public:
  Vector(int s) :elem{new double(s)}, sz{s} {}
  double& operator[](int i) { return elem[i]; }
  int size() { return sz; }
private:
  double* elem;
  int sz;
}
</syntaxhighlight>
===== Double colons =====
* Double colons was used to define/refer a class's function/method. Sometimes it can be used to resolve namespace problem. See [http://stackoverflow.com/questions/4269034/what-is-the-meaning-of-prepended-double-colon-to-class-name here].
<pre>
void MyClass::setText() {}
</pre>
==== :: operator ====
The :: in the function name is the same scope operator. For example, [https://github.com/arraytools/C/blob/master/accelerated_unix/chapter09/Student_info.cc <Student_info.cc>] in Accelerated C++,
<syntaxhighlight lang='cpp'>
double Student_info::grade() const
{
return ::grade(midterm, final, homework);
}
</syntaxhighlight>
Note that the :: in front of a name insists on using a version of that name that is not a member of anything. In this case, we call the version of ''grade'' that takes two doubles and a vector<double>.
The ''const'' in is a promise that calling the grade function will not change any of the data member of the Student_info object (Accelerated C++ 9.2.1). We can understand this usage by comparing the new function declaration with the original:
<syntaxhighlight lang='cpp'>
double Student_info::grade() const { ... }  // member-function version
double grade(const Student_info&) { ... }    // original
</syntaxhighlight>
The grade() declared in Stduent_info::grade() is a '''const member function'''.
Another more explicit example is on [http://stackoverflow.com/questions/9338217/why-does-c-need-the-scope-resolution-operator Why does C++ need the scope resolution operator].
<syntaxhighlight lang='cpp'>
#include <iostream>
int a = 10;
namespace M
{
    int a = 20;
    namespace N
    {
        int a = 30;
        void f()
        {
            int x = a; //a refers to the name inside N, same as M::N::a
            int y = M::a; //M::a refers to the name inside M
            int z = ::a; //::a refers to the name in the global namespace
            std::cout << x << ", "<< y << ", " << z << std::endl; //30,20,10
        }
    }
}
int main()
{
    M::N::f();
}
</syntaxhighlight>
==== Accessor and Mutator ====
[https://stackoverflow.com/questions/9627093/what-is-the-difference-between-accessor-and-mutator-methods What is the difference between accessor and mutator methods?] An '''accessor''' is a class method used to read data members, while a '''mutator''' is a class method used to change data members.
<syntaxhighlight lang='cpp'>
class foo
{
    private:
        int a;
    public:
        int  accessA() const { return(a);}
        void mutateA(const int A) { a = A;}
}
</syntaxhighlight>
==== Access Member Functions ====
Use "." dot for regular objects. See the next session.
For pointers to a class, use arrows "->".
=====  Pointers to classes =====
<syntaxhighlight lang='cpp'>
// http://www.cplusplus.com/doc/tutorial/classes/
#include <iostream>
using namespace std;
class Rectangle {
  int width, height;
public:
  Rectangle(int x, int y) : width(x), height(y) {}
  int area(void) { return width * height; }
};
int main() {
  Rectangle obj (3, 4);
  Rectangle * foo, * bar, * baz;
  foo = &obj;
  bar = new Rectangle (5, 6);
  baz = new Rectangle[2] { {2,5}, {3,6} };
  cout << "obj's area: " << obj.area() << '\n';
  cout << "*foo's area: " << foo->area() << '\n';
  cout << "*bar's area: " << bar->area() << '\n';
  cout << "baz[0]'s area:" << baz[0].area() << '\n';
  cout << "baz[1]'s area:" << baz[1].area() << '\n';     
  delete bar;
  delete[] baz;
  return 0;
}
</syntaxhighlight>
==== Access parent class data member ====
http://stackoverflow.com/questions/11525418/how-to-access-parent-classs-data-member-from-child-class-when-both-parent-and
==== Default constructor ====
The default constructor is the constructor that takes no parameters. It is called when an object is declared but is not initialized with any arguments. <span style="color: red">In fact, empty parentheses cannot be used to call the default constructor</span>.
<syntaxhighlight lang='cpp'>
// http://www.cplusplus.com/doc/tutorial/classes/
// overloading class constructors
#include <iostream>
using namespace std;
class Rectangle {
    int width, height;
  public:
    Rectangle ();
    Rectangle (int,int);
    int area (void) {return (width*height);}
};
Rectangle::Rectangle () {
  width = 5;
  height = 5;
}
Rectangle::Rectangle (int a, int b) {
  width = a;
  height = b;
}
int main () {
  Rectangle rect (3,4);
  Rectangle rectb;    // ok, default constructor called
  Rectangle rectc();  // function declaration, default constructor NOT called
  cout << "rect area: " << rect.area() << endl;
  cout << "rectb area: " << rectb.area() << endl;
  cout << "rectc area: " << rectc.area() << endl; // error
  return 0;
}
</syntaxhighlight>
When we compile it, we will get an error
<pre>
In function 'int main()': 29:35: error: request for member 'area' in 'rectc', which is of non-class type 'Rectangle()'
</pre>
==== Defining class member functions and member initializer list====
Use the single colon and a parenthesis. The parenthesis method can be used in the [http://www.cplusplus.com/doc/tutorial/variables/ general variables intialization].
This method is particular necessary when we want to initialize a constant member value. See [http://stackoverflow.com/questions/14495536/how-to-initialize-const-member-variable-in-a-class-c how to initialize const member variable in a class C++] in stackoverflow.com. '''The constant assignment happens in initializer list, much before the class initilization occurs'''.
See [[C#Single_colon|Single colon]] or 9.4.4 of Programming: principles and practice, 9.5.1 of Accelerated C++.
<syntaxhighlight lang='cpp'>
class Date {
public:
    Date(int yy, int mm, int dd)
        :y(yy), m(mm), d(dd)       
    {  ...  }
    void add_day(int n)
    {  ...  }
    int month() { return m; }
    ...
private:
    int y, m, d;            // year, month, day
};
</syntaxhighlight>
The  :y(yy), m(mm), d(dd) notation is how we initialize members. It is called a '''member initializer list''' (See also [http://www.cplusplus.com/forum/articles/17820/ this article] in cplusplus.com).
We could also use ''assignment'' to do the same job (not as good as the above way)
<syntaxhighlight lang='cpp'>
Date(int yy, int mm, int dd)
{
    y = yy;
    m = mm;
    d = dd;
    // ...
}
</syntaxhighlight>
If we want to initialize private variables with value 0, we can do it too now.
<syntaxhighlight lang='cpp'>
class Student_info {
public:
  Student_info();
  Student_info(std::istream&);
private:
  std::string n;
  double midterm, final;
  std::vector<double> homework;
};
Student_info::Student_info() : midterm(0), final(0) { }
</syntaxhighlight>
==== explicit keyword and implicit conversion to resolve the parameters to a function ====
http://stackoverflow.com/questions/121162/what-does-the-explicit-keyword-in-c-mean
==== Class constructor can take parameter via assignment ====
For example (see p6 & p76 of the [[http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-096-introduction-to-c-january-iap-2011/lecture-notes/ MIT lecture note])
<syntaxhighlight lang='cpp'>
class Integer {
  public:
  int val;
  Integer(int v) {
    val = v; cout << "constructor with arg " << v << endl;
  }
};
int main() {
  Integer i(3);
  Integer j = 5;
}
</syntaxhighlight>
The output will be
<syntaxhighlight lang='cpp'>
constructor with arg 3
constructor with arg 5
</syntaxhighlight>
==== copy constructor and copy assignment (operator=) ====
* See Chapter 11.3 of Accelerate C++
* [http://www.cplusplus.com/articles/y8hv0pDG/ Cplusplus.com]
* See Chapter 18.3.1 and 18.3.2 of Programming: Principles and Practice (2nd ed).
'''copy constructor'''
<syntaxhighlight lang='cpp'>
MyClass(const MyClass& arg);
</pre>
To use it,
<pre>
MyClass v1(3);
MyClass v2 = v1; // OR MyClass v2 {v1};
</syntaxhighlight>
'''copy assignment'''
<syntaxhighlight lang='cpp'>
MyClass& MyClass::operator=(const MyClass& a)
</pre>
To use it,
<pre>
MyClass v1(3);
v1.set(2, 2.2);
MyClass v2(4);
v2 = v1;
</syntaxhighlight>
==== Operator Overloading ====
* http://www.learncpp.com/cpp-tutorial/91-introduction-to-operator-overloading/
* Chapter 11 Working with Classes -> Operator Overloading of C++ Primer Plus
* Chapter 11 Working with Classes -> Overloaded Operators: Member vs Nonmember Functions of C++ Primer Plus.
An operator function has the form
<syntaxhighlight lang='cpp'>
operatorOP(argument-list)
</syntaxhighlight>
For many operators, you have a choice between using member functions or nonmember functions to implement operator overloading. Typically, the nonmember version is a friend function so it can directly access the private data for a class.
Case 1 (member function): Suppose both object1 and object2 belong to some class T, then the operation
<pre>
object1 + object2
</pre>
is the same as
<pre>
object1.operator+(object2)
</pre>
if we have implemented an appropriate '''operator+(const T &t)''' function in class T.
Case 2 (nonmember function):
<pre>
friend T operator+(const T & t1, const T & t2);
</pre>
Case 3 (nonmember function): Another example is object1 and object2 belong to different classes.
<pre>
cout << object
</pre>
comes from a nonmember friend function
<pre>
void operator<<(ostream & os, const Time &t)  {  ...  }
// OR better with (see p519-520)
friend std::ostream & operator<<(std::ostream &os, const Time &t) { ... }
</pre>
Case 4 (nonmember function): In another situation (see C++ Primer Plus -> Chapter 11 -> Creating Friends), the statement
<pre>
ConstantVariable + object
// OR
operator+(ConstantVariable, object)
</pre>
where operator+() is a nonmember friend function; e.g.
<pre>
friend T operator+(double m, const T &t) { ... }
</pre>
===== An example =====
From Chapter 11 of the C++ Primer Plus.
{| class="wikitable"
!
! Class
! Main
|-
| Ex1
| <syntaxhighlight lang='cpp'>
class Time {
public:
  Time Sum(const Time &t) const;
}
</syntaxhighlight>
| | <syntaxhighlight lang='cpp'>
Time time1, tim2, total;
total = time1.Sum(time2);
</syntaxhighlight>
|-
| Ex2
| <syntaxhighlight lang='cpp'>
class Time {
public:
  Time operator+(const Time &t) const;
}
</syntaxhighlight>
| | <syntaxhighlight lang='cpp'>
total = time1.operator+(time2);
// OR
total = time1 + time2;
</syntaxhighlight>
|}
==== Friend functions & classes ====
* http://www.cplusplus.com/doc/tutorial/inheritance/ (w/ a complete example, the friend function does not below any any class)
* <strike>http://www.cprogramming.com/tutorial/friends.html</strike> (the friend function belongs to anothe class but why does it not include an argument??, confusing)
* http://www.learncpp.com/cpp-tutorial/813-friend-functions-and-classes/ (the examples are Ok)
* Chapter 19.4 Friends of The C++ Programming Language.
* Chapter 11 Working with Classes/Introducing Friends of C++ Primer Plus (it uses overloading the '''<<''' and '''* operator''' as an example; see the code <mytime3.h>, <mytime3.cpp> and <usetime3.cpp> [https://github.com/arraytools/C/blob/master/c%2B%2Bprimerplus/Chapter%2011/mytime3.h here])
* Chapter 15 Friend classes in C++ Primer Plus. Friend classes are probably more practical than friend functions.
===== Idea of friend functions =====
We want some general function (see examples below) or a member function of some class (see http://www.cprogramming.com/tutorial/friends.html) to access the (private, protected and public) members of one or more classes. 
By declaring a nonmember function a '''friend''', we can give it to access the private part of the class declaration. A '''friend''' declaration can be placed in either the private or the public part of a class declaration; it does not matter where. (Chapter 19.4 Friends of The C++ Programming Language)
Note that when friends are specified within a class, this does not give the class itself access to the friend function. That function is not within the scope of the class; it's only an indication that the class will grant access to the function (http://www.cprogramming.com/tutorial/friends.html).
For example, we could define an operator that multiplies a ''Matrix'' and a ''Vector''. The implementation routine cannot be a member of both. Also, we don't want to provide low-level access function to allow every user to both read and write the complete representation of both ''Matrix'' and ''Vector''. To avoid this, we declare the operator* a friend of both.
<syntaxhighlight lang='cpp'>
constexpr int rc_max {4]; // row and column size
class Matrix;
class Vector {
    float c[rc_max];
    // ...
    friend Vector operator*(const Matrix&, const Vector&);
    // operator*() can reach into the implementation of Vector.
};
class Matrix {
    Vector v[rc_max];
    // ...
    friend Vector operator*(const Matrix&, const Vector&);
    // operator*() can reach into the implementation of Matrix.
};
Vector operator*(const Matrix& m, const Vector& v)
{
    Vector r;
    ...
    return r;
}
</syntaxhighlight>
Now ''operator*()'' can reach into the implementation of both ''Vector'' and ''Matrix''.
Other examples: '''std::ostream& operator<<''' operator as in [https://github.com/ekg/freebayes/search?utf8=%E2%9C%93&q=friend&type=Code freebayes] and [https://github.com/s-u/Rserve/search?utf8=%E2%9C%93&q=friend Rserve].
===== More examples from operator functions =====
Consider the following (C++ Primer Plus)
<pre>
time2 = time1 * 2.75;
// is the same as
time2 = time1.operator*(2.75);
</pre>
But the following statement is unclear because 2.75 is not a type ''Time'' object.
<pre>
time2 = 2.75 * time1;
</pre>
One solution is to use a nonmember function. So the compiler could match the expression ''time2 = 2.75 * time1'' to the following nonmember function call: ''time2 = operator*(2.75, time2)''. However, nonmember functions can't directly access private data in a class. This is how '''friends''' comes.
See [https://github.com/arraytools/C/blob/master/c%2B%2Bprimerplus/Chapter%2011/mytime3.h <mytime3.h>] (the '''friend''' keyword in front of line 20 is required). It is a little tricky to understand the lines 20&21 at first. The function is a nonmember function (written in an inline way) and thus the keyword '''friend''' is necessary.
===== Overloading the << Operator =====
A common use of '''friend''' is overloading the << operator.
<syntaxhighlight lang='cpp'>
// mytime3.h
class Time
{
public:
    friend std::ostream & operator<<(std::ostream &os, const Time &t);
};
// mytime3.cpp
std::ostream & operator<<(std::ostream& os, const Time &t)
{
  os << t.hours << " hours, " << t.minutes << " minutes";
}
// usetime3.cpp
int main()
{
    Time aida, tosca;
    cout << aida << "; " << tosca << endl;
}
</syntaxhighlight>
=== Inheritance ===
* http://www.cplusplus.com/doc/tutorial/inheritance/
==== protected keyword in the base class  ====
See an [https://github.com/arraytools/C/blob/master/accelerated_unix/chapter13/Core.h example] from Accelerated C++ Chapter 13.1.
Suppose there is a base class and a derived class. Then the private functions and members in the base class cannot be accessed from outside of the base class. But if we declare these functions and members as '''protected''', then we give derived classes access to the ''protected'' members of their constituent base-class objects, but keeps these elements inaccessible to users of the classes.
That is, members declared in '''protected''' are designed for the derived class. This should not be confused with the '''friend''' keyword.
See the example in [http://www.cplusplus.com/doc/tutorial/inheritance/ cplusplus.com].
==== Polymorphism ====
http://www.cplusplus.com/doc/tutorial/polymorphism/
A function's '''signature''' is its argument list. You can define two functions having the same name, provided that they have different signatures. This is called '''function polymorphism'''.
'''Function polymorphism''' is also called '''function overloading'''.
==== '''Virtual''' functions ====
http://www.cplusplus.com/doc/tutorial/polymorphism/
<pre>
  Class 1
    ^
    |
    |
    v
  Class 2 (child)
</pre>
(See 13.2.1 in Accelerated C++) Suppose we have two classes: one is a base class and the other is a derived class.  Both of them have defined their own function called grade(). Then if we have defined a function as
<syntaxhighlight lang='cpp'>
bool compare_grades(const Base& c1, const Base& c2) {
  return c1.grade() < c2.grade();
}
</syntaxhighlight>
then there is no way to distinguish between these two versions of grade() function. When we execute the compare_grades() function, it will always execute the Base::grade() member.
If we want C++ to use Derived::grade() member in compare_grades(), we can use C++ '''virtual''' functions:
<syntaxhighlight lang='cpp'>
class Base {
public:
    virtual double grade() const; 
    ...
}
</syntaxhighlight>
Now when we call compare_grades(), the implementation will determine the version of grade() to execute by looking at the actual types of the objects to which the reference ''c1'' and ''c2'' are bound. That is, if the argument is a ''Derived'' object, it will run Derived::grade() function; if the argument is a ''Base'' object it will run the Base::grade() function.
The '''virtual''' keyword may be used only inside the class definition. If the functions are defined separately from their declaration, we do not repeat '''virtual''' in the definitions.
(See OOP Demystified Chapter 4.3: Run-time polymorphism) Virtual functions may be actual functions or merely placeholders for real functions that derived classes must provide. If you define a virtual function without a body, that means the derived class must provide it (it has no choice, and the program will not compile otherwise). Classes with such functions are called '''abstract''' classes, because they aren’t complete classes and are more a guideline for creating actual classes. (For example, an abstract class might state “you must create the Display() method.”) In C++, you can create a virtual function without a body by appending =0 after its signature (also known as a pure virtual function).
([https://github.com/s-u/Rserve/ Rserve] R package). The cxx client header file [https://github.com/s-u/Rserve/blob/master/src/client/cxx/Rconnection.h <Rconnection.h>]
<syntaxhighlight lang='cpp'>
class Rexp {
public:
    virtual Rsize_t length() { return len; }
    virtual std::ostream& os_print(std::ostream& os) {
        return os << "Rexp[type=" << type << ",len=" << len <<"]";
    }
};
class Rinteger : public Rexp {
public:
    virtual Rsize_t length() { return len/4; }
    virtual std::ostream& os_print (std::ostream& os) {
        return os << "Rinteger[" << (len/4) <<"]"; 
    }
};
class Rdouble : public Rexp {
public:
    virtual Rsize_t length() { return len/8; }
    virtual std::ostream& os_print (std::ostream& os) {
        return os << "Rdouble[" << (len/8) <<"]";
    }   
};
</syntaxhighlight>
=== Template ===
C++ uses '''templates''' to enable '''generic programming''' techniques. The C++ Standard Library includes the Standard Template Library (STL) that provides a framework of templates for common data structures and algorithms.
There are two kinds of templates: function templates (e.g. algorithm library) and class templates (e.g. array, vector, list containers).
==== Generic Programming & STL ====
* '''Generic programming''' is an approach to programming that focuses on algorithm reuse (to contrast, '''OOP''' focuses on data reuse). Read [http://www.cplusplus.com/forum/general/187031/  What is generic Programming ?]. For example, a ''find'' function would work with arrays (int, double,...) or linked lists or any other '''container''' type. That is, not only should the function be independent of the data type stored in the container, it should be independent of the data structure of the container itself.
* A goal of generic programming is to write code that is independent of data types and data structures/'''containers'''. Templates are the C++ tools for creating generic programs. The '''STL''' goes further by providing a generic representation of algorithms. Chapter 16 of C++ Primer Plus.
==== Iterators (an extension of pointers) ====
'''Accelerated C++: Chapter 5. Using sequential containers and analyzing strings -> Iterators'''
<syntaxhighlight lang='cpp'>
// use indices
for (vector<Student_info>::size_type i=0; i != students.size(); ++i) }
    cout << students[i].name << endl;
}
// use iterators
for (vector<Student_info>::const_iterator iter = students.begin();
    iter != students.end(); ++iter) {
    cout << (*iter).name << endl;
    // OR
    // cout << iter->name << endl;
}
</syntaxhighlight>
Note that
* [http://www.cplusplus.com/forum/beginner/126145/ const_iterator] means the iterator is used for access only, and cannot be used for modification.
* students.begin() and students.end() return an iterator.
* Iterators can be used in a while() loop in addition to a for() loop; see [https://github.com/arraytools/C/blob/master/accelerated_unix/chapter05/fails_iters.cc <fails_iters.cc>].
* We can use iter (not *iter) to specify the element that is to be erased; see [[#Common_operations|common operations]] and [[#list|an example]].
'''C++ Primer Plus Chapter 16. The ''string'' class and the STL -> Generic programming -> Why iterators'''
Understanding iterators is the key to understanding the STL.
Just as template make algorithms independent of the type of data stored, iterators make the algorithms independent of the type of container (array, list, set, map, ...) used.
For example consider the ''find'' function which would work on different container types. We need a generic representation of the process of moving through the values in a container. The '''iterator''' is that generalized representation.
An iterator should contain some properties. See also [[#iterator_.26_sequence|Iterators]].
* Able to dereference an iterator in order to access the value. If p is an interator, *p should be defined.
* Able to assign one iterator to another. The expression p = q should be defined.
* Able to compare two iterator for equality. The expression p == q and p != q should be defined.
* Able to move an iterator through all the element of a container. ++p and p++ Should be defined.
The following example shows how the pointers can be extended to iterators.
<syntaxhighlight lang='cpp'>
// Method 1. pointers
// If the function finds the value in the array, it returns the address in the array
// where the value is found; otherise, it returns the null pointer.
double * find_ar(double * ar, int n, const double * val)
{
    for (int i = 0; i < n; i++)
        if (ar[i] == val)
            return &ar[i];
    return 0;
}
// Method 2. Self-defined iterator type
typedef double * iterator;
iterator find_ar(iterator ar, int n, const double & val)
{
    for (int i = 0; i < n; i++, ar++)
        if (*ar == val)
            return ar;
    return 0;
}
// Method 3. More like a STL function style
// The function can return the end of pointer as a sign the value was not found.         
// http://www.cplusplus.com/reference/algorithm/find/?kw=find
typedef double * iterator;
iterator find_ar(iterator begin, iterator end, const double & val)
