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 CopyConstructible 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

constructs a new std::function instance
(public member function)
destroys a std::function instance
(public member function)
assigns a new target
(public member function)
swaps the contents
(public member function)
(removed in C++17)
assigns a new target
(public member function)
checks if a target is contained
(public member function)
invokes the target
(public member function)
Target access
obtains the typeid of the stored target
(public member function)
obtains a pointer to the stored target
(public member function)

Non-member functions

specializes the std::swap algorithm
(function template)
(removed in C++20)
compares a std::function with nullptr
(function template)

Helper classes

(C++11) (until C++17)
specializes the std::uses_allocator type trait
(class template specialization)

Deduction guides(since C++17)


Care should be taken when a std::function, whose result type is a reference, is initialized from a lambda expression without a trailing-return-type. Due to the way auto deduction works, such lambda expression will always return a prvalue. Hence, the resulting reference will usually bind to a temporary whose lifetime ends when std::function::operator() returns.

(until C++23)

If a std::function returning a reference is initialized from a function or function object returning a prvalue (including a lambda expression without a trailing-return-type), the program is ill-formed because binding the returned referenced to a temporary object is forbidden.

(since C++23)
std::function<const int&()> F([]{ return 42; }); // Error since C++23: can't bind
                                                 // the returned reference to a temporary
int x = F(); // Undefined behavior until C++23: the result of F() is a dangling reference
std::function<int&()> G([]()->int& { static int i{0x2A}; return i; }); // OK
std::function<const int&()> H([i{052}]->const int& { return i; }); // OK


#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;
    // store a lambda
    std::function<void()> f_display_42 = []() { print_num(42); };
    // store the result of a call to std::bind
    std::function<void()> f_display_31337 = std::bind(print_num, 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 );
    // store a call to a member function and object ptr
    std::function<void(int)> f_add_display3 = std::bind( &Foo::print_add, &foo, _1 );
    // store a call to a function object
    std::function<void(int)> f_display_obj = PrintNum();
    auto factorial = [](int n) {
        // store a lambda object to emulate "recursive lambda"; aware of extra overhead
        std::function<int(int)> fac = [&](int n){ return (n < 2) ? 1 : n*fac(n-1); };
        // note that "auto fac = [&](int n){...};" does not work in recursive calls
        return fac(n);
    for (int i{5}; i != 8; ++i) { std::cout << i << "! = " << factorial(i) << ";  "; }

Possible output:

num_: 314159
5! = 120;  6! = 720;  7! = 5040;

See also

wraps callable object of any type with specified function call signature
(class template)
the exception thrown when invoking an empty std::function
creates a function object out of a pointer to a member
(function template)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.