C++ Library -

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

S.N.	Member functions	Definition
1	(constructor)	It is used to construct a new std::function instance
2	(destructor)	It is used to destroy a std::function instance
3	operator=	It is used to assign a new target
3	swap	It is used to swap the contents
4	assign	It is used to assign a new target
5	operator bool	It is used to check if a valid target is contained
6	operator()	It is used to invoke the target

Non-member functions

S.N.	Non-member functions	Definition
1	std::swap	It specializes the std::swap algorithm
2	operator== operator!=	It compares an std::function with nullptr

Operator classes

S.N.	Operator classes	Definition
1	bit_and	It is a bitwise AND function object class
2	bit_or	It is a bitwise OR function object class
3	bit_xor	It is a bitwise XOR function object class
3	divides	It is a division function object class
4	equal_to	It is a function object class for equality comparison
5	greater	It is a function object class for greater-than inequality comparison
6	greater_equal	It is a function object class for greater-than-or-equal-to comparison
7	less	It is a function object class for less-than inequality comparison
8	less_equal	It is a function object class for less-than-or-equal-to comparison
9	logical_and	It is a logical AND function object class
10	logical_not	It is a logical NOT function object class
11	logical_or	It is a logical OR function object class
12	minus	It is a subtraction function object class
13	modulus	It is a modulus function object class
14	multiplies	It is a multiplication function object class
15	negate	It is a negative function object class
16	not_equal_to	It is a function object class for non-equality comparison
17	plus	It is an addition function object class