{
    iterator ar;
    for (ar = begin; ar != end; ar++)
        if (*ar == val)
            return ar;
    return end; // indicates val not found
}
</syntaxhighlight>
==== Function templates ====
http://www.cplusplus.com/doc/tutorial/templates/
<syntaxhighlight lang='cpp'>
// function template
#include <iostream>
using namespace std;
template <class T>
T GetMax (T a, T b) {
  T result;
  result = (a>b)? a : b;
  return (result);
}
int main () {
  int i=5, j=6, k;
  long l=10, m=5, n;
  k=GetMax<int>(i,j);
  n=GetMax<long>(l,m);
  cout << k << endl;
  cout << n << endl;
  return 0;
}
</syntaxhighlight>
Another example is exchanging two variables. See Listing 8.11 <funtemp.cpp> in C++ Primer Plus (5th ed).
<syntaxhighlight lang='cpp'>
template <class Any>
void Swap(Any &a, Any &b)
{
// References as Function Arguments
  Any temp;
  temp = a;
  a = b;
  b = temp;
}
int main()
{
  int i = 10, j= 20;
  Swap(i, j);
  cout << "New i, j = " << i << ", " << j << ".\n";
  double x = 24.5, y = 81.7;
  Swap(x, y);
  cout << "New x, y = " << x << ", " << y << ".\n";
}
</syntaxhighlight>
Or Overloaded Templates. See Listing 8.12 <twotemps.cpp> in C++ Primer Plus (5th ed).
<syntaxhighlight lang='cpp'>
template <class Any>
void Swap(Any &a, Any &b)
template <class Any>
void Swap(Any *a, Any *b, int n);
int main()
{
  int i=10, j=20;
  Swap(i, j);
  int d1[8] = {0, 7, 0, 4, 1, 7, 7, 6};
  int d2[8] = {0, 6, 2 ,0, 1, 9, 6, 9};
  Swap(d1, d2, 8);
}
template <class Any>
void Swap(Any &a, Any &b)
{
  Any temp; temp=a; a=b; b=temp;
}
template <class Any>
void Swap(Any a[], Any b[], int n)
{
  Any temp;
  for (int i=0; i<n; i++)
  {
    temp = a[i]; a[i] = b[i]; b[i]=temp;
  }
}
</syntaxhighlight>
==== Class templates ====
http://www.cplusplus.com/doc/tutorial/templates/
<syntaxhighlight lang='cpp'>
// class templates
#include <iostream>
using namespace std;
template <class T>
class mypair {
    T a, b;
  public:
    mypair (T first, T second)
      {a=first; b=second;}
    T getmax ();
};
template <class T>
T mypair<T>::getmax ()
{
  T retval;
  retval = a>b? a : b;
  return retval;
}
int main () {
  mypair <int> myobject (100, 75);
  cout << myobject.getmax();
  return 0;
}
</syntaxhighlight>
=== Alternative to int ===
==== std::size_t ====
http://en.cppreference.com/w/cpp/types/size_t and http://www.cplusplus.com/reference/cstring/size_t/
According to http://www.cplusplus.com/forum/beginner/15959/, '''size_type''' was most used inside the STD containers.
<syntaxhighlight lang='cpp'>
#include <iostream>
#include <string>
int main ()
{
  std::string str ("This is a book.");
  std::size_t i, j;
  std::cout << "The size of str is " << str.size() << " bytes.\n"; // 15
  i=0;
  j=4;
  std::cout << str.substr(i, j) << std::endl; // "This"
  return 0;
}
</syntaxhighlight>
==== std::string::size_type ====
http://www.cplusplus.com/reference/string/string/
size_type is an '''unsigned''' type of container subscripts, element counts, etc. It is conceivable that an int is insufficient to contain the length.
<syntaxhighlight lang='cpp'>
const std::string::size_type cols = myString.size() + 2;
</syntaxhighlight>
==== std::vector<TYPE>::size_type ====
http://www.cplusplus.com/reference/vector/vector/
In vector, size_type is used as an index in a for-loop. See an example at [[#size_type|vector]] case.
=== String, iterator and printing ===
* http://www.cplusplus.com/reference/string/string/begin/
The '''iterator''' is one of string's member types and '''begin()''' & '''end()''' are two string's member functions with return type of iterators.
An iterator is a generalization of a pointer. You can print the value by using star ('''*''') to deference, use '''+/-''' to move it and use '''!=''' to compare different iterators. Moreover, we can use subset operator '''[]''' to get elements (eg iter[-1]).
Usually an iterator is used in sequences or vectors, see [[#iterator_.26_sequence|Sequence]].
<syntaxhighlight lang='cpp'>
// string::begin/end
#include <iostream>
#include <string>
int main ()
{
  std::string str ("Test string");
  for ( std::string::iterator it=str.begin(); it!=str.end(); ++it)
    std::cout << *it;
  std::cout << '\n';
  return 0;
}
</syntaxhighlight>
=== snprintf() and sprintf() ===
See http://www.cplusplus.com/reference/cstdio/sprintf/
* int snprintf(char *s, size_t n, const char * format, ...)
* int sprintf(char *s, const char * format, ...)
=== String, string, string ===
http://stackoverflow.com/questions/11322200/unable-to-build-my-c-code-with-g-4-6-3
http://stackoverflow.com/questions/2258561/getting-the-length-of-an-array-using-strlen-in-g-compiler
[https://www.raspberrypi.org/magpi/issues/essentials-c-v1/ Learn C Essentials] by MagPi
* C++ uses <cstring> for char*, strlen, strcpy ...
* C uses <string.h> for char*
* C++ uses <string> for string class & <sstream> for stringstream class.
* Qt uses [http://doc.qt.io/qt-4.8/qstring.html QString] & [http://doc.qt.io/qt-4.8/qstringlist.html QStringList] from <QtCore>, [http://doc.qt.io/qt-4.8/qtextstream.html QTextStream] from <QTextStream>. Cf [http://doc.qt.io/qt-4.8/qdatastream.html QDataStream] from <QDataStream>.
==== Convert a c-style string to a c++ string ====
<syntaxhighlight lang='cpp'>
char * mystr = "klajlfdjlajfd";
std::string mycppstr(mystr);
</syntaxhighlight>
==== string::size_type ====
==== padding ====
* Use std::setw() http://stackoverflow.com/questions/667183/padding-stl-strings-in-c
* Use "+" operation
Example:
<syntaxhighlight lang='cpp'>
#include <iostream>
#include <string>
#include <iomanip>      // std::setw
using namespace std;
int main() {
string str;
str = "abcd";
cout << setw (10) << str << endl;
cout << str + string(4, ' ') + str << endl;
return 0;
}
/* Output:
      abcd
abcd    efgh
*/
</syntaxhighlight>
==== Comparison ====
[http://www.cplusplus.com/reference/cstring/strcmp/ strcmp] and [http://www.cplusplus.com/reference/string/string/compare/ compare]
P50 of [https://www.raspberrypi.org/magpi/issues/essentials-c-v1/ Learn C Essentials] - <string.h> library.
* C-string
<pre>
  char key[] = "apple";
  char buffer[80];
  do {
    printf ("Guess my favorite fruit? ");
    fflush (stdout);
    scanf ("%79s",buffer);
  } while (strcmp (key,buffer) != 0);</pre>
* C++ string
<pre>
  std::string str1 ("green apple");
  std::string str2 ("red apple");
  if (str1.compare(str2) != 0)
    std::cout << str1 << " is not " << str2 << '\n';
</pre>
==== Remove file name & Get the basename from a full path: substr(), find_last_of()  ====
* These two actions correspond to the [https://en.wikipedia.org/wiki/Dirname dirname] and [https://en.wikipedia.org/wiki/Basename basename] commands in Linux OS.
* Use [http://en.cppreference.com/w/cpp/string/basic_string/find_last_of string::find_last_of()] and [http://www.cplusplus.com/reference/string/string/substr/ string::substr()] functions. See http://stackoverflow.com/questions/10364877/c-how-to-remove-filename-from-path-string
<syntaxhighlight lang='cpp'>
#include <iostream>
using namespace std;
int main() {
    // This solution works with both forward and back slashes.
    string s1("../somepath/somemorepath/somefile.ext");
    string s2("..\\somepath\\somemorepath\\somefile.ext");
    // Remove file name from a full path
    cout << s1.substr(0, s1.find_last_of("\\/")) << endl; // s1 is not changed yet
    cout << s2.substr(0, s2.find_last_of("\\/")) << endl; // use s1=s1.substr() to change it
    // Get the basename
    cout << s1.substr(s1.find_last_of("\\/")+1) << endl;
    cout << s2.substr(s2.find_last_of("\\/")+1) << endl;
    return 0;
}
</syntaxhighlight>
The output is
<pre>
../somepath/somemorepath
..\somepath\somemorepath
somefile.ext
somefile.ext
</pre>
==== Extract the file name and extension from a string: find_last_of() ====
https://stackoverflow.com/questions/4430780/how-can-i-extract-the-file-name-and-extension-from-a-path-in-c
==== Replace a substring ====
* Use the [http://en.cppreference.com/w/cpp/string/basic_string/find string::find()] or ([http://www.cplusplus.com/reference/string/string/find/ this]) and [http://en.cppreference.com/w/cpp/string/basic_string/replace string::replace()] functions. See http://stackoverflow.com/questions/3418231/replace-part-of-a-string-with-another-string
<syntaxhighlight lang='cpp'>
#include <iostream>
using namespace std;
bool replace(std::string& str, const std::string& from, const std::string& to) {
    size_t start_pos = str.find(from);
    if(start_pos == std::string::npos)
        return false;
    str.replace(start_pos, from.length(), to);
    return true;
}
int main() {
    std::string string("/home/brb/igenomes/Homo_sapiens/Ensembl/GRCh37/Sequence/BWAIndex/");
    replace(string, "GRCh37", "GRCh38");
    cout << string << endl;
    return 0;
}
</syntaxhighlight>
The output is
<pre>
/home/brb/igenomes/Homo_sapiens/Ensembl/GRCh38/Sequence/BWAIndex/
</pre>
==== convert backslash to forward slash in string ====
In C++ (not tested)
<syntaxhighlight lang='cpp'>
string path = "C:\Program Files";
std::replace(path.begin(), path.end(), '\\', '/');
</syntaxhighlight>
In Qt (tested), we can use
<syntaxhighlight lang='cpp'>
QString sReturnedValue = "C:\Program Files";
sReturnedValue.replace("\\", "/");
</syntaxhighlight>
=== size method ===
* [http://www.cplusplus.com/reference/string/string/size/ string object]: Returns the length of the string, in terms of bytes.
* [http://www.cplusplus.com/reference/vector/vector/size/ vector object]: Return the number of elements.
=== Convert ===
==== Danger of implicit type conversion ====
Implicit type conversion (coercion) can result in unexpected results. See the following example.
<syntaxhighlight lang='bash'>
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
  float myFloat;
  long int myInt = 123456789;
  // long int  are 32-bit signed integers with a range -2147483648 to 2147483647.
  myFloat = myInt;
  streamsize prec=cout.precision();
  cout << "myInt is changed to" << setprecision(9) << myFloat << setprecision(prec) << endl; 
  // 123456792. It is not a typo!
  return 0;
}
</syntaxhighlight>
Implicit types conversion rules:
* long int to float can cause wrong results
* float to int removes the decimal part
* double to float rounds digit of double
* long int to int drops the encoded higher bits
==== Convert char* to string ====
std::string has a constructor that takes a char*.
<pre>
char *path = "Eggs on toast.";
std::string str = std::string(path);
</pre>
====  Convert [http://en.wikipedia.org/wiki/String_(C%2B%2B) std::string] to [https://en.wikipedia.org/wiki/C_string_handling c-string/char *] ====
See
* [http://stackoverflow.com/questions/347949/convert-stdstring-to-const-char-or-char stackoverflow A]
* [http://stackoverflow.com/questions/1195675/convert-a-char-to-stdstring stackoverflow B]
* [http://www.cplusplus.com/reference/string/string/c_str/ www.cplusplus.com]
* [http://forums.codeguru.com/showthread.php?231155-C-String-How-to-convert-between-CString-and-std-string codeguru.com].
* http://stackoverflow.com/questions/12862739/convert-string-to-char
<syntaxhighlight lang="cpp">
char * S = new char[R.length() + 1];
std::strcpy(S,R.c_str());
</syntaxhighlight>
This is useful when we want to use the '''ifstream''' class to open a file where the file name is a string. The standard streams doesn't accept a standard string, only c-string! So pass the string using '''c_str()''':
<syntaxhighlight lang="cpp">
Table::Table(string filename)
{
    ifstream fin;
    fin.open(filename.c_str());
    ...
}
</syntaxhighlight>
==== Convert an integer to character string ====
See [http://www.cplusplus.com/articles/D9j2Nwbp/ www.cplusplus.com].
We need to include
<pre>
#include <sstream>
</pre>
==== Convert a numerical number to char ====
Use
<pre>
char mychar[256]="";
double number;
sprintf_s(mychar, "%0.2f", number);
</pre>
This will save a number for example -7.035425 to -7.03 as characters.
=== C++ IO Streams ===
* http://www.cprogramming.com/tutorial/c++-iostreams.html
* http://www.cplusplus.com/doc/tutorial/files/
* The filename used in the '''open()''' method requires a C-style string argument:
** argv[i] from main()
** explicit string like "myfile"
** convert from a string object by using the '''c_str()''' method like filename.c_str()
The first example is to read the input filename using command line argument (Accelerated C++ Chapter 10.5).
<syntaxhighlight lang='cpp'>
#include <iostream>
#include <fstream>
int main(int argc, char **argv)
{
    int fail_count = 0;
    // for each file in the input list
    for (int i = 1; i < argc; ++i) {
        ifstream in(argv[i]);
        // if it exists, write its contents, otherwise generate an error message
        // another way is to use (in.is_open() == false) to check if the file can be opened; see other places
        if (in) {
            string s;
            while (getline(in, s))
                cout << s << endl;
        } else {
            cerr << "cannot open file " << argv[i] << endl;
            ++fail_count;
        }
    }
    return fail_count;
}
</syntaxhighlight>
The 2nd example is to hard-code the input filename.
<syntaxhighlight lang='cpp'>
#include <fstream>
#include <string>
using std::endl;
using std::getline;
using std::ifstream;
using std::ofstream;
using std::string;
int main()
{
    ifstream infile("in");
    ofstream outfile("out");
    string s;
    while (getline(infile, s))
  outfile << s << endl;
    infile.close();
    outfile.close();
    return 0;
}
</syntaxhighlight>
==== No matching function - ifstream open() ====
If we want to use a '''string''' type as a file name instead of specifying the file name in the code (hard-code), we need to use the '''c_str''' member function first.
See [http://stackoverflow.com/questions/16552753/no-matching-function-ifstream-open here]. It
Change to:
<syntaxhighlight lang='cpp'>
std::string filename
std::ifstream infile;
infile.open(filename.c_str());
# Or
std::ifstream infile(filename.c_str());
</syntaxhighlight>
An argument natural to ifstream is argv[i] from main(). For example,
<syntaxhighlight lang='cpp'>
ifstream in(arv[i]);
</syntaxhighlight>
If we want to check if the open operation is successful or not, we can not use statement like
<syntaxhighlight lang='cpp'>
if (file.open())  { ... }
</syntaxhighlight>
We need to use ether '''if (infile)''' or '''infile.is_open()''' function or declare filename as filebuf* type and the open() function will return NULL if the file cannot be opened. See an example in
[http://www.cplusplus.com/reference/istream/basic_istream/basic_istream/ cplusplus.com].
Support for std::string argument was added in c++11.
==== fail(), bad() and eof() functions ====
* http://www.cplusplus.com/reference/ios/ios/bad/
* http://en.cppreference.com/w/cpp/io/basic_ifstream
* fail() means logical error. An example of its usage after  [http://www.physics.utah.edu/~detar/lessons/c++/canned_classes/node9.html .open()]
* bad() means read/write error. Possible causes are more complex like memory shortage or buffer throws an exception. See an [http://stackoverflow.com/questions/2547355/c-file-bad-bit answer] from stackoverflow.com.
[http://en.cppreference.com/w/cpp/io/basic_ios/bad Here] is an example from cppreference.com to test bad(), eof() and fail() methods.
=== stringstream ===
* http://www.cplusplus.com/reference/sstream/stringstream/
* http://en.cppreference.com/w/cpp/io/basic_stringstream
Since stringstream is based on sstream, a stringstream is closer to stream (or file) rather than a string.
==== Simple example (using multiple << and >> operators) ====
<pre>
// swapping ostringstream objects
#include <string>      // std::string
#include <iostream>    // std::cout
#include <sstream>      // std::stringstream
int main () {
  std::stringstream ss;
  ss << 100 << ' ' << 200;
  int foo,bar;
  ss >> foo >> bar;
  std::cout << "foo: " << foo << '\n';
  std::cout << "bar: " << bar << '\n';
  return 0;
}
// Output:
// foo: 100
// bar: 200
</pre>
==== stringstream::str ====
<syntaxhighlight lang='c'>
#include <string>      // std::string
#include <iostream>    // std::cout
#include <sstream>      // std::stringstream, std::stringbuf
int main () {
  std::stringstream ss;
  ss.str ("Example string");
  std::string s = ss.str();
  std::cout << s << '\n';
  return 0;
}
// Output:
// Example string
</syntaxhighlight>
==== Clean/empty a stringstream ====
See [http://en.cppreference.com/w/cpp/io/basic_stringstream/str stringstream].
<syntaxhighlight lang='c'>
std::stringstream ss;
ss.str("");
</syntaxhighlight>
P.S. The clear() member function is inherited from ios and is used to clear the error state of the stream only. For some reason, it is necessary to use the clear() function inside a loop call. Another method is to declare a stringstream variable inside a loop.
==== clear() and rdstate() functions ====
'''stringstream''' is related to reading. We call clear() to reset the error state, which tells the library that input can continue despite the failure. Note that clear() is also used in '''vector''' objects.
* http://www.cplusplus.com/reference/ios/ios/clear/
<syntaxhighlight lang='cpp'>
#include <iostream>    // std::cout
#include <fstream>      // std::fstream
int main () {
  char buffer [80];
  std::fstream myfile;
  myfile.open ("test.txt",std::fstream::in);
  myfile << "test";
  if (myfile.fail())
  {
    std::cout << "Error writing to test.txt\n";
    myfile.clear();
  }
  myfile.getline (buffer,80);
  std::cout << buffer << " successfully read from file.\n";
  return 0;
}
</syntaxhighlight>
And convert a string from in input to integers.
* http://www.cplusplus.com/forum/beginner/47814/
* http://stackoverflow.com/questions/541022/stringstream-extract-integer
<syntaxhighlight lang='cpp'>
#include <sstream>
#include <vector>
#include <iostream>
using namespace std;
int main()
{
    string Digits("11 22 33");
    stringstream ss(Digits);
    string Temp;
    vector<string>Tokens;
    while(ss >> Temp)
        Tokens.push_back(Temp);
    if (ss.rdstate() != 0) {
        cout << "not goodbit" << endl;
    } else cout << "goodbit" << endl;
    // When the stream extracts the last of the 3 digist "1 2 3", the eof state will be set. 
    // You have to reset all status flags (eofbit) and bring the stream into a good state (goodbit):
    // After that, read operations will be canceled and you have to clear that flag out again.
    // Anyway, after clearing and resetting the string, you can then go on extracting the integers.
    ss.clear(); // clear the flag; it is not needed for the next line (ss.str(Tokens[0]),
                // but for the line of ss >> Num;
                // Without this line, building the program is OK but the output of Num is 0.
    if (ss.rdstate() != 0) {
        cout << "not goodbit" << endl;
    } else cout << "goodbit" << endl;
    ss.str(Tokens[0]);
    cout << ss.str() << endl;
    int Num = 0;
    ss >> Num;
    cout << Num << endl;
}
// Output:
// 11
</syntaxhighlight>
And an example to verify if the string is an integer (by converting a string to a stringstream, and then use the ">>" extract operator)
<syntaxhighlight lang='cpp'>
#include <sstream>
#include <iostream>
using namespace std;
int main() {
std::stringstream ss;
std::string input = "a b c 4 e";
ss << input;
if (ss.rdstate() != 0) {
        cout << "not goodbit" << endl;
    } else cout << "goodbit" << endl;
int found;
std::string temp;
while(std::getline(ss, temp,' ')) {
    if(std::stringstream(temp)>>found)
    {
        std::cout<<found<<std::endl;
    }
}
if (ss.rdstate() != 0) {
        cout << "not goodbit" << endl;
    } else cout << "goodbit" << endl;
return 0;
}
// Output:
// goodbit
// 4
// not goodbit
</syntaxhighlight>
==== getline() function to extract ====
* See [[#Read_a_text_file_with_multiple_rows_.28using_stringstream_and_getline.28.29.29|this section]].
* http://en.cppreference.com/w/cpp/io/basic_istream/getline
=== istream/ostream, iostream and fstream headers ===
See 4.1.2 the standard-library headers and namespace in The C++ Programming Language. The standard library is defined in a namespace called '''std'''.
# [http://www.cplusplus.com/reference/iostream/ iostream] include cin, cout, cerr, clog, [http://www.cplusplus.com/reference/istream/ istream] and [http://www.cplusplus.com/reference/ostream/ ostream]
# [http://www.cplusplus.com/reference/fstream/ fstream] includes [http://www.cplusplus.com/reference/fstream/ifstream/ ifstream] and ofstream. These two are useful for files i/o. Note '''ostream''' is termed a base class and '''ofstream''' class is based on it. '''ofstream''' is termed a '''derived class'''.
(10.5.2 in Accelerated C++) It is possible to use an ''ifstream'' wherever the library expects an ''istream'' and an ''ofstream'' wherever the library expects an ''ostream''???
=== Read from console: '''[http://www.cplusplus.com/reference/iostream/cin/ std::cin >>]'''  ===
We need to include the header file <iostream>.
Note that this is different from '''[http://www.cplusplus.com/reference/istream/istream/ std::istream read()]''' which requires the header file <fstream> and it is used for accessing files.
==== What is std::cin and if (cin >> x) ====
https://stackoverflow.com/questions/6791520/if-cin-x-why-can-you-use-that-condition
<syntaxhighlight lang='cpp'>
int x;
if (cin >> x){}
</syntaxhighlight>
is equivalent to
<syntaxhighlight lang='cpp'>
cin >> x;
if (cin){}
</syntaxhighlight>
==== Create a vector: input from cin ====
<syntaxhighlight lang='cpp'>
#include <iostream>
#include <vector>
using namespace std;
int main()
{
    vector<double> temps;        // temperatures
    double temp;
    while (cin >> temp)            // read
        temps.push_back(temp);    // put into vector
    cout << "length of the vector is " << temps.size() << endl;
    for (const auto tmp : temps)
      cout << tmp << " ";
    cout << "\n";
}
</syntaxhighlight>
Hit the 'Enter' key first and then use 'ctrl+d' to end the input.
<syntaxhighlight lang='bash'>
$ g++ -std=c++11 foo.cpp
$ cat > input.txt
4 3 2 1 9.0
$ ./a.out < input.txt
length of the vector is 5
4 3 2 1 9
</syntaxhighlight>
Another example is <[https://github.com/arraytools/C/blob/master/accelerated_unix/chapter04/main1.cc main1.cc]> from Accelerated C++ Chapter 4. The function ''read_hw()'' uses '''std::istream''' as the type instead of std::cin to read from the console. It also has a return type '''std::istream'''.
<syntaxhighlight lang='cpp'>
#include <iostream>  // std::cin, std::istream
using std::cin;
using std::istream;
istream& read_hw(istream& in, vector<double>& hw) {
  ...
}
int main() {
  ...
  vector<double> homework;
  read_hw(cin, homework);
  ...
}
</syntaxhighlight>
==== Create a map (key-index vector) object (word count) ====
This is example code from Chapter 21.6.1 "Map" of PPP.
<syntaxhighlight lang='cpp'>
#include <iostream>
#include <map>
#include <string>
using namespace std;
//------------------------------------------------------------------------------
int main()
{
    map<string,int> words;    // keep (word,frequency) pairs
    string s;
    while (cin>>s) ++words[s];  // reads every whitespace-separated word on input   
                                // note: words is subscripted by a string
                                   
