C++ Library - <functional>

Introduction

Function objects are objects specifically designed to be used with a syntax similar to that of functions. Instances of std::function can store, copy, and invoke any Callable target -- functions, lambda expressions, bind expressions, or other function objects, as well as pointers to member functions and pointers to data members.

Declaration

Following is the declaration for std::function.

template<class >
class function; 

C++11

template< class R, class... Args >
class function<R(Args...)>

Parameters

• R − result_type.

• argument_type − T if sizeof...(Args)==1 and T is the first and only type in Args.

Example

In below example for std::function.

#include <functional>
#include <iostream>

struct Foo {
   Foo(int num) : num_(num) {}
   void print_add(int i) const { std::cout << num_+i << '\n'; }
   int num_;
};
 
void print_num(int i) {
   std::cout << i << '\n';
}

struct PrintNum {
   void operator()(int i) const {
      std::cout << i << '\n';
   }
};

int main() {
   std::function<void(int)> f_display = print_num;
   f_display(-9);

   std::function<void()> f_display_42 = []() { print_num(42); };
   f_display_42();

   std::function<void()> f_display_31337 = std::bind(print_num, 31337);
   f_display_31337();

   std::function<void(const Foo&, int)> f_add_display = &Foo::print_add;
   const Foo foo(314159);
   f_add_display(foo, 1);

   std::function<int(Foo const&)> f_num = &Foo::num_;
   std::cout << "num_: " << f_num(foo) << '\n';

   using std::placeholders::_1;
   std::function<void(int)> f_add_display2= std::bind( &Foo::print_add, foo, _1 );
   f_add_display2(2);
 
   std::function<void(int)> f_add_display3= std::bind( &Foo::print_add, &foo, _1 );
   f_add_display3(3);

   std::function<void(int)> f_display_obj = PrintNum();
   f_display_obj(18);
}

The sample output should be like this −

-9
42
31337
314160
num_: 314159
314161
314162
18

Member functions

Non-member functions

Operator classes