std::function

Defined in header `<functional>`
template< class > class function; /* undefined */		(since C++11)
template< class R, class... Args > class function<R(Args...)>;		(since C++11)

Class template std::function is a general-purpose polymorphic function wrapper. 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.

The stored callable object is called the target of std::function. If a std::function contains no target, it is called empty. Invoking the target of an empty std::function results in std::bad_function_call exception being thrown.

std::function satisfies the requirements of CopyConstructible and CopyAssignable.

Member types

Type	Definition
`result_type`	`R`
`argument_type`(deprecated in C++17)(removed in C++20)	`T` if sizeof...(Args)==1 and `T` is the first and only type in `Args...`
`first_argument_type`(deprecated in C++17)(removed in C++20)	`T1` if sizeof...(Args)==2 and `T1` is the first of the two types in `Args...`
`second_argument_type`(deprecated in C++17)(removed in C++20)	`T2` if sizeof...(Args)==2 and `T2` is the second of the two types in `Args...`

Member functions

(constructor)	constructs a new `std::function` instance (public member function)
(destructor)	destroys a `std::function` instance (public member function)
operator=	assigns a new target (public member function)
swap	swaps the contents (public member function)
assign (until C++17)	assigns a new target (public member function)
operator bool	checks if a valid target is contained (public member function)
operator()	invokes the target (public member function)
Target access
target_type	obtains the typeid of the stored target (public member function)
target	obtains a pointer to the stored target (public member function)

Non-member functions

std::swap(std::function) (C++11)	specializes the std::swap algorithm (function template)
operator==operator!=	compares a std::function with nullptr (function template)

Helper classes

std::uses_allocator<std::function>

(C++11) (until C++17)

specializes the std::uses_allocator type trait
(class template specialization)

Deduction guides(since C++17)

Example

Run this code

#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()
{
    // store a free function
    std::function<void(int)> f_display = print_num;
    f_display(-9);

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

    // store the result of a call to std::bind
    std::function<void()> f_display_31337 = std::bind(print_num, 31337);
    f_display_31337();

    // store a call to a member function
    std::function<void(const Foo&, int)> f_add_display = &Foo::print_add;
    const Foo foo(314159);
    f_add_display(foo, 1);
    f_add_display(314159, 1);

    // store a call to a data member accessor
    std::function<int(Foo const&)> f_num = &Foo::num_;
    std::cout << "num_: " << f_num(foo) << '\n';

    // store a call to a member function and object
    using std::placeholders::_1;
    std::function<void(int)> f_add_display2 = std::bind( &Foo::print_add, foo, _1 );
    f_add_display2(2);

    // store a call to a member function and object ptr
    std::function<void(int)> f_add_display3 = std::bind( &Foo::print_add, &foo, _1 );
    f_add_display3(3);

    // store a call to a function object
    std::function<void(int)> f_display_obj = PrintNum();
    f_display_obj(18);
}

Output:

-9
42
31337
314160
314160
num_: 314159
314161
314162
18

Language
Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

Member functions
function::function
function::~function
function::operator=
function::swap
function::assign (until C++17)
function::operator bool
function::operator()
function::target_type
function::target
Non-member functions
operator==operator!=
std::swap(std::function)
Helper classes
std::uses_allocator<std::function> (until C++17)
Deduction guides(C++17)

bad_function_call (C++11)	the exception thrown when invoking an empty std::function (class)
mem_fn (C++11)	creates a function object out of a pointer to a member (function template)