    typedef map<string,int>::const_iterator Iter;
    for (Iter  p = words.begin(); p!=words.end(); ++p)
        cout << p->first << ": " << p->second << '\n';
}
</syntaxhighlight>
<syntaxhighlight lang='bash'>
$ ./chapter.21.6.1.exe
b b c c a ab
ab
a: 1
ab: 2
b: 2
c: 2
</syntaxhighlight>
It seems the words are sorted too.
=== Directory ===
==== Check if a directory exists or not ====
http://stackoverflow.com/questions/3828192/checking-if-a-directory-exists-in-unix-system-call
<syntaxhighlight lang='cpp'>
#include <sys/stat.h>
struct stat sb;
if (stat(pathname, &sb) == 0 && S_ISDIR(sb.st_mode))
{
    ...it is a directory...
}
</syntaxhighlight>
==== Create a directory in Linux ====
http://codeyarns.com/2014/08/07/how-to-create-directory-using-c-on-linux/
<syntaxhighlight lang='bash'>
#include <sys/stat.h>
const int dir_err = mkdir("foo", S_IRWXU | S_IRWXG | S_IROTH | S_IXOTH);
if (-1 == dir_err)
{
    printf("Error creating directory!n");
    exit(1);
}
</syntaxhighlight>
Note that this does not create parent directories .
==== List files in a directory ====
http://www.linuxquestions.org/questions/programming-9/c-list-files-in-directory-379323/
=== File ===
==== Delete a file ====
Use C library [http://www.cplusplus.com/reference/cstdio/ '''stdio''']; i.e. include <stdio.h> or <cstdio>. See also [http://en.cppreference.com/w/cpp/io/c/remove remove]. Note that according to '''The Linux Programming Interface''' book, remove() function removes a file or an empty directory. The empty directory part was not mentioned on website??
<syntaxhighlight lang='bash'>
#include <iostream>
#include <stdio.h>
int main() {
remove("myfile.txt");
return 0;
}
</syntaxhighlight>
A convenient way to see the manual is to use '''man remove'''.
On LInux,
<syntaxhighlight lang='bash'>
REMOVE(3)                Linux Programmer's Manual                  REMOVE(3)
NAME
      remove - remove a file or directory
SYNOPSIS
      #include <stdio.h>
      int remove(const char *pathname);
</syntaxhighlight>
On Mac,
<syntaxhighlight lang='bash'>
REMOVE(3)                BSD Library Functions Manual                REMOVE(3)
NAME
    remove -- remove directory entry
SYNOPSIS
    #include <stdio.h>
    int
    remove(const char *path);
</syntaxhighlight>
=== Reading a table file ===
==== Line ending difference between DOS and UNIX text file ====
The DOS text file has two ending characters (CR + LF) while UNIX text file has only 1 ending character (LF). This may create two answers when we want to count the number of columns in a row or compare the elements from the last column.
For example, if a text file is created from DOS, it will look like below on Linux OS.
<pre>
"ID 1 2 3 4 5 6 7 8 9 10
"
</pre>
In this case, the last element is read in as '''10[CR]''' instead of '''10''' on Linux. So if you need to run string/character comparison, you may not able to get what you want.
But if we remove the trailing \r character ( tr -d '\r' < INPUT > OUTPUT), we will obtain
<pre>
"ID 1 2 3 4 5 6 7 8 9 10"
</pre>
Interestingly, if we use '''winscp''' to transfer text files from Linux to Windows, it automatically add \r character to text files.
* If we want to convert files using dos2unix/unix2dos, we just need to specify the input file. By default, the input file will be overwritten.
<pre>
dos2unix inputoutputfile
unix2dos inputoutputfile
</pre>
* Normally when we create a file, the file has a LF character (or CR + LF). So when we using '''cat'' command, the output is normal.
<pre>
brb@brb-T3500:~/Downloads$ cat combobox.txt
ID 1 2 3
1 -0.80 -0.30 -0.41
brb@brb-T3500:~/Downloads$
</pre>
But if we remove the last line's LF character (eg using Window's notepad or geany editor)
<pre>
brb@brb-T3500:~/Downloads$ cat combobox.txt
ID 1 2 3
1 -0.80 -0.30 -0.41brb@brb-T3500:~/Downloads$
</pre>
==== Count number of lines in a text file --- '''[http://www.cplusplus.com/reference/fstream/ifstream/?kw=ifstream std::ifstream]''' & getline() ====
We need to include the header <fstream>.
http://stackoverflow.com/questions/3482064/counting-the-number-of-lines-in-a-text-file
Method 1. Succinct
<syntaxhighlight lang="cpp">
#include <iostream>    // std::cout
#include <fstream>      // std::ifstream
#include <string>      // std::getline
using namespace std;
int main() {
    int number_of_lines = 0;
    std::string line;
    ifstream fin;
    fin.open("combobox.txt"); // assume the file uses tab as delimiter
    while (std::getline(fin, line))
        ++number_of_lines;
    std::cout << "Number of lines in text file: " << number_of_lines;
    return 0;
}
</syntaxhighlight>
Very reliable. Don't need to worry about an empty line at the end of the file.
Method 2. Good, but lenghy
<syntaxhighlight lang="cpp">
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main() {
    int number_of_lines = 0;
    std::string line;
    ifstream fin;
    fin.open("combobox.txt"); // assume the file uses tab as delimiter
    std::getline(fin, line);
    while (fin)
    {
  ++number_of_lines;
  std::getline(fin, line);
    }
    std::cout << "Number of lines in text file: " << number_of_lines;
    return 0;
}
</syntaxhighlight>
Method 3. Not good. Need extra correction
<syntaxhighlight lang="cpp">
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main() {
    int number_of_lines = 0;
    std::string line;
    ifstream fin;
    fin.open("combobox.txt"); // assume the file uses tab as delimiter
    while (fin.good())
    {
  ++number_of_lines;
  std::getline(fin, line);
    }   
    --number_of_lines; // need an extra step
    std::cout << "Number of lines in text file: " << number_of_lines;
    return 0;
}
</syntaxhighlight>
==== Read a text file and one character at a time (using get()) ====
<syntaxhighlight lang="cpp">
void foo(ifstream& fin, ofstrea& fout)
{
    char next;
    fin.get(next);
    while (! fin.eof())
    {
        if (next == 'A')
            fout << "A++";
        else
            fout << next;
        fin.get(next);
    }
}
</syntaxhighlight>
==== Read a text file with one row only (using getline()) ====
<syntaxhighlight lang="cpp">
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
  ifstream fin;
  fin.open("combobox.txt"); // assume the file uses tab as delimiter
  if (fin.is_open() == false)
  {
    cout << "Can't open file combobox.txt. Bye." << endl;
    return 1;
  }
  string item;
  int count = 0;
  getline(fin, item, '\t');
  while (fin)
  {
    ++count;
    cout << count << ": " << item << endl;
    getline(fin, item, '\t');
  }
  cout << "Done\n";
  fin.close();
}
</syntaxhighlight>
If the input file 'combobox.txt' looks like (the [LF] is line feedback, hidden character),
<pre>
ID 1 2 3[LF]
1 -0.80 -0.30 -0.41[LF]
</pre>
the output will look like
<pre>
1: ID
2: 1
3: 2
4: 3
1
5: -0.80
6: -0.30
7: -0.41
Done
</pre>
Explanation
* After the element 2, the next element is 3[LF]1. So the output looks a little strange.
* After the element -0.30, the next element is -0.41[LF]. The there is an extra blank line there.
* The output is the same if the end of line is in DOS format (CR + LF).
==== Read a text file with multiple columns --- std::stringstream and getline() ====
With the following examples, we can count the number of columns and number of rows of a text file.
'''Solution 1.'''
<syntaxhighlight lang="cpp">
#include <iostream>
#include <fstream>
#include <string>
#include <sstream> // stringstream
#include <stdlib.h>  /* exit, EXIT_FAILURE */
using namespace std;
int main()
{
  ifstream fin;
  fin.open("combobox.txt"); // step 1. open a file
  if (fin.is_open() == false)
  {
    cout << "Can't open file. Bye." << endl;
    exit(EXIT_FAILURE);
  }
  string line, item;
  int ncol = 0, nrow = 0;
  stringstream iss;
  while(getline(fin, line)) {  // step 2. extract a line ("\n" is the default delimiter) from a (file) stream
    nrow++;                  //        accumulate row number.
    iss << line;            // step 3. insert a string to a stringstream 
    // Assume data is tab delimited
    // http://www.cplusplus.com/reference/string/string/getline/
    while (getline(iss, item, '\t')) { // step 4. extract a string from a stringstream
        if (nrow == 1 ) ++ncol;        //        accumulate column number.
        cout << item << endl;
    }
    iss.clear();              // step 5.  clear the error state of the stream?
  }
  cout << "There are " << nrow << " rows and " << ncol " columns\n";
  fin.close();
}
</syntaxhighlight>
If we want to assign the column names to an array of string and elements to 2 2D string arrays, we need to determine the dimension and then declare the variables first.
<syntaxhighlight lang="cpp">
col_names = new string[ncol];
for(int i=0;i<ncol;i++){
    getline(iss, col_names[i], '\t');
}
fin.clear();
fin.seekg(0, ios::beg);
element = new string *[nrow];
stringstream iss;
string str;
for(int i=0;i<nrow;i++){
    getline(fin, line);
    iss << line;
    element[i] = new string[ncol];
    for(int j=0;j< ncol;j++){
        if(getline(iss, str, '\t') ) {
    element[i][j]=str;
        }else{
    element[i][j]=string("");
}
    }
    iss.clear()
}
</syntaxhighlight>
'''Solution 2'''
* http://stackoverflow.com/questions/1075712/reading-delimited-files-in-c
* http://www.cplusplus.com/forum/general/17771/
<syntaxhighlight lang="cpp">
typedef vector<vector<string> > Rows;
Rows rows;
ifstream input("filename.csv");
char const row_delim = '\n';
char const field_delim = '\t';
for (string row; getline(input, row, row_delim); ) {
  rows.push_back(Rows::value_type());
  istringstream ss(row);
  for (string field; getline(ss, field, field_delim); ) {
    rows.back().push_back(field);
  }
}
</syntaxhighlight>
==== reset position to the beginning of file ====
<pre>
    ifstream orderfile;
    orderfile.open (fullpath);
    std::string line;
    long total_count=0;
    while (orderfile.good()){
        getline(orderfile, line);
        if (!line.empty()) total_count++;
    }
    orderfile.clear();
    orderfile.seekg(0,ios::beg);
    getline(orderfile, line);
</pre>
=== Write a Function to Contain an Argument of Output Device (eg file, screen) ===
See Listing 8.8 [https://github.com/arraytools/C/blob/master/c%2B%2Bprimerplus/Chapter%208/filefunct.cpp <filefunc.cpp>] in C++ Primer Plus (5th ed). It uses '''ostream &''' as the class of output device type in the function definition. For the main function, we can use objects of class '''ofstream''' or '''istream'''. Recall, ostream is a '''base class''' and ofstream is a '''derived class'''.
The program teaches
# use reference ('''ostream &''') as a function argument to refer to an '''ostream''' object such as '''cout''' (#include <iostream>) and to an '''ofstream''' (#include <fstream>) object. 
# how '''ostream''' formatting methods such as [http://www.cplusplus.com/reference/ios/ios_base/precision/ precisions()], [http://www.cplusplus.com/reference/ios/ios_base/setf/ setf()] and [http://www.cplusplus.com/reference/ios/ios_base/width/ width()] can be used for both types.
<syntaxhighlight lang='cpp'>
int main() {
    ofstream fout;
    ...
    file_it(fout, objective, eps, LIMIT);
    ...
}
void file_it(ostream & os, double fo, const double fe[],int n) {}
</syntaxhighlight>
Another example [https://github.com/arraytools/C/blob/master/accelerated_unix/chapter06/analysis.cc <analysis.gcc>] can be found on Chapter 6 of Accelerated C++.
<syntaxhighlight lang='cpp'>
using std::cout;
int main() {
    ...
    write_analysis(cout, "median", median_analysis, did, didnt);
    ...
}   
void write_analysis(ostream& out, const string& name,
                    double analysis(const vector<Student_info>&),
                    const vector<Student_info>& did,
                    const vector<Student_info>& didnt) {}
</syntaxhighlight>
=== Sorting only ===
* [http://stackoverflow.com/questions/12459290/how-to-compare-c-string-using-qsort-in-c C++ string sorting] Note: qsort() in C won't work for string type in C++.
<pre>
#include <iostream>
#include <string>
#include <iterator>
#include <algorithm>
int main() {
  std::string obj[4] = {"fine", "ppoq", "tri", "get"};
  std::sort(obj, obj + 4);
  std::copy(obj, obj + 4, std::ostream_iterator<std::string>(std::cout, "\n"));
// And for vector
// #include <vector>
// std::vector<std::string> stringarray;
// std::sort(stringarray.begin(), stringarray.end());
}
</pre>
and
<pre>
#include <stdlib.h>
#include <string.h>
int compare_cstr(const void* c1, const void* c2)
{
  return strcmp(*(const char**)(c1), *(const char**)(c2));
}
int main() {
  const char* obj[4] = {"fine", "ppoq", "tri", "get"};
  qsort(obj, 4, sizeof(obj[0]), compare_cstr);
  std::copy(obj, obj + 4, std::ostream_iterator<const char*>(std::cout, "\n"));
}
</pre>
Note: The use of '''ostream_iterator<string>(cout, "\n")''' was explained & used in Accelerated C++ Chapter 8.3 (Input and output iterators) & 8.4. It was also explained in [http://www.cplusplus.com/reference/iterator/ostream_iterator/ cplusplus.com].
=== Return permutation (R's order() function) using 3 approaches ===
* http://stackoverflow.com/questions/4523220/sorting-a-vector-of-double-precision-reals-and-obtain-their-order
Good example. This is using '''[http://candrews.net/blog/2011/07/understanding-c-0x-lambda-functions/ lambda]''' from C++0x but it can be replaced with simple functor object.
<pre>
#include <vector>
#include <algorithm>
#include <iostream>
template<class Vals>
void sortingPermutation(const Vals& values, std::vector<int>& v){
  int size = values.size();
  v.clear(); v.reserve(size);
  for(int i=0; i < size; ++i)
    v.push_back(i);
  std::sort(v.begin(), v.end(), [&values](int a, int b) -> bool {
    return values[a] < values[b];
  });
}
int main()
{
    std::vector<double> values;
    values.push_back(24);
    values.push_back(55);
    values.push_back(22);
    values.push_back(1);
    std::vector<int> permutation;
    sortingPermutation(values, permutation);
    typedef std::vector<int>::const_iterator I;
    for (I p = permutation.begin(); p != permutation.end(); ++p)
        std::cout << *p << " ";
    std::cout << "\n";
}
</pre>
This will return values 3, 2, 0, 1. In fact, the code is so general: if I add #include <string> and change double to std::string in the declaration of values, the code works for string data type.
Method 2. You can use std::sort to sort the list of pairs {(24, 0), (55, 2), (22, 0), (1, 1)}.
<pre>
#include <vector>
#include <algorithm>
#include <utility>
typedef std::pair<double, int> Pair;
struct CmpPair
{
    bool operator()(const Pair& a, const Pair& b)
    { return a.first < b.first; }
};
void sortingPermutation(
    const std::vector<double>& values,
    std::vector<int>& permutation)
{
    std::vector<Pair> pairs;
    for (int i = 0; i < (int)values.size(); i++)
        pairs.push_back(Pair(values[i], i));
    std::sort(pairs.begin(), pairs.end(), CmpPair());
    typedef std::vector<Pair>::const_iterator I;
    for (I p = pairs.begin(); p != pairs.end(); ++p)
        permutation.push_back(p->second);
}
#include <iostream>
int main()
{
    std::vector<double> values;
    values.push_back(24);
    values.push_back(55);
    values.push_back(22);
    values.push_back(1);
    std::vector<int> permutation;
    sortingPermutation(values, permutation);
    typedef std::vector<int>::const_iterator I;
    for (I p = permutation.begin(); p != permutation.end(); ++p)
        std::cout << *p << " ";
    std::cout << "\n";
}
</pre>
This will give the same result as above. In fact, the code is so general: if I add '''#include <string>''' and change '''double''' to '''std::string''' in the declaration of ''values'', the code works for string data type. See also c version
http://stackoverflow.com/questions/2804493/finding-unique-elements-in-an-string-array-in-c.
Method 3. Create a vector of ints 0..N and then sort that array with a comparison function that compares the corresponding elements of the vector you're trying to find the sorted permutation of. Something like:
<pre>
#include <iostream>
#include <algorithm>
#include <vector>
template<class T> class sorter {
    const std::vector<T> &values;
public:
    sorter(const std::vector<T> &v) : values(v) {}
    bool operator()(int a, int b) { return values[a] < values[b]; }
};
template<class T> std::vector<int> order(const std::vector<T> &values)
{
    std::vector<int> rv(values.size());
    int idx = 0;
    for (std::vector<int>::iterator i = rv.begin(); i != rv.end(); i++)
        *i = idx++;
    std::sort(rv.begin(), rv.end(), sorter<T>(values));
    return rv;
}
int main()
{
    std::vector<double> values;
    values.push_back(24);
    values.push_back(55);
    values.push_back(22);
    values.push_back(1);
    std::vector<int> permutation;
    permutation = order(values);
    typedef std::vector<int>::const_iterator I;
    for (I p = permutation.begin(); p != permutation.end(); ++p)
        std::cout << *p << " ";
    std::cout << "\n";
}
</pre>
This also gives the same result. In fact, the code is so general: if I add '''#include <string>''' and change '''double''' to '''std::string''' in the declaration of ''values'', the code works for string data type.
=== Infinity value ===
http://en.cppreference.com/w/cpp/types/numeric_limits/infinity
<syntaxhighlight lang='cpp'>
    double max = std::numeric_limits<double>::max();
    double inf = std::numeric_limits<double>::infinity();
    if(inf > max)
        std::cout << inf << " is greater than " << max << '\n';
</syntaxhighlight>
=== Scope ===
Method 1: Created object cannot be used in other places.
<syntaxhighlight lang='cpp'>
Class aClass {
  aClass();
  void foo();
};
aClass::aClass() {
  bClass *obj = new bClass;
}
void aClass::foo() {
  obj->myfunction(); // Won't work!!
}
</syntaxhighlight>
Method 2: Created object can be used within the class.
<syntaxhighlight lang='cpp'>
Class aClass {
  aClass();
  void foo();
};
aClass::aClass() {
  obj = new bClass;
}
void aClass::foo() {
  obj->myfunction();
}
</syntaxhighlight>
=== C++11 ===
* http://www.stroustrup.com/ and [http://www.stroustrup.com/C++11FAQ.html C++FAQ]
* [http://www.codeproject.com/Articles/570638/Ten-Cplusplus-Features-Every-Cplusplus-Developer Ten C++11 Features Every C++ Developer Should Use]
* http://en.wikipedia.org/wiki/C%2B%2B11
* http://www.stroustrup.com/C++11FAQ.html
* http://www.cprogramming.com/c++11/what-is-c++0x.html
To compile a code containing C++11 features (gcc 4.7 and up), using '''-std=c++11''' or '''-std=c++0x''' option in g++
<pre>
g++ -std=c++11 MYCODE.cc
</pre>
==== regex ====
* http://www.cplusplus.com/reference/regex/
* http://en.cppreference.com/w/cpp/regex/
* http://www.johndcook.com/blog/cpp_regex/ (interesting but seems not working on my gcc 4.7)
* Need [http://gcc.gnu.org/gcc-4.9/changes.html gcc 4.9] to support regex. See [http://stackoverflow.com/questions/11269766/no-matches-with-c11-regex this post] on stackoverflow.com.
regex_match() function
<pre>
#include <algorithm>
#include <iostream>
#include <string>
#include <regex>
int main()
{
    if (std::regex_match("yx12", std::regex("(.*x1.*)")))
        std::cout << "string yx12 contains x1.\n";
    if (std::regex_match("yx12", std::regex("(.*x2.*)")))
        std::cout << "string yx12 contains x2.\n";
    if (std::regex_match("yx12", std::regex("(.*yx1.*)")))
        std::cout << "string yx12 contains yx1.\n";
}
</pre>
See [http://stackoverflow.com/questions/1318279/regex-to-match-a-string-that-doesnt-contain-a-certain-string this post] to match a string that does not contain some string.
==== Lambda functions ====
A lambda function is essentially an anonymous function (a function without a name) that’s defined inline.
* http://www.cprogramming.com/c++11/c++11-lambda-closures.html
* http://msdn.microsoft.com/en-us/library/dd293608(v=vs.100).aspx
* http://en.cppreference.com/w/cpp/language/lambda
* http://candrews.net/blog/2011/07/understanding-c-0x-lambda-functions/
* [http://stackoverflow.com/questions/1577475/c-sorting-and-keeping-track-of-indexes sorting and index]
A simple example from an [http://www.oracle.com/technetwork/articles/servers-storage-dev/howto-use-lambda-exp-cpp11-2189895.html article] from oracle.com.
<syntaxhighlight lang='cpp'>
#include <stdlib.h>
#include <stdio.h>
#include <algorithm>
int main()
{
  int a[10] = { 9, 8, 7, 6, 5, 4, 3, 2, 1, 0 };
  std::sort( a, &a[10], [](int x, int y){ return x < y; } );
  for(int i=0; i<10; i++) { printf("%i ", a[i]); }
  printf("\n");
  return 0;
}
</syntaxhighlight>
=== STL vector vs C++ new ===
* http://lemire.me/blog/archives/2012/06/20/do-not-waste-time-with-stl-vectors/
Basic STL vector
* http://www.cprogramming.com/tutorial/stl/vector.html
* http://www.cplusplus.com/reference/vector/vector/
Vectors examples:
<syntaxhighlight lang='cpp'>
#include <vector>
using std::vector;
vector<int> ivec;            // ivec holds objects of type int
vector<Sales_item> Sales_vec; // holds Sales_items
vector<vector<string>> file;  // vector whose elements are vectors
# Initialize vectors
vector<int> ivec;            // initially empty
vector<int> ivec2(ivec);      // copy elements of ivec into ivec2
vector<int> ivec3 = ivec;    // copy elements of ivec into ivec3
vector<string> svec(ivec2);  // error: svec holds strings, not ints
vector<string> articles = {"a", "an", "the"};
vector<int> ivec(10, -1);      // ten int elements, each initialized to -1
vector<string> svec(10, "hi!"); // ten strings; each element is "hi!"
vector<int> ivec(10);    // ten elements, each initialized to 0
vector<string> svec(10); // ten elements, each an empty string
vector<int> v1(10);    // v1 has ten elements with value 0
vector<int> v2{10};    // v2 has one element with value 10
vector<int> v3(10, 1); // v3 has ten elements with value 1
vector<int> v4{10, 1}; // v4 has two elements with values 10 and 1
# Add an element to a vector
vector<int> v2;        // empty vector
for (int i = 0; i != 100; ++i)
    v2.push_back(i);    // append sequential integers to v2
                  // at end of loop v2 has 100 elements, values 0 . . . 99
# vector operators
# v.empty()
# v.size()
# v.push_back(t)
# v[n]
# v1 = v2
# v1 = {a, b, c}
# v1 == v2
# v1 != v2
# <, <=, >, >=
vector<int> v{1,2,3,4,5,6,7,8,9};
for (auto &i : v)    // for each element in v (note: i is a reference)
    i *= i;          // square the element value
for (auto i : v)      // for each element in v
    cout << i << " "; // print the elemen
vector<int> ivec;  // empty vector
for (decltype(ivec.size()) ix = 0; ix != 10; ++ix)
    ivec.push_back(ix);  // ok: adds a new element with value ix
</syntaxhighlight>
Iterator examples:
<syntaxhighlight lang='cpp'>
auto b = v.begin(), e = v.end(); // begin and end are two iterator members.
                                // Return type is an iterator.
# standard container iterator operators
# *iter        returns a reference to the element denoted by the iterator iter
# iter->mem
# ++iter
# --iter
# iter1 == iter2
# iter1 != iter2
string s("some string");
if (s.begin() != s.end()) { // make sure s is not empty
    auto it = s.begin();    // it denotes the first character in s
    *it = toupper(*it);    // make that character uppercase
}
// changed the case of the first word in a string to use iterators instead
// process characters in s until we run out of characters or we hit a whitespace
for (auto it = s.begin(); it != s.end() && !isspace(*it); ++it)
    *it = toupper(*it); // capitalize the current character
# Combining Dereference and Member Access
(*it).empty() // dereferences it and calls the member empty on the resulting object
# Iterator arithmetic
# iter + n
# iter -n
# iter1 += n
iter1 -= n
# iter1 - iter2
# >, >=, <, <=
// compute an iterator to the element closest to the midpoint of vi
auto mid = vi.begin() + vi.size() / 2;
</syntaxhighlight>
==== convert a C-style array to a c++ vector ====
<syntaxhighlight lang='cpp'>
#include <vector>
int data[] = { 1, 2, 3, 4, 4, 3, 7, 8, 9, 10 };
std::vector<int> v(data, data+10);
</syntaxhighlight>
=== WINVER ===
http://stackoverflow.com/questions/1439752/what-is-winver
WINVER determines the minimum platform SDK required to build your application, which in turn will determine at compile time which routines are found by the headers.
You can use this to verify, at compile time, that your application will work on Windows 2000 (0x0500), for example, or on Windows XP (0x0501).
This was used in win32 disk imager program.
=== Period ===
[http://stackoverflow.com/questions/1238613/what-is-the-difference-between-the-dot-operator-and-in-c What is the difference between the dot (.) operator and -> in C++?]
foo->bar() is the same as (*foo).bar().
Member Access Operators: . and -> from [http://msdn.microsoft.com/en-us/library/b930c881.aspx msdn.microsoft.com]
=== Protected vs private members ===
See [http://stackoverflow.com/questions/224966/private-and-protected-members-c here] or [http://www.cs.rit.edu/~mjh/docs/c++-faq/private-inheritance.html C++ FAQ]
Private members are only accessible within the class defining them.
Protected members are accessible in the class that defines them and in classes that inherit from that class.
Edit: Both are also accessible by friends of their class, and in the case of protected members, by friends of their derived classes.
Edit 2: Use whatever makes sense in the context of your problem. You should try to make members private whenever you can to reduce coupling and protect the implementation of the base class, but if that's not possible then use protected members. Check C++ FAQ Lite for a better understanding of the issue. This question about protected variables might also help.
=== alignment of pointers ===
https://stat.ethz.ch/pipermail/r-devel/2013-August/067314.html
=== Generate random numbers ===
[http://www.cplusplus.com/reference/cstdlib/srand/ srand()],  rand() and [http://www.cplusplus.com/reference/cstdlib/RAND_MAX/ RAND_MAX]
[https://stackoverflow.com/a/6219525 How to generate a random number between 0 and 1?]
=== Find sample quantiles (percentiles) using STL ===
STL algorithms defined in the <algorithm> and <numeric> libraries.
* http://stackoverflow.com/questions/10769743/determine-the-third-quartile-from-a-collection-of-integers-in-c
* http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=290
* http://www.java2s.com/Code/Cpp/STL-Algorithms-Non-modifying-sequence-operations/findanddisplayvinlowest20thpercentile.htm
<pre>
#include <iostream>
#include <numeric>      // accumulate
#include <algorithm>    // std::sort, std::nth_element
#include <vector>      // std::vector
using namespace std;
int main()
{
  int grades[]={89, 74, 89, 63, 100};
 
