function (general)

A function is a callable piece of code that performs a specific general task with as little information as possible (Why would you want this? Go to this page to view the purpose of using functions).

A function declaration states what a function needs and returns. A function definition states how a function uses its arguments and calculates what to return.


#include <iostream>



//SayHello is a function that takes no arguments and returns nothing

void SayHello()

{

  std::cout << "Hello world" << std::endl;

}



//main is a special function: this form of main takes no arguments

//and returns the program's error code

int main()

{

  SayHello();

}

A function that accompanies a class (and is non-friend) is called a free function.

Consider using proper function design. Avoid writing long functions [1-2].

boost::function: the Boost.Function library

boost::function is part of the Boost.Function library for working with functions in general, including member functions.

boost::function example 1

Below are two pieces of near-identical code, the first using function pointers to global functions, the second using boost::function.


#include <algorithm>

#include <iostream>

#include <vector>

#include <boost/foreach.hpp>



int f1(const int i) { return i + 1; }

int f2(const int i) { return i + 2; }

int f3(const int i) { return i + 3; }



int main()

{

  typedef int(*Function)(const int);

  std::vector<Function> v;

  v.push_back(f1);

  v.push_back(f2);

  v.push_back(f3);

  std::random_shuffle(v.begin(),v.end());



  int x = 0;

  BOOST_FOREACH(const Function& f,v)

  {

    x = f(x);

    std::cout << x << '\n';

  }

}


#include <algorithm>

#include <iostream>

#include <vector>

#include <boost/foreach.hpp>

#include <boost/function.hpp>



int f1(const int i) { return i + 1; }

int f2(const int i) { return i + 2; }

int f3(const int i) { return i + 3; }



int main()

{

  std::vector<boost::function<int(const int)> > v;

  v.push_back(f1);

  v.push_back(f2);

  v.push_back(f3);

  std::random_shuffle(v.begin(),v.end());



  int x = 0;

  BOOST_FOREACH(const boost::function<int(const int)>& f,v)

  {

    x = f(x);

    std::cout << x << '\n';

  }

}

Possible screen output:

boost::function example 2


#include <iostream>

#include <boost/function.hpp>



struct Speaker

{

  Speaker(const std::string hello_message, const std::string bye_message)

    : m_hello_message(hello_message),

      m_bye_message(bye_message)

  {



  }



  void SayHello() const { std::cout << m_hello_message << '\n'; }

  void SayBye() const { std::cout << m_bye_message << '\n'; }



  const std::string m_hello_message;

  const std::string m_bye_message;

};



int main()

{

  const Speaker s1("Hello!","Bye!");

  const Speaker s2("HELLO!","BYE!");



  const boost::function<void (const Speaker*)> say_hello_function = &Speaker::SayHello;

  const boost::function<void (const Speaker*)> say_bye_function = &Speaker::SayBye;



  say_hello_function(&s1);

  say_bye_function(&s1);



  say_hello_function(&s2);

  say_bye_function(&s2);

}

Screen output:


Hello!

Bye!

HELLO!

BYE!

External links

WikiPedia's page about functions

References

Herb Sutter, Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Item 20: 'Avoid long functions. Avoid deep nesting'
Joint Strike Fighter Air Vehicle C++ Coding Standards for the System Development and Demonstration Program. Document Number 2RDU00001 Rev C. December 2005. AV Rule 1: 'Any one function (or method) will contain no more than 200 logical source lines of code.'

Go back to Richel Bilderbeek's C++ page.