  size_t elements=sizeof(grades)/sizeof(grades[0]);
  double res= accumulate(grades, grades+ elements, 0)/double(elements);
  std::cout << "Raw data:";
  for(int i=0; i< elements; i++) std::cout << ' ' << grades[i];
  std::cout << '\n';
  cout << res << " is the mean" << endl << endl;
   
  // the iterator constructor can also be used to construct from arrays: 
  std::vector<int> myvector (grades, grades + elements); 
  std::sort(myvector.begin(), myvector.end());
  std::cout << "myvector contains:";
  for (std::vector<int>::iterator it = myvector.begin(); it!=myvector.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';
  double median= *(myvector.begin()+myvector.size()/2); //89
  cout<<median << " is the median" << endl  << endl;
  std::vector<int> myvector2 (grades, grades + elements); 
  // None of the elements preceding nth (25-th perct) are greater than it,
  // and none of the elements following it are less.
  nth_element(myvector2.begin(),
              myvector2.begin()+ (int)(myvector2.size()*0.25),
              myvector2.end());
  std::cout << "myvector2 contains:";
  for (std::vector<int>::iterator it = myvector2.begin(); it!=myvector2.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';           
  // This method will select one data as the percentile! 
  int p_25= *(myvector2.begin()+ (int)(myvector2.size()*.25));
  cout<<p_25<< " is the 25th percentile" << endl;
  return 0;
}
</pre>
The output looks like (note in myvector2 data preceding 74 is less than 74 and data following 74 is greater than 74)
<pre>
$ g++ example.cpp
$ ./a.out
Raw data: 89 74 89 63 100
83 is the mean
myvector contains: 63 74 89 89 100
89 is the median
myvector2 contains: 63 74 89 89 100
74 is the 25th percentile
$
</pre>
=== Calculate execution time  ===
http://stackoverflow.com/questions/5248915/execution-time-of-c-program. The following code is from [http://www.simonmonk.org/?page_id=370 Programming Arduino Next Steps]. It took .04 seconds on Xeon W3690 @ 3.47GHz and 28 seconds on Arduino Uno.
<syntaxhighlight lang='c'>
#include <stdio.h>
#include <time.h> 
main()
{
  printf("\nStarting Test\n");
  time_t startTime = clock();
 
  // test code here
  long  i = 0;
  long j = 0;
  for (i = 0; i < 20000000; i ++)
  {
    j = i + i * 10;
    if (j > 10) j = 0;
  }
  // end of test code
  time_t endTime = clock();
 
  printf("%ld\n", j); // prevent loop being optimized out
  printf("Finished Test\n");
  double timeSpent = (double)(endTime - startTime) / CLOCKS_PER_SEC;
  printf("seconds taken: %f\n", timeSpent);
  return 0;
}
</syntaxhighlight>
==== Time the iterations from 0 to 2147483647 ====
<syntaxhighlight lang='c'>
#include <iostream>
using namespace std;
int main() {
    /* Purpose: time the iterations from 0 to
    * the maximum positive integer.
    * See
    * http://en.cppreference.com/w/cpp/language/types
    * http://stackoverflow.com/questions/728068/how-to-calculate-a-time-difference-in-c
    */
    const clock_t begin_time = clock();
    int flag=0;
    for(int i=0; i<2147483647; i++)
      if (!(i%50)) flag++;
    std::cout << float( clock () - begin_time ) /  CLOCKS_PER_SEC;
    cout << "\nflag " <<  flag << endl;
    int flag2 = 2147483647;
    cout << "\nflag2=" << flag2 <<endl;
    flag2++;
    cout << "\nflag2 + 1=" << flag2 << endl;
    flag2++;
    cout << "\nflag2 + 2=" << flag2 << endl;
    return 0;
}
</syntaxhighlight>
On Xeon(R) CPU E5-1650 0 @ 3.20GHz, it took about 7 seconds and on my [[Udoo|UDOO Dual]], it took about 1 minute. On Phenom(tm) II X6 1055T, it took 9 seconds. On Raspberry Pi 2, it took 97 seconds. On ODroid xu4, it took 13 seconds.
<syntaxhighlight lang='bash'>
brb@T3600 /tmp $ g++ tmp.cpp; ./a.out
6.83841
flag 42949673
flag2=2147483647
flag2 + 1=-2147483648
flag2 + 2=-2147483647
</syntaxhighlight>
=== execute a command line command from a C++ program ===
* Use the [http://linux.die.net/man/3/system system()] function - http://stackoverflow.com/questions/10931134/return-value-of-system-function-call-in-c-used-to-run-a-python-program
<syntaxhighlight lang='bash'>
#include <stdlib.h>
int main() {
  system("cp ~/Downloads/testInt.cpp ~/Downloads/testInt2.cpp");
}
</syntaxhighlight>
* Executing a shell command in c/c++ and getting its output http://en.cppreference.com/w/cpp/utility/program/system
<syntaxhighlight lang='bash'>
FILE* pipe = popen("your shell command here", "r");
if (!pipe)
{
cerr<<"popen error"<<endl;
}   
char buffer[128];
    while(!feof(pipe))
    {
      if(fgets(buffer, 128, pipe) != NULL){}
    }
pclose(pipe);
buffer[strlen(buffer)-1] = '\0';
</syntaxhighlight>
=== Check OS ===
http://stackoverflow.com/questions/142508/how-do-i-check-os-with-a-preprocessor-directive
<syntaxhighlight lang='cpp'>
#ifdef _WIN32
...
#else
...
#endif
</syntaxhighlight>
=== Check Visual Studio compiler ===
'''_MSC_VER''' macro. See
* https://stackoverflow.com/questions/70013/how-to-detect-if-im-compiling-code-with-visual-studio-2008
* https://msdn.microsoft.com/en-us/library/b0084kay.aspx
* http://www.cplusplus.com/forum/beginner/155465/
* https://github.com/arraytools/C/blob/master/accelerated_unix/chapter03/med.cc
<syntaxhighlight lang='cpp'>
#ifdef _MSC_VER
typedef std::vector<double>::size_type vec_sz;
#else
typedef vector<double>::size_type vec_sz;
#endif
</syntaxhighlight>
== C libraries ==
* http://www.cplusplus.com/reference/clibrary/
=== cctype ===
=== cerrno ===
=== climits ===
=== cmath ===
=== cstddef ===
=== cstdio ===
=== cstdlib ===
http://www.cplusplus.com/reference/cstdlib/
abs(), free(), malloc(), exit(), rand(), system(), getenv(), NULL and some string function.
=== cstring ===
=== ctime ===
== C++ standard libraries ==
* [http://en.cppreference.com/w/cpp cppreference.com]
* [http://en.wikipedia.org/wiki/C%2B%2B_Standard_Library wikipedia]
* [http://www.cplusplus.com/reference/ cplusplus.com]
* [http://cppstdlib.com/ The C++ Standard Library - A Tutorial and Reference] by Nicolai M. Josuttis and [https://www.amazon.com/Standard-Library-Tutorial-Reference-2nd/dp/0321623215 reviews] from amazon.
=== STL and c++ standard library ===
* [http://en.wikipedia.org/wiki/C%2B%2B_Standard_Library C++ standard library] from wikipedia.
* [http://stackoverflow.com/questions/5205491/whats-this-stl-vs-c-standard-library-fight-all-about STL vs C++ standard library]. STL was written in the days long before C++ was standardized. Parts of the C++ Standard Library were based on parts of the STL, and it is these parts that many people (including several authors and the notoriously error-ridden cplusplus.com) still refer to as "the STL". However, this is inaccurate; indeed, the C++ standard never mentions "STL", and there are content differences between the two.
=== Containers ===
==== Four categories ====
# <span style="color: blue">'''Sequence containers'''</span>: vector, array, deque, list, forward_list.
# <span style="color: blue">'''Associative containers'''</span>: map, set, multimap, multiset.
# <span style="color: blue">'''Unordered associative containers'''</span>: unordered_map, unordered_set, unordered_multimap, unordered_multiset.
# <span style="color: blue">'''Container adaptors'''</span>: stack, queue, priority_queue.
The most important containers are '''vector''', '''list''' and '''map'''.
==== Common operations ====
See Accelerated C++ 5.9.
<syntaxhighlight lang='cpp'>
container<T>::iterator
container<T>::const_iterator
container<T>::size_type
c.begin()
c.end()
container<T> c;
container<T> c(c2);
container<T> c(n);
container<T> c(n, t);
c=c2;
c.size()
c.empty()
c.insert(d, b, e)
c.erase(it)
c.erase(b, e);    // remove [b, e)
c.push_back(t)
c[n]
// iterator
*it
(*it).x
it->x
++it;
b == e;
b != e;
// string type
s.substr(i, j)
getline(is, s)
s += s2;
// vector
v.reserve(n)
v.resize(n)
// list
l.sort()
l.sort(cmp)
</syntaxhighlight>
// <cctype> header
isspace(c)
isalpha(c)
isdigit(c)
isalnum(c)
isupper(c)
islower(c)
toupper(c)
tolower(c)
==== vector ====
http://www.cplusplus.com/reference/vector/vector/
===== push_back() method =====
http://www.cplusplus.com/reference/vector/vector/push_back/
The vector length does not have to be determined beforehand. The '''push_back()''' method can be used to insert an element to the end of a vector object.
===== clear() method =====
Removes all elements from the vector (which are destroyed), leaving the container with a size of 0. See http://www.cplusplus.com/reference/vector/vector/clear/.
===== resize() method =====
See an application of [http://www.cplusplus.com/reference/algorithm/transform/ transform].
===== Vector of own type =====
* See Accelerated C++ Chapter 4. See <[https://github.com/arraytools/C/tree/master/accelerated_unix/chapter04 main2.cc]> program which is a standalone program which does not depend on other source. Below it gives an example of the output:
<syntaxhighlight lang='bash'>
brb@T3600 ~/github/C/accelerated_unix/chapter04 $ ./main2
Taylor 80 90
1 2 3
Jones 90 80
3 2 1
Jones  50.8
Taylor 52.8
</syntaxhighlight>
===== size_type =====
<syntaxhighlight lang='cpp'>
vector<int>::size_type x;
</syntaxhighlight>
size_type is a (static) member type of the type vector<int>. Usually, it is a typedef for std::size_t, which itself is usually a typedef for unsigned int or unsigned long long.
Declare x as a variable of a type suitable for holding the size of a vector
For example,
<syntaxhighlight lang='cpp'>
string::size_type width(const vector<string>& v)
{
string::size_type maxlen = 0;
#ifdef _MSC_VER
for(std::vector<string>::size_type i = 0; i != v.size(); ++i)
#else
for(vector<string>::size_type i = 0; i != v.size(); ++i)
#endif
maxlen = max(maxlen, v[i].size());
return maxlen;
}
</syntaxhighlight>
===== square bracket operator =====
For example,
<syntaxhighlight lang='cpp'>
const vector<double> v;
double d = v[1];
</syntaxhighlight>
The square bracket operation is actually doing
<syntaxhighlight lang='cpp'>
double d1 = v.operator[](1)
</syntaxhighlight>
===== 2 dimensional matrix =====
* http://www.cplusplus.com/forum/articles/7459/
* http://stackoverflow.com/questions/12841170/how-to-define-a-2d-array-in-c-and-stl-without-memory-manipulation
The first example uses resize() method to specify the number of rows and columns.
<syntaxhighlight lang='cpp'>
#include <vector>
using std::vector;
#define HEIGHT 5
#define WIDTH 3
int main() {
  vector<vector<double> > array2D;
  // Set up sizes. (HEIGHT x WIDTH)
  array2D.resize(HEIGHT);
  for (int i = 0; i < HEIGHT; ++i)
    array2D[i].resize(WIDTH);
  // Put some values in
  array2D[1][2] = 6.0;
  array2D[3][1] = 5.5;
  return 0;
}
</syntaxhighlight>
The second example (Method 2 below) needs to use push_back() twice. So it is probably less efficient.
<syntaxhighlight lang='cpp'>
int main ()
{
    /**************
        1  2  3
        4  5  6
    ***************/
    // Method 1
    const int ROW = 2;
    const int COL = 3;
    int array1[ROW][COL];
    for(int i=0; i<ROW; i++)
        for(int j=0; j<COL; j++)
            array1[i][j] = i*COL+j+1;
    // Method 2
    typedef vector<vector<int> > ARRAY;
    ARRAY array2;
    vector<int> rowvector;
    for(int i=0; i<ROW; i++)
    {
        rowvector.clear();
        for(int j=0; j<COL; j++)
            rowvector.push_back(i*COL+j+1);
        array2.push_back(rowvector);
    }
    return 0;
}
</syntaxhighlight>
The first example also works on string type.
<syntaxhighlight lang='cpp'>
#include <iostream>
#include <vector>
#include <string>
using namespace std;
void foo1(vector<vector<string>> &vs) {
    vs.resize(2);
    for(int i=0; i < 2; ++i)
        vs[i].resize(4);
    vs[0][0] = "This";
    vs[0][1] = "Is";
    vs[0][2] = "A";
    vs[0][3] = "BOok";
    vs[1][0] = "That";
    vs[1][1] = "Is not";
    vs[1][2] = "An";
    vs[1][3] = "Apple";
}
void foo2(vector<vector<string>> &vs) {
    cout << vs.size() << endl;
    for(unsigned int i=0; i < vs.size(); ++i) cout << "row " << i << ", size " << vs[i].size() << endl;
    for(unsigned int i=0; i < vs.size(); ++i)
    {
        for(unsigned int j=0; j < vs[i].size(); ++j)
        {
            cout << vs[i][j] << " ";
        }
        cout << endl;
    }
}
int main()
{
    vector<vector<std::string>> vs;
    foo1(vs);
    foo2(vs);
    cout << endl;
    return 0;
}
</syntaxhighlight>
===== Comparison of a C++ array and C++ '''vector''' =====
See the example from PPP Chapter 20.1.
<syntaxhighlight lang='cpp'>
double* get_from_jack(int* count);  // jack puts doubles into an array
                                    // and returns the number of elements in *count
vector<double>* get_from_jill();    // Jill fills the vector
int main()
{
    int jack_count = 0;
    double* jack_data = get_from_jack(&jack_count);   
    vector<double>* jill_data = get_from_jill();   
    // ... Process ...
    // jack_data[i]    ---- value
    // &jack_data[i]    ---- address
    //
    // (*jill_data)[i]  ---- value, deference the pointer first
    // &(*jill_data)[i] ---- address, deference the pointer first
    //
    // note *jill_data[i] is not what we want; that means *(jill_data[i])
    //
    delete[] jack_data;
    delete jill_data;
}
double* get_from_jack(int* count)
{
    if (!count)
        return 0;
    const int n = 10;
    double* arr = new double[n];
    if (arr)  {
        *count = n;
        for (int i = 0; i < n; ++i)
            arr[i] = i;
    }
    return arr;
}
vector<double>* get_from_jill()
{
    const int n = 10;
    vector<double>* arr = new vector<double>(n);
    if (arr)
    {
        for (int i = 0; i < n; ++i)
            (*arr)[i] = i;
    }
    return arr;
}
</syntaxhighlight>
==== list ====
(From The C++ PL) A '''list''' is a double-linked list. We use a '''list''' for sequences where we want to insert and delete elements without moving other elements.
(From Acceleted C++) Just as '''vector'''s are optimized for fast random access, '''list'''s are optimized for fast insertion and deletion anywhere within the container. Because '''list'''s have to maintain a more complicated structure, they are slower than '''vector'''s if the container is accessed only sequentially. That is, if the container grows and shrinks only or primarily from the end, a '''vector''' will outperform a '''list'''. However, if a program deletes many elements from the middle of the container, then '''list'''s will be faster for large inputs. ... '''list'''s and '''vector'''s share many operations. As a result, we can often translate programs that operate on '''vector'''s into programs that operate on '''list'''s, and vice versa.
One key operation that '''vector'''s support, but '''list'''s do not, is indexing. But if we use 'iterators''' instead of indices, these two types will be even similar. See 5.5.1 for some important differences between these two.
<syntaxhighlight lang='cpp'>
list<Student_info> extract_fails(list<Student_info>& students)
{
  list<Student_info> fail;
  list<Student_info>::iterator iter = students.begin();
  while (iter != students.end()) {
    if (fgrade(*iter)) {
        fail.push_back(*iter);
        iter = students.erase(iter);
    } else
        ++iter;
  }
}
</syntaxhighlight>
(From The C++ PL) When we use a linked list, we end not to access elements using subscripting the way we do for vectors. Instead, we might search the list looking for an element with a given value.
<syntaxhighlight lang='cpp'>
struct Entry {
  string name;
  int number;
}
list<Entry> phone_book = {
  {"David Hume", 123456},
  {"Karl Popper", 234547},
  {"Bert Arthur", 345678}
};
int get_number(const string& s)
{
  for (const auto& x : phone_book)
    if (x.name == s)
      return x.number;
  return 0; // use 0 to represent 'number not found'
}
// OR using the iterator
int get_number(const string& s)
{
  for (auto p = phone_book.begin(); p!= phone_book.end(); ++p)
      if (p->name == s)
          return p->number;   
  return 0;
}
// To delete or insert an element
void f(const Entry& ss, list<Entry>::iterator p, list<Entry>::iterator q)
{
  phone_book.insert(p, ee); // add ee before the element referred to by p
  phone_book.erase(q);      // remove the element referred to by q
}
</syntaxhighlight>
==== map ====
A map is like a vector but using a key instead of an integer to index it (R's vector object can do it already; see [https://cran.r-project.org/doc/manuals/r-release/R-intro.html#Index-vectors An introduction to R]).
The following example is from Chapter 7 of Accelerated C++. See also the [http://www.cplusplus.com/reference/map/map/begin/ example] and a list of [http://www.cplusplus.com/reference/map/map/?kw=map member types/functions] from  cplusplus.com. The interesting thing is the elements are ordered by their ''key'' at all times (see the example below).
<syntaxhighlight lang='cpp'>
#include <iostream>
#include <map>
#include <string>
using std::cin;
using std::cout;
using std::endl;
using std::map;
using std::string;
int main()
{
string s;
map<string, int> counters; // store each word and an associated counter
// read the input, keeping track of each word and how often we see it
while (cin >> s)
++counters[s];
// write the words and associated counts
for (map<string, int>::const_iterator it = counters.begin();
    it != counters.end(); ++it) {
cout << it->first << "\t" << it->second << endl;
}
return 0;
}
</syntaxhighlight>
And the output
<pre>
brb@T3600 ~/Downloads $ g++ mapTest.cpp
brb@T3600 ~/Downloads $ ./a.out
this is a book
a      1
book    1
is      1
this    1
brb@T3600 ~/Downloads $
</pre>
==== array ====
==== deque ====
==== queue ====
==== set ====
==== stack ====
==== unordered_map ====
==== unordered_set ====
=== [http://www.cplusplus.com/reference/algorithm/ Algorithms] ===
* [http://www.cplusplus.com/reference/algorithm/ cplusplus.com]
* [http://en.cppreference.com/w/cpp/algorithm cppreference.com]
Accelerated C++ 6.4 says <span style="color: red">Algorithms act on container elements; they do not act on containers</span>. For example, remove_if() and partition() do not change the size of the container on which it operates. The '''size''' of the container is the same. To really shorten a vector, we need to use the [http://www.cplusplus.com/reference/vector/vector/erase/ erase()] method from container operations. That is to say, when we use remove() or remove_if() function, it is likely we want to apply erase() method to the container.
<syntaxhighlight lang='cpp'>
/* 6.4 Algorithms, containers and iterators in Accelerated C++ */
students.erase(remove_if(students.begin(), students.end(), fgrade),
              students.end());
</syntaxhighlight>
==== all_of/ any_of/ none_of/ for_each ====
* http://www.cplusplus.com/reference/algorithm/all_of/
* http://www.cplusplus.com/reference/algorithm/any_of/
* http://www.cplusplus.com/reference/algorithm/none_of/
* http://www.cplusplus.com/reference/algorithm/for_each/
==== count/ count_if/ find/ find_if/ mismatch/ search/ search_n ====
'''N.B.'''
# string, map and set types also has a member function called '''find()'''. See [http://www.cplusplus.com/reference/string/string/find/ string::find], [http://www.cplusplus.com/reference/map/map/find/ map::find] and [http://www.cplusplus.com/reference/set/set/find/ set::find].
# Qt has its own solution: [[Qt#QRegExp|QRegExp]].
The algorithms '''find()''' and '''find_if()''' return an iterator to the first element that matches a value and a predicate (bool return type), respectively.
<syntaxhighlight lang='cpp'>
void f(const string& s)
{
  auto p_space = find(s.begin(), s.end(), ' ');
  auto p_whitespace = find(s.begin(), s.end(), isspace);
}
</syntaxhighlight>
See also the split() function example in 6.1.1 of Accelerated C++ that uses find_if() function.
Example of using [http://www.cplusplus.com/reference/algorithm/find/ std::find()] function to <u>return the index</u> of a string (only the first match) in a vector and [http://www.cplusplus.com/reference/iterator/distance/ std::distance] to find the distance between two iterators.
<syntaxhighlight lang='cpp'>
#include <algorithm>
#include <iostream>
#include <vector>
// #include <iterator>
int main()
{
    int data[] = { 1, 2, 3, 4, 4, 3, 7, 8, 9, 10 };
    std::vector<int> v(data, data+10);
    std::vector<int> indexResult;
    std::cout << "Original data is ";
    for(std::vector<int>::iterator it=v.begin(); it != v.end(); ++it)
      std::cout << *it << " ";
    std::cout << '\n';
    for(std::vector<int>::iterator it=v.begin(); it != v.end(); ++it) {
      // std::cout << "Begin shift= " << std::distance(v.begin(), it) << ", ";
      std::cout << "Current data is ";
      for(std::vector<int>::iterator it2=it; it2 != v.end(); ++it2) std::cout << *it2 << " ";
      std::cout << ", ";
      auto p = std::find(it, v.end(), 3);
      if (p != std::end(v))
        std::cout << "element 3 was found in v at position (starting from 0): " <<
          std::distance(it, p) << std::endl;
      else
        std::cout << "element 3 was not found in Data\n";
    } 
}
</syntaxhighlight>
And the output
<pre>
$ g++ -std=c++11 count.cpp; ./a.out
Original data is 1 2 3 4 4 3 7 8 9 10
Current data is 1 2 3 4 4 3 7 8 9 10 , element 3 was found in v at position (starting from 0): 2
Current data is 2 3 4 4 3 7 8 9 10 , element 3 was found in v at position (starting from 0): 1
Current data is 3 4 4 3 7 8 9 10 , element 3 was found in v at position (starting from 0): 0
Current data is 4 4 3 7 8 9 10 , element 3 was found in v at position (starting from 0): 2
Current data is 4 3 7 8 9 10 , element 3 was found in v at position (starting from 0): 1
Current data is 3 7 8 9 10 , element 3 was found in v at position (starting from 0): 0
Current data is 7 8 9 10 , element 3 was not found in Data
Current data is 8 9 10 , element 3 was not found in Data
Current data is 9 10 , element 3 was not found in Data
Current data is 10 , element 3 was not found in Data
</pre>
But the string type case is more complicated. It only finds the EXACT match string.
<syntaxhighlight lang='cpp'>
#include <algorithm>
#include <iostream>
#include <vector>
#include <string>
// #include <iterator>
int main()
{
    std::vector<std::string> v = {"x1", "yx12", "x1", "x3", "x4"};
    std::vector<int> indexResult;
    std::cout << "Search position (starting from 0)" << std::endl;
    std::cout << "Original data is ";
    for(auto it : v)
      std::cout << it << " ";
    std::cout << '\n';
    for(std::vector<std::string>::iterator it=v.begin(); it != v.end(); ++it) {
      std::cout << "Current data is ";
      for(std::vector<std::string>::iterator it2=it; it2 != v.end(); ++it2)
          std::cout << *it2 << " ";
      std::cout << ", ";
      auto p = std::find(it, v.end(), "x1");
      if (p != std::end(v))
        std::cout << "element x1 was found in v at position: " <<
                    std::distance(it, p) << std::endl;
      else
        std::cout << "element x1 was not found in Data\n";
    }
}
</syntaxhighlight>
After compiling & running it, we will see the find() function cannot pick up the 2nd case.
<pre>
$ g++ -std=c++11 count.cpp; ./a.out
Search position (starting from 0)
Original data is x1 yx12 x3 x4
Current data is x1 yx12 x1 x3 x4 , element x1 was found in v at position: 0
Current data is yx12 x1 x3 x4 , element x1 was found in v at position: 1
Current data is x1 x3 x4 , element x1 was found in v at position: 0
Current data is x3 x4 , element x1 was not found in Data
Current data is x4 , element x1 was not found in Data
</pre>
==== copy/ copy_if/ swap/ swap_ranges/ remove/ remove_if/ fill/ replace/ replace_if/ shuffle/ unique/ transform ====
The [http://www.cplusplus.com/reference/algorithm/transform/ transform()] function is similar to the [http://www.cplusplus.com/reference/algorithm/for_each/ for_each()] function.
An application of '''[http://www.cplusplus.com/reference/algorithm/transform/ transform]''' function is to copy data from the vector to the set and convert that to lowercase. PS. the following example will overwrite the original string because the output iterator is the same as the input iterator.
<syntaxhighlight lang='cpp'>
#include <iostream>
#include <algorithm>
#include <string>
int main() {
    std::string data = "Abc";
    std::transform(data.begin(), data.end(), data.begin(), ::tolower);
    std::cout << data << std::endl;
    return 0;
}
</syntaxhighlight>
If the output length is known we can create a new object of the desired size as in [http://www.cplusplus.com/reference/algorithm/transform/ this example] in cplusplus.com. If the output is unknown, we can use [http://www.cplusplus.com/reference/iterator/back_inserter back_inserter()] function as in 6.2.3 of Accelerated C++.
Examples of '''remove_copy()''' can be found in 6.2.4 of Accelerated C++.
Examples of '''remove_copy_if()''' and '''remove_if()''' can be found in 6.3.1 of Accelerated C++.
==== sort/ stable_sort/ is_sorted/ partial_sort ====
==== merge/ set_difference/ set_intersection/ set_union ====
==== max/ max_element/ min/ min_element/ next_permutation ====
==== accumulate/ inner_product/ partial_sum ====
=== iterator & sequence ===
<pre>
Container  -------  Iterator  --------  Algorithm
</pre>
(PPP 20.3 Sequences and iterators) The central concept of the STL is the '''sequence'''. From the STL point of view, a collection of data is a sequence.
The reason '''STL algorithms''' and '''containers''' work so well together is that they don't know anything about each other. Instead, both understand about sequences defined by pairs of iterators.
A sequence has a beginning and an end. We identify the beginning and the end of a sequence by a pair of iterators. An '''iterator''' is an object that identifies an element of a sequence. An STL sequence is what is usually called "half-open"; the element identified by ''begin'' is part of the sequence, but the ''end'' iterator points one beyond the end of the sequence.
What is an iterator?
* An iterator points to an element of a sequence
* You can compare two iterators using == and !=
* You can refer to the value of the element pointed to by an iterator using the unary * operator ("dereference")
* You can get an iterator to the next element by using ++.
* The idea of an iterator is related to the idea of a '''pointer'''. However, many iterators are not just pointers; for example, we could define a range-checked iterator that throws an exception if you try to make it point outside its ''[begin:end)'' sequence or dereference ''end''. We get enormous flexibility and generality from having iterator as an abstract notion rather than as a specific type.
Advantages of using an iterator
* [http://stackoverflow.com/questions/14373934/iterator-loop-vs-index-loop Iterator Loop vs index loop]
* [http://stackoverflow.com/questions/131241/why-use-iterators-instead-of-array-indices Why use iterators instead of array indices?]
* [http://stackoverflow.com/questions/3046050/stl-iterators-purpose STL iterators - purpose]
Basic standard iterator operations
<pre>
// if p and q are two iterators
p == q
p != q
*p
*p = val
val = *p
++p
</pre>
==== Using iterators in string type ====
<syntaxhighlight lang='cpp'>
auto b=v.begin(), e=v.end();
string s("some string");
for (auto it = s.begin(); it != s.end() && !isspace(*it); ++it) {
    *it = toupper(*it);      // make that char uppercase
}
</syntaxhighlight>
==== Using iterator in vector type ====
<syntaxhighlight lang='cpp'>
vector<int>::iterator it;  // it can read and write vector<int> elements
string::iterator it2;      // it2 can read and write characters in a string
vector<int>::const_iterator it3; // it3 can read but not write elements
string::const_iterator it4;      // it4 can read but not write characters
vector<int> v;
const vector<int> cv;
auto it1 = v.begin();  // it1 has type vector<int>::iterator
auto it2 = cv.begin(); // it2 has type vector<int>::const_iterator
auto it3 = v.cbegin(); // it3 has type vector<int>::const_iterator
</syntaxhighlight>
Assuming ''it'' is an iterator into this ''vector'', we can check whether the string that ''it'' denotes is empty as follows:
<syntaxhighlight lang='cpp'>
(*it).empty()  # *it.empty() will gives an error
</syntaxhighlight>
Print each element of a vector of strings.
<syntaxhighlight lang='cpp'>
vector<string> text;
for (auto it = text.cbegin(); it != text.cend() && !it->empty(); ++it)
    cout << *it << endl;
</syntaxhighlight>
==== Examples ====
* [https://github.com/arraytools/C/blob/master/accelerated_unix/chapter06/split.cc split()] function (and [http://www.cplusplus.com/reference/algorithm/find_if/ find_if()] library function) in 6.1.1 of Accelerated C++ where the '''find_if()''' function is used to find the position of non-space characters (an auxiliary function). Compare with the [https://github.com/arraytools/C/blob/master/accelerated_unix/chapter05/split.cc split()] function in Chapter 5 where the search of a new word is done by searching the space key in a while() loop.
* [https://github.com/arraytools/C/blob/master/accelerated_unix/chapter06/urls.cc find_urls()] function in 6.1.3 of Accelerated C++
=== numeric ===
=== string ===
=== utilities ===
==== Function objects ====
==== pair ====
== Common errors ==
=== array out of bound error & memory ===
* [https://stackoverflow.com/questions/671703/array-index-out-of-bound-in-c Array index out of bound in C]
* [https://stackoverflow.com/questions/9154115/how-to-set-memory-use-limit-when-writing-c-program-and-what-happens-if-once-it-e How to set memory use limit when writing C program and what happens if once it exceeds this limit?]
* [http://tldp.org/LDP/solrhe/Securing-Optimizing-Linux-RH-Edition-v1.3/x4733.html ulimit]
* [https://ss64.com/bash/ulimit.html ulimit keys]
=== bad_alloc error ===
http://stackoverflow.com/questions/6833143/how-to-check-memory-allocation-failures-with-new-operator
In C++ there are 2 primary ways in which new allocates memory and each requires different error checking.
The standard new operator will throw a std::bad_alloc exception on failure and this can be handled like a normal exception
<pre>
try {
  char* c = new char[100];
} catch (std::bad_alloc&) {
  // Handle error
}
</pre>
Or alternative the nothrow version of new will simply return NULL on failure
<pre>
char* c = new (std::nothrow) char[100];
if (!c) {
  // Handle error
}
</pre>
== Debugging ==
* http://beej.us/guide/bggdb/
* An article from [http://www.drdobbs.com/testing/13-linux-debuggers-for-c-reviewed/240156817 Dr. Dobb's]
* [http://www.amazon.com/Art-Debugging-GDB-DDD-Eclipse/dp/1593271743 The Art of Debugging] and [http://www.cs.ucdavis.edu/people/faculty/matloff.html Matloff personal website].
* [https://msdn.microsoft.com/en-us/library/windows/hardware/ff551063(v=vs.85).aspx Debugging Tools for Windows (WinDbg, KD, CDB, NTSD)]
=== valgrind ===
http://valgrind.org/docs/manual/quick-start.html
For example,
<pre>
valgrind --leak-check=yes build-Qheatmap-Desktop_Qt_4_8_5-Debug/Qheatmap /home/brb/Qt/example/BRCACC/
</pre>
=== GDB, DDD, Nemiver ===
* http://beej.us/guide/bggdb/
* http://calcul.math.cnrs.fr/Documents/Ecoles/PF-2011/Cours/debug_autrans2011.pdf
* http://stackoverflow.com/questions/79023/is-there-a-c-gdb-gui-for-linux
==== gdb ====
* http://beej.us/guide/bggdb/ mentioned gdb has a built-in ''GUI'' mode called [https://sourceware.org/gdb/onlinedocs/gdb/TUI.html '''Text User Interface'''].
* https://en.wikipedia.org/wiki/GNU_Debugger
* http://babbage.cs.qc.edu/courses/cs701/Handouts/using_gdb.html. It mentioned ''assert()'', '''-DNDEBUG''' option in g++.
* http://www.cs.umd.edu/~srhuang/teaching/cmsc212/gdb-tutorial-handout.pdf
* https://blogs.oracle.com/ksplice/entry/8_gdb_tricks_you_should
* http://web.stanford.edu/class/cs107/gdb_coredump1.pdf Breakpoints tricks
* http://weldon.whipple.org/technotes/?page_id=33 Breakpoint condition with string
* http://blog.vinceliu.com/2009/07/gdbs-conditional-breakpoints.html Breakpoint condition with string
* http://www.thegeekstuff.com/2014/02/gdb-breakpoins-watchpoints/
A C program with a function: [https://www.programiz.com/c-programming/examples/prime-interval-function Display Prime Numbers Between Intervals Using Function]. A copy of the code is at [https://gist.github.com/arraytools/818e533754130153e37b9c19bcd8c9c6 Github].
To run gdb with its TUI mode,
<syntaxhighlight lang='bash'>
gcc -g -o foo foo.c
gdb -tui foo
# Press return key to see the source showing on the top panel
</syntaxhighlight>
To run a command with arguments, do
<syntaxhighlight lang='bash'>
gdb --args executablename arg1 arg2 arg3
</syntaxhighlight>
To combine tui and args parameters, do
<syntaxhighlight lang='bash'>
gdb -tui --args executablename arg1 arg2 arg3
</syntaxhighlight>
To run a command with redirect input (e.g. the source code contains e.g. '''std::cin'''), do
<syntaxhighlight lang='bash'>
gdb executablename
run < input
</syntaxhighlight>
{| class="wikitable"
! file a.out
! Load an executable file by name
|-
| '''b'''reak hello.c:100
| set a break point (a function, line number)
|-
| '''b'''reak hello.c:100 if i == 5
| set a break point if a condition is true
|-
| cond N Condition
| N is the break point number, Condition is any condition (eg i < .5, *p == 'a', strcmp(msg,"OK") == 0)
|-
| '''i'''nfo '''b'''reak
|
|-
| '''d'''elete Location
|
|-
| clear Location
| delete any breakpoints set at or within the code of the specified line
|-
| '''l'''ist
|
|-
| '''r'''un
|
|-
| '''n'''ext
|
|-
| '''s'''tep
| step in
|-
| '''u'''ntil
| run until the program reaches a source line greater than the current (eg loop)
|-
| '''fin'''ish
| Run until the end of the current 'function'. It'll jump back to the caller.
|-
| '''c'''ontinue
|
|-
| '''p'''rint Expression
|
|-
| '''set''' Variable=Expression
| For example, set x=5
|-
| backtrace ('''bt''')
| #0 is where the code broke.
|-
| frame N
| switch to frame N (see backtrace output)
|-
| '''q'''uit
|
|}
==== ddd ====
* [http://knuth.luther.edu/~leekent/tutorials/ddd.html A Brief Introduction to DDD] by knuth.luther.edu.
* To use command line argument, go to Program -> Run where you can specify your command line arguments.
* Check Source -> Display Line Numbers.
* It seems there is not keyboard shorts for ddd. And trying to change the fonts will give you errors.
* A good feature in ddd is once a program aborted, ddd can show the '''backtrace (Status -> Backtrace...)'''. So it is easy to find out which line of code broke the program and how the line was called. I did not see this feature in Nemiver.
For example,
<pre>
g++ -Wall -g -o XXX.o -c XXX.cpp
g++ -o XXX.exe XXX.o -lstdc++
ddd XXX.exe
</pre>
==== nemiver ====
Nemiver is an on going effort to write an easy to use standalone C/C++ debugger that integrates well in the GNOME environment.
<pre>
sudo apt-get install nemiver
nemiver
</pre>
* Qt Creator. See the above discussion link for an instruction.
=== Qt Creator ===
A screenshot based on Qt Creator 3.3 and Qt 5.4.
[[File:QtCreatorDebug.png|200px]]
=== Write a Linux Debugger ===
https://blog.tartanllama.xyz/c++/2017/06/19/writing-a-linux-debugger-source-break/
== Tools ==
=== Online compilers ===
* https://wandbox.org for GCC
* http://webcompiler.cloudapp.net/ for VC++
=== IDE editor ===
==== [http://www.geany.org/ Geany] ====
Pros: show a list of symbols/functions on the left hand side panel. Show the open files as a tree structure on the left hand side. Code folding. Con: [http://plugins.geany.org/debugger.html the GDB debugger?] To install the latest version of geany and geany-plugin-debugger:
<pre>
sudo add-apt-repository ppa:geany-dev/ppa
sudo apt-get update
sudo apt-get install geany
sudo apt-get install geany-plugin-debugger
</pre>
==== [http://www.codeblocks.org/ Code::Blocks] ====
* http://wiki.codeblocks.org/index.php/Main_Page
* http://wiki.codeblocks.org/index.php/Debugging_with_Code::Blocks
<syntaxhighlight lang='bash'>
sudo apt-get install codeblocks
</syntaxhighlight>
Pros: built-in GDB debugger.
This is the one people recommend.
If you have already source code and Makefile. Then
# Put source code together with Makefile under the same directory as ProjectName.cbp file.
# Project->Properties check 'This is a custom Makefile' and modify the makefile name as needed.
# Don't worry about the Execution directory shown there.
# Go to the 'Build targets' tab, change the Output filename to the one we want to run or debug.
# Go to Project->Build Options. Go to "Make" commands tab and change the 'Build project/target:' to $make -f $makefile (i.e. remove $target). This needs to be done for 'Debug' build.
Now we can click the build, run or debug button on the toolbar.
As we can see, the required step to debug an executable in codeblocks is much more complicated than '''ddd''' program.
The only thing I want to change is the font of the execution terminal. The font is too small.
==== Qt Creator ====
I haven't found a way to use own Makefile to run debugging.
=== GCC ===
[http://www.network-theory.co.uk/docs/gccintro/ Introduction to GCC]
==== Creating shared libraries ====
http://www.cprogramming.com/tutorial/shared-libraries-linux-gcc.html
==== Show all libraries used (dynamically linked) by an executable program ====
Use '''ldd''' ('''L'''ist '''D'''ynamic '''D'''ependencies) command on Linux environment. On Windows OS, we can use [http://dependencywalker.com/ Dependency Walker]; see [http://stackoverflow.com/questions/1993673/what-is-the-equivalent-of-linuxs-ldd-on-windows this post].
* http://www.cyberciti.biz/tips/linux-shared-library-management.html
* http://www.ibm.com/developerworks/library/l-lpic1-v3-102-3/
* http://qt-project.org/wiki/Show_library_dependencies
* http://www.linuxintro.org/wiki/Ldd
See also the '''[[Ubuntu#Check_if_a_library_is_installed_or_not_.28ldconfig.29|ldconfig]]''' command to check if a library is installed or not.
<syntaxhighlight lang='bash'>
debian@beaglebone:~$ ldd /usr/bin/netsurf
libjpeg.so.8 => /usr/lib/arm-linux-gnueabihf/libjpeg.so.8 (0xb6f1c000)
libz.so.1 => /lib/arm-linux-gnueabihf/libz.so.1 (0xb6f02000)
libxml2.so.2 => /usr/lib/arm-linux-gnueabihf/libxml2.so.2 (0xb6e22000)
        ....
libcurl.so.4 => /usr/lib/arm-linux-gnueabihf/libcurl.so.4 (0xb6dd7000)
libtasn1.so.3 => /usr/lib/arm-linux-gnueabihf/libtasn1.so.3 (0xb5d5b000)
libp11-kit.so.0 => /usr/lib/arm-linux-gnueabihf/libp11-kit.so.0 (0xb5d47000)
</syntaxhighlight>
==== Linking with external libraries ====
<pre>
$ gcc -Wall calc.c /usr/lib/libm.a -o calc
$ gcc -Wall calc.c -lm -o calc
</pre>
To specify the library path, we can use "-L", and/or "-Wl,-rpath" in gcc/g++. 
When shared libraries are present in nondefault directories, the option "-Wl,-rpath" is needed in linker options. See [http://gcc.gnu.org/ml/gcc-help/2005-12/msg00017.html here]. If we don't specify "-Wl,-rpath" in linker options, we need to define  "LD_LIBRARY_PATH" environment variable. See an example in [[R#RInside:_Embed_R_in_C.2B.2B|RInside]].
==== Header files/Include path ====
The list of directories for '''header''' files is often referred to as the '''include''' path, and the list of directories for '''libraries''' as the '''library''' search path or '''link''' path.
When additional libraries are installed in other directories it is necessary to extend the search paths, in order for the libraries to be found. The compiler options '''-I''' and '''-L''' add new directories to the beginning of the include path and library search path respectively.
<pre>
$ gcc -Wall -I/opt/gdbm-1.8.3/include -L/opt/gdbm-1.8.3/lib dbmain.c -lgdbm
</pre>
----
==== Environment variables ====
We can use some environment variables to replace '''-I''' and '''-L''' flags. See some basic introduction on
* http://www.ntu.edu.sg/home/ehchua/programming/cpp/gcc_make.html
* http://www.network-theory.co.uk/docs/gccintro/gccintro_23.html
Header files (CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH)
<pre>
$ CPLUS_INCLUDE_PATH=/opt/gdbm-1.8.3/include
$ export CPLUS_INCLUDE_PATH
</pre>
Library files during link time
<pre>
$ LIBRARY_PATH=/opt/gdbm-1.8.3/lib
$ export LIBRARY_PATH
</pre>
Library files during run time (needed only for ''dynamic'' libraries). On Windows platform, the '''PATH''' variable will be used.
<pre>
$ export LD_LIBRARY_PATH=/opt/gdbm-1.8.3/lib
</pre>
Check environment variables
<pre>
echo $SHELL
echo $PATH
echo $LIBRARY_PATH
env
</pre>
=== Makefile ===
This is an example of <Makefile>. The source code files are Parent.cpp, Parent.h, Child.cpp, Child.h, and main.cpp. Remember the indents should be a single tab.
<pre>
CC=g++
TARGET=pc
OBJECTS=main.o Parent.o Child.o
$(TARGET): $(OBJECTS)
@echo "** Linking Executable"
$(CC) $(OBJECTS) -o $(TARGET)
clean:
@rm -f *.o *~
veryclean: clean
@rm -f $(TARGET)
%.o: %.cpp
@echo "** Compiling C++ Source"
$(CC) -c $(INCFLAGS) $<
</pre>
==== 'all' and '.PHONY' targets ====
* (all) http://stackoverflow.com/questions/2514903/makefile-what-does-all-stand-for
* (.PHONY) http://stackoverflow.com/questions/2145590/what-is-the-purpose-of-phony-in-a-makefile. It means the target is NOT a file name.
==== minimal make ====
http://kbroman.org/minimal_make/
==== [http://www.cmake.org/ cmake] ====
On Windows, it will be installed on ''C:\Program Files (x86)\Cmake 2.8'' folder. By default, it is not added to system PATH. The 'Cmake' program will ask for source, binary folders and the Compiler option. After clicking 'configure' and 'generate' buttons, it will create VS solution file and we can double click the solution file to open the project in Visual Studio. In Visual Studio, we can just build the solution (Ctrl + Shift + B). When we want to debug the code, we should 1. right click on project and select property. Change the working directory to the source code (note that .exe file will be generated there). 2. Set the project as the starting project.
The idea of cmake is it can create <Makefile> file from <CMakeLists.txt> file. See 
* https://www.cs.swarthmore.edu/~adanner/tips/cmake.php.
* http://derekmolloy.ie/hello-world-introductions-to-cmake/
For example, if we use git clone to get files from [https://github.com/BartVandewoestyne/Effective-Modern-Cpp Effective Modern C++].
<syntaxhighlight lang='bash'>
$ git clone https://github.com/BartVandewoestyne/Effective-Modern-Cpp.git
$ cd Effective-Modern-Cpp/Item01_Understand_template_type_deduction
$ ls
CMakeLists.txt
[and other cpp files]
$ sudo apt-get install cmake
$ mkdir build
$ cd build
$ cmake ..
$ ls MakeCache.txt CMakeFiles  cmake_install.cmake  Makefile
$ make
</syntaxhighlight>
=== Autotools ===
* [https://www.ghacks.net/2017/05/24/how-to-build-from-source-code-in-gnulinux/ How to build from source code in GNU/Linux]
* http://stackoverflow.com/questions/2531827/what-are-makefile-am-and-makefile-in
'''Makefile.am''' is a programmer-defined file and is used by '''automake''' to generate the '''Makefile.in''' file. The '''./configure''' script typically seen in source tarballs will use the Makefile.in to generate a Makefile.
The '''./configure''' script itself is generated from a programmer-defined file named either '''configure.ac''' or '''configure.in''' , I prefer .ac (for AutoConf) since it differentiates it from the generated Makefile.in files and that way I can have rules such as make dist-clean which rm -f *.in . Since it is a generated file it is not typically stored in a revision system such as SVN or CVS, rather the .ac file would be.
Read more on [http://en.wikipedia.org/wiki/GNU_build_system GNU build system/Autotools]. Read about make and Makefile first, then learn about automake, autoconf, libtool, etc.
=== Profiling ===
* [https://www.howtoforge.com/tutorial/how-to-install-and-use-profiling-tool-gprof/ How to install and use profiling tool Gprof]
=== [http://llvm.org/ LLVM] Compiler Infrastructure ===
[http://clang.llvm.org/ clang]: a C language family frontend for LLVM
== C++ Libraries ==
* See [http://www.richelbilderbeek.nl/Cpp.htm Richel Bilderbeek] page for more listing.
=== [https://computing.llnl.gov/tutorials/pthreads/ pthreads - POSIX Threads Programming] ===
* [http://compression.ca/pbzip2/ pbzip2] - a parallel implementation of the bzip2 block-sorting file compressor that uses pthreads and achieves near-linear speedup on SMP machines.
* [http://bowtie-bio.sourceforge.net/manual.shtml#building-from-source bowtie software] which makes use of pthread library. Similar [https://ccb.jhu.edu/software/tophat/manual.shtml tophat] and [http://cole-trapnell-lab.github.io/cufflinks/cuffmerge/index.html cuffmerge] also have a '-p' argument to support pthread.
=== [http://www.webtoolkit.eu/wt Wt] ===
C++ library for developing web applications
=== [http://trac.seqan.de/ SeqAn] ===
C++ for sequencing data. On Windows OS, it requires Python 2 and CMake in addition to VS.
=== [http://www.boost.org/ Boost] ===
Boost is a set of C++ libraries for numerical computation that provide support for tasks and structures such as linear algebra, pseudorandom number generation, multithreading, image processing, regular expressions, and unit testing. Release 1.52 contains over eighty individual libraries.
The R library [http://cran.r-project.org/web/packages/BH/index.html BH] provides template use in R. See http://gallery.rcpp.org/articles/using-boost-with-bh/ for a gallery.
Boost contains a lot of libraries.
<pre>
brb@brb-VirtualBox:~/Downloads/boost_1_56_0$ ./bootstrap.sh --show-libraries
Building Boost.Build engine with toolset gcc... tools/build/src/engine/bin.linuxx86/b2
The following Boost libraries have portions that require a separate build
and installation step. Any library not listed here can be used by including
the headers only.
The Boost libraries requiring separate building and installation are:
    - atomic
    - chrono
    - container
    - context
    - coroutine
    - date_time
    - exception
    - filesystem
    - graph
    - graph_parallel
    - iostreams
    - locale
    - log
    - math
    - mpi
    - program_options
    - python
    - random
    - regex
    - serialization
    - signals
    - system
    - test
    - thread
    - timer
    - wave
</pre>
==== build under Ubuntu OS ====
To install via apt-get is 'sudo apt-get install libboost-all-dev'. The software center shows the version is 1.48 (kind of old) on Ubuntu 12.04 and 1.54 on Ubuntu 14.04.
We can install it by downloading its source code and build it by ourselves. See [http://ubuntuforums.org/showthread.php?t=1180792 this post] on ubuntuforums.org.
<pre>
./bootstrap.sh
./b2 install
</pre>
At the end of buliding, it will show the paths to header files (sometimes it goes to /usr/local/include) and the libraries themselves (sometimes it is /usr/local/lib). If we use apt-get to install boost, the header files go to /usr/include and the libraries files libboost*.a and libboost*.so go to /usr/lib.
<pre>
The Boost C++ Libraries were successfully built!
The following directory should be added to compiler include paths:
    /home/brb/Downloads/boost_1_55_0
The following directory should be added to linker library paths:
    /home/brb/Downloads/boost_1_55_0/stage/lib
</pre>
The source directory contains <index.html> file. It tells us how to run a simple test program. For example, the following example test headers-only libraries and require no separately-compiled library binaries or special treatment when linking.
<pre>
g++ -I /home/brb/Downloads/boost_1_55_0 example.cpp -o example
echo 1 2 3 | ./example
</pre>
To link to boost binary libraries, we can do
<pre>
$ g++ -I /home/brb/Downloads/boost_1_55_0 example.cpp -o example2 \
  -L~/Downloads/boost_1_55_0/stage/lib/ -lboost_regex
$ ./example2 < jayne.txt
Will Success Spoil Rock Hunter?
</pre>
Note that under boost_1_55_0/stage/lib directory, both static and dynamic libraries are available for boost_regex. My system picks the static library. If we want to link to dynamic library, we can
<pre>
$ g++ -I /home/brb/Downloads/boost_1_55_0/ example2.cpp -o example2 \
    ~/Downloads/boost_1_55_0/stage/lib/libboost_regex.so
$ ./example2 < jayne.txt
./example2: error while loading shared libraries: libboost_regex.so.1.55.0: cannot open shared object file:
No such file or directory
$ export LD_LIBRARY_PATH=/home/brb/Downloads/boost_1_55_0/stage/lib
</pre>
Here we see the purpose of specifying the environment variable '''LD_LIBRARY_PATH'''.
To try an example (Math and numerics > math/statistical distributions > Calculating confidence intervals on the mean with the Students-t distribution), we can compile and generate the executable file by (no need to link to library in this case)
<pre>
g++ -I /home/brb/Downloads/boost_1_55_0/ \
    ~/Downloads/boost_1_55_0/libs/math/example/students_t_single_sample.cpp
</pre>
If we like the headers and libraries automatically available in linux environment without using -I and -L parameters in g++, we can run
<pre>
./bootstrap.sh --prefix=/usr/local
sudo ./b2 install
</pre>
Running ./b2 took about 1 hours on my single core VM. At the end, it will create /usr/local/include/boost subdirectory and a bunch of libboost*.a and libboost*.so will be created under /usr/local/lib directory.
==== Check BOOST version ====
* On Ubuntu, use ' tail /usr/include/boost/version.hpp'
* IN boost C++ code, use boost version macro 'BOOST_LIB_VERSION'
<pre>
#include <boost/version.hpp>
#include <iostream>
using namespace std;
int main()
{
    cout << "Boost version: " << BOOST_LIB_VERSION << endl;
    return 0;
}
</pre>
==== Build boost from source on Windows OS ====
* http://andres.jaimes.net/718/how-to-install-the-c-boost-libraries-on-windows/
* http://www.rodneybeede.com/Boost_C___libraries_on_Windows_with_MinGW.html
* http://www.boost.org/doc/libs/1_55_0/more/getting_started/windows.html
MinGW
<pre>
boostrap --with-toolset=gcc
.\b2 --build-type=complete toolset=gcc link=shared runtime-link=shared install
</pre>
==== use of statistical distributions in Boost ====
See an article in [https://www.quantnet.com/threads/c-statistical-distributions-in-boost.10016/ www.quantnet.com]
For each of [http://www.boost.org/doc/libs/1_55_0/libs/math/doc/html/dist.html distribution functions], we can compute cdf, quantile, pdf, mean, mode, median, variance, etc.
* [http://www.boost.org/doc/libs/1_55_0/libs/math/doc/html/math_toolkit/stat_tut/weg/normal_example/normal_misc.html Normal distribution]
* [http://www.boost.org/doc/libs/1_55_0/libs/math/doc/html/math_toolkit/stat_tut/weg/st_eg/tut_mean_intervals.html T distribution]
To compute t(alpha/2, df), we can
<pre>
#include <boost/math/distributions/students_t.hpp>
#include <iostream>
using namespace std;
using boost::math::students_t;
int main()
{
  int df = 195;
  double alpha = .1;
  students_t dist(df);
  double T = quantile(complement(dist, alpha / 2));
  cout << T << endl;
  return 0;
}
</pre>
We can compile the code and get the result (check by using R statement '''qt(.95, 195)''').
<pre>
$ g++ example.cpp; ./a.out
1.65271
</pre>
PS For some reason, I don't need to worry about environment variable.
Googling shows we can use '''g++ --print-search-dirs''' to show LIBRARY search paths and adding '''-v''' to show include search paths to g++ command.
==== Boost thread example ====
http://ashishgrover.com/boost-multi-threadingfor-c/
<pre>
#include <boost/thread.hpp>
void readerApi()
{
  for (int i=0; i < 10; i++) {
    usleep(400);
    std::cout << "readerApi: " << i
              << std::endl;
  }
}
void writerApi()
{
  for (int i=0; i < 10; i++) {
    std::cout << "writerApi: " << i
              << std::endl;
    usleep(400);
  }
}
int main()
{
  boost::thread readerThread(readerApi);
  boost::thread writerThread(writerApi);
  readerThread.join();
  writerThread.join();
}
</pre>
Then compile the code by
<pre>
$ g++ -o b1 boost1.cpp -lboost_thread
</pre>
Before you can run it, specify LD_LIBRARY_PATH env variable if you [[#build_under_Ubuntu_OS|build boost library by yourself]].
<pre>
$ export LD_LIBRARY_PATH=/usr/local/lib:$LD_LIBRARY_PATH
$ ./b1
writerApi: 0
readerApi: 0
writerApi: 1
readerApi: 1
writerApi: 2
readerApi: 2
writerApi: 3
writerApi: 4
readerApi: 3
writerApi: 5
readerApi: 4
writerApi: 6
readerApi: 5
writerApi: 7
readerApi: 6
writerApi: 8
readerApi: 7
writerApi: 9
readerApi: 8
readerApi: 9
</pre>
To make changes permanent in Ubuntu you have to add a new configuration file for ldconfig:
<pre>
sudo nano /etc/ld.so.conf.d/libboost.conf
</pre>
(my editor of choice is nano, and the name of the file itself doesn’t matter)
Add the library path to that file, i.e. /usr/local/lib/.
Save the file, quit and reload your configuration by calling
<pre>
sudo ldconfig
</pre>
Note that your LD_LIBRARY_PATH won’t change, but your program will now run!
Another working example is from http://www.codeproject.com/Articles/279053/How-to-get-started-using-Boost-threads
<pre>
#include <iostream> 
#include <boost/thread.hpp> 
#include <boost/date_time.hpp>     
     
void workerFunc() 
{ 
    boost::posix_time::seconds workTime(3);         
    std::cout << "Worker: running" << std::endl;   
     
    // Pretend to do something useful...
    boost::this_thread::sleep(workTime);         
    std::cout << "Worker: finished" << std::endl; 
}   
     
int main(int argc, char* argv[]) 
{ 
    std::cout << "main: startup" << std::endl;         
    boost::thread workerThread(workerFunc);   
     
    std::cout << "main: waiting for thread" << std::endl;         
    workerThread.join();   
     
    std::cout << "main: done" << std::endl;         
    return 0; 
}
</pre>
It can be built by g++ test.cpp -lboost_thread -lboost_system.
Another excellent example is from Jeff Benshetler. Check out this page http://advancedcplusplus.com/5min-threads/ (I cannot build it successfully).
==== Books ====
* The Boost C++ Libraries by Boris Schäling [http://en.highscore.de/cpp/boost/ Online ebook] from an old version of the book and [http://www.amazon.com/The-Boost-Libraries-Boris-Sch%C3%A4ling/dp/0982219199/ref=pd_sim_b_4 Amazon]
* [https://www.packtpub.com/application-development/boost-c-application-development-cookbook Boost C++ Application Development Cookbook]
==== Boost and Rcpp package ====
* http://gallery.rcpp.org/articles/a-first-boost-example/
* [http://cran.r-project.org/web/packages/BH/index.html BH] package
==== [http://www.boost.org/doc/libs/1_55_0/libs/python/doc/index.html Boost.python] ====
http://edyfox.codecarver.org/html/boost_python.html
Rcpp was inspired by Boost.python. See the [http://cran.r-project.org/web/packages/Rcpp/vignettes/Rcpp-modules.pdf Rcpp module] vignette.
=== [http://scythe.wustl.edu/ The Scythe Statistical Library] ===
[http://www.jstatsoft.org/v42/i12 Scythe: An Open Source C++ Library for Statistical Computation] from J of Stat Software.
=== [http://arma.sourceforge.net/ Armadillo] ===
Armadillo is a C++ linear algebra library (matrix maths) aiming towards a good balance between speed and ease of use.
=== GNU Scientific Library  ===
=== BLAS, LAPACK ===
=== IMSL ===
=== Numerical Recipes ===
== Some C++ Projects ==
[https://github.com/trending/c++ Trending in Github]
=== [http://www.cs.umd.edu/~mount/ANN/ Approximate nearest neighbor search] ===
The R wrap is [http://cran.r-project.org/web/packages/RANN/index.html here]
=== BEDTools ===
https://github.com/arq5x/bedtools2
=== freebayes ===
https://github.com/ekg/freebayes/
=== [https://github.com/NGS-lib/NGSplusplus NGS++] ===
A programming library in C++11 specialized in manipulating both next-generation sequencing (NGS) datasets and genomic information files. See the [http://bioinformatics.oxfordjournals.org/content/29/15/1893.short?rss=1 paper].
=== [http://www.csie.ntu.edu.tw/~cjlin/libsvm/ LIBSVM] ===
=== SAMTools (in C only) ===
* http://www.htslib.org/
* https://github.com/samtools/ (new)
* http://samtools.sourceforge.net/ (old)
=== [http://tophat.cbcb.umd.edu/tutorial.shtml Tophat] ===
It also requires the packages
* Boost
* [http://samtools.sourceforge.net/ SamTools] and its [https://github.com/samtools/samtools source code]
=== [https://github.com/COMBINE-lab/Salmon Salmon - fast and bias-aware quantification of transcript expression] ===
http://www.rna-seqblog.com/salmon-fast-and-bias-aware-quantification-of-transcript-expression/
=== [http://www.cs.cmu.edu/~ckingsf/software/parana2/ Parana2] ===
It also depends on a few other tools.
* Bio++ - a set of C++ libraries for Bioinformatics, including sequence analysis, phylogenetics, molecular evolution and population genetics.
* Boost
* GMP - The GNU Multiple Precision Arithmetic Library
* MPFR - C library for multiple-precision floating-point computations with correct rounding.
* pugixml -  light-weight C++ XML processing library.
=== [http://csse.szu.edu.cn/staff/zhuzx/CompMap/ Comprehensive short read mapping] ===
=== [http://viq854.github.io/bwbble Short read alignment with populations of genomes] ===
https://github.com/viq854/bwbble
=== [http://www.ikmb.uni-kiel.de/janus/janus.html Janus-comprehensive tool investigating the two faces of transcription] ===
It depends on bamtools.
=== Rcount-simple and flexible RNA-Seq read counting ===
* [http://bioinformatics.oxfordjournals.org/content/31/3/436.short?rss=1 Paper]
* https://github.com/MWSchmid/Rcount (based on C++ and Qt)
=== RSEM/RNA-Seq by Expectation-Maximization ===
* http://deweylab.biostat.wisc.edu/rsem/
* https://github.com/bli25wisc/RSEM
=== [http://sysbio.uni-ulm.de/soft/RNA-Pareto/ RNA-Pareto] ===
Interactive Analysis of Pareto-optimal RNA Sequence-Structure Alignments
The software is written in Java 6 (graphical user interface) and C++ (dynamic programming algorithms). To run, a Java Runtime Environment, version ≥1.6.0 is required. It is well tested with GCC 4.6.
=== [https://github.com/ANGSD/angsd ANGSD] ===
Analysis of Next Generation Sequencing Data
=== Open MS ===
http://open-ms.sourceforge.net/. It used external libraries such as: (i) Qt, which provides visualization and database support; (ii) Xerces for XML file parsing; (iii) libSVM, for machine learning algorithms; and (iv) the GNU Scientific Library (GSL), used for mathematical and statistical analysis. One of the strong points of OpenMS is a complete set of examples to extend and use the libraries, the TOPP (The OpenMS Proteomics Pipeline) and TOPPView tutorials describe in detail the OpenMS.
=== MACAU - Differential Expression Analysis for RNAseq using Poisson Mixed Models ===
http://www.xzlab.org/software.html. GSL and Lapack are used.
== GUI Programming ==
=== Windows Programming ===
==== Resource ====
* http://www.stanford.edu/class/cs193w/
* [http://www.charlespetzold.com/books.html Petzold Programming Windows 1998] with source code for download
* Kruglinski Programming Microsoft Visual C++, 1998
* [http://xoax.net/cpp/index.php Xoax] which contains tutorials for C++, OpenGL, Win32 C++ and more.
*[http://www.youtube.com/playlist?list=PLS1QulWo1RIZz6uDid--I09EOImRmPHS0 ProgrammingKnowledge] about Visual C++ Windows Forms Application Tutorial in Youtube.
* [http://www.youtube.com/watch?v=-SXdzxWngPg Simple GUI calculator using VS C++/CLR]
==== Difference between Win32 project and CLR (common language runtime) project ====
See [http://stackoverflow.com/questions/3739392/what-is-the-difference-between-a-win32-project-clr-empty-project-and-empt here].
A Win32 project is used if you want to end up with a DLL or a Win32 application usually using the bare WinAPI. A CLR project is used to create C++/CLI project, i.e. to use C++/CLI to target the .NET platform.
The main difference between projects is what Visual Studio comes up with in terms of pre-created files. A windowed Win32 application for example (what you get when you choose Win32 project, but not a DLL) is created with a file for resources (menus, acceleators, icons etc.) and some default code to create and register a window class and to instantiate this window.
Quoted from http://en.wikipedia.org/wiki/Common_Language_Runtime
''The Common Language Runtime (CLR) is the virtual machine component of Microsoft's .NET framework and is responsible for managing the execution of .NET programs. In a process known as Just-in-time compilation, the compiled code is converted into machine instructions that, in turn, are executed by the computer's CPU. The CLR provides additional services including memory management, type safety and exception handling. All programs written for the .NET framework, regardless of programming language, are executed by the CLR. It provides exception handling, garbage collection and thread management. CLR is common to all versions of the .NET framework.''
''The CLR is Microsoft's implementation of the Common Language Infrastructure (CLI) standard.''
==== Difference between Win32, MFC and .NET ====
http://stackoverflow.com/questions/821676/how-do-i-decide-whether-to-use-atl-mfc-win32-or-clr-for-a-new-c-project
Using the CLR will provide you with the most expressive set of libraries (the entire .NET framework), at the cost of restricting your executable to requiring the .NET framework to be installed at runtime, as well as limiting you to the Windows platform (however, all 4 listed technologies are windows only, so the platform limitation is probably the least troublesome).
However, CLR requires you to use the C++/CLI extensions to the C++ language, so you'll, in essense, need to learn some extra language features in order to use this. Doing so gives you many "extras," such as access to the .net libraries, full garbage collection, etc.
Using Win32 directly provides the smallest executables, with the fewest dependencies, but is more work to write. You have the least amount of helper libraries, so you're writing more of the code.
Win32 is the raw, bare-metal way of doing it. It's tedious, difficult to use, and has alot of small details you need to remember otherwise things will fail in relatively mysterious ways.
MFC builds upon Win32 to provide you an object oriented way of building your application. It's not a replacement for Win32, but rather an enhancement - it does alot of the hard work for you.
System.Windows.Forms (which is what I assume you meant by CLR) is completely different, but has large similarities to MFC from its basic structure. It's by far the easiest to use, but requires the .NET framework, which may or may not be a hindrance in your case.
==== Why not MFC ====
http://win32-framework.sourceforge.net/explanation.htm The website also provides an alternative software called Win32++ to replace MFC. It also provides useful links for C++ compilers, tools, tutorial and references.
=== [[Qt]] ===
* [http://www.informit.com/store/c-plus-plus-gui-programming-with-qt4-9780132354165 C++ GUI Programming with Qt 4]
* Qt Graphics http://qt-project.org/doc/qt-5.0/qtdoc/topics-graphics.html
* [http://qt-project.org/doc/qt-5.0/qtopengl/2dpainting.html 2D Painting Example] QPainter and QGLWidget can be used together to display accelerated 2D graphics on supported hardware. OpenGL examples are [http://qt-project.org/doc/qt-5.0/qtopengl/examples-widgets-opengl.html here].
=== [http://wxwidgets.org/ wxwidgets] ===
Some projects:
* [http://sourceforge.net/projects/freefilesync/ FreeFileSync]
=== Simple OpenGL GUI ===
As mentioned in http://www.oppi.uef.fi/bioinformatics/forg3d/downloads.php
== OpenGL Programming on Windows ==
We need to include
<pre>
#include <gl/gl.h>
#include <gl/glu.h>
</pre>
And go to project's link properties and enter <opengl32.lib> & <glu32.lib>. Check the directory ''C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Include'' and ''C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\lib''
header files are: gl\gl.h and glu.h
libraries are: openGL32.lib and GLU32.lib
x64 libs can be found in  ''C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\lib\x64''
you can put freeglut lib and header files to those locations to use freeglut with visual studio 2010
when you copy freeglut DLLs to C:\Windows\System32 don’t copy 64 bit DLL to syswow64 this gives a freaky error 0xc000007b when running code. Don’t know what it mean, but if you have freeglut only in system32 you going to be fine.
=== Resource ===
* [Book] OpenGL Programming Guide: The Official Guide to Learning OpenGL. [http://www.glprogramming.com/red/ Online book ver 1.1]
* http://nehe.gamedev.net/
* http://homepage.cs.uiowa.edu/~cwyman/classes/common/howto/winGLUT.html. The instruction there assumes the Windows XP. On my Windows 7 and VS2010 machine, I see <glu32.lib> in ''C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Lib''.
* http://www.cprogramming.com/tutorial/opengl_first_opengl_program.html
* http://www.devshoe.net/2012/08/opengl-programming-in-windows-7-64-bit.html
* http://www3.ntu.edu.sg/home/ehchua/programming/opengl/HowTo_OpenGL_C.html (Notice the comment at the end)
* http://www.opengl-tutorial.org/beginners-tutorials/tutorial-1-opening-a-window/
* [http://msdn.microsoft.com/en-us/library/windows/desktop/dd374293(v=vs.85).aspx OpenGL on Windows]
* http://www.nullterminator.net/opengl32.html (hmv, without using glut). See also a discussion on [http://stackoverflow.com/questions/879035/initializing-opengl-without-glut here] why it is platform specific to initialize OpenGL.
* Check OpenGL version: [http://www.realtech-vr.com/glview/download.php glview]
* [http://lazyfoo.net/tutorials/OpenGL/01_hello_opengl/windows/msvsnet2010u/index.php Setting up freeGLUT on Visual Studio 2010]
* Google: windows opengl programming tutorial visual studio
=== Example 1 ===
http://openglbook.com/setting-up-opengl-glew-and-freeglut-in-visual-c/
# Download freeglut (freeglut-MSVC-2.8.0-1.mp.zip) & glew (glew-1.9.0-win32.zip)
# Copy files include and lib to appropriate location
# Copy freeglut.dll to the Project's Release or Debug folder
I don't need Step 5 (Compiler) and Step 6 (Linker).
I keep a copy of the instruction in Evernote.
=== Example 2 ===
Teapot and Glut shapes (WireTeapot, SolidTeapot, SolidCube & SolidSphere) from http://openglsamples.sourceforge.net/
=== Example 3 (no Glut, Windows OS only) ===
http://www.nullterminator.net/opengl32.html
=== Examples from [http://www.opengl.org opengl.org] ===
http://www.opengl.org/sdk/docs/tutorials/
=== Example of American Flag ===
http://www.youtube.com/watch?v=9xjBlde4Cew
== Computer Systems ==
=== MIPS Instruction Set ===
* http://www.cs.columbia.edu/~sedwards/classes/2012/3827-spring/mips-isa.pdf
* http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-823-computer-system-architecture-fall-2005/lecture-notes/l05_singlecycle.pdf

Revision as of 20:12, 23 June 2019

Socket Programming

Terms

There are two types of address domains.

  • AF_UNIX: the unix domain for two processes which share a common file system, and
  • AF_INET: the Internet domain for any two hosts on the Internet.

There are two types of sockets.

  • a stream socket in which characters are read in a continuous stream as if from a file or pipe, and
  • a datagram socket, in which messages are read in chunks.

The two symbolic constants are SOCK_STREAM and SOCK_DGRAM.

There are two types of ports:

Retrieved from "https://wiki.taichimd.us/index.php?title=C&oldid=20965"