std::function<R(Args...)>::function

function() noexcept;	(1)	(since C++11)
function( std::nullptr_t ) noexcept;	(2)	(since C++11)
function( const function& other );	(3)	(since C++11)
	(4)
function( function&& other );			(since C++11) (until C++20)
function( function&& other ) noexcept;			(since C++20)
template< class F > function( F&& f );		(5)	(since C++11)
template< class Alloc > function( std::allocator_arg_t, const Alloc& alloc ) noexcept;		(6)	(since C++11) (removed in C++17)
template< class Alloc > function( std::allocator_arg_t, const Alloc& alloc, std::nullptr_t ) noexcept;		(7)	(since C++11) (removed in C++17)
template< class Alloc > function( std::allocator_arg_t, const Alloc& alloc, const function& other );		(8)	(since C++11) (removed in C++17)
template< class Alloc > function( std::allocator_arg_t, const Alloc& alloc, function&& other );		(9)	(since C++11) (removed in C++17)
template< class F, class Alloc > function( std::allocator_arg_t, const Alloc& alloc, F f );		(10)	(since C++11) (removed in C++17)

Constructs a std::function from a variety of sources.

1,2) Creates an empty std::function.

3) Copies the target of other to the target of *this.

If other is empty, *this will be empty right after the call too.

4) Moves the target of other to the target of *this.

If other is empty, *this will be empty right after the call too.

other is in a valid but unspecified state right after the call.

5) Initializes the target with std::forward<F>(f). The target is of type std::decay<F>::type.

If f is a null pointer to function, a null pointer to member, or an empty value of some std::function specialization, *this will be empty right after the call.

This overload participates in overload resolution only if all following conditions are satisfied:

std::is_same_v<std::remove_cvref_t<F>, std::function<R(Args...)> is false.

(since C++23)

An lvalue of type std::decay<F>::type is callable for argument types Args... and return type R.

If std::is_copy_constructible_v<std::decay_t<F>> or std::is_constructible_v<std::decay_t<F>, F> is false, the program is ill-formed.

(since C++23)

If F is not CopyConstructible, the behavior is undefined.

6-10) Same as (1-5) except that alloc is used to allocate memory for any internal data structures that the std::function might use.

When the target is a function pointer or a std::reference_wrapper, small object optimization is guaranteed, that is, these targets are always directly stored inside the std::function object, no dynamic allocation takes place. Other large objects may be constructed in dynamic allocated storage and accessed by the std::function object through a pointer.

Parameters

other	-	the function object used to initialize *this
f	-	a callable object used to initialize *this
alloc	-	an Allocator used for internal memory allocation
Type requirements
- `Alloc` must meet the requirements of Allocator.

Exceptions

3,8,9) Does not throw if other's target is a function pointer or a std::reference_wrapper, otherwise may throw std::bad_alloc or any exception thrown by the constructor used to copy or move the stored callable object.

4) Does not throw if other's target is a function pointer or a std::reference_wrapper, otherwise may throw std::bad_alloc or any exception thrown by the constructor used to copy or move the stored callable object.

(until C++20)

5,10) Does not throw if f is a function pointer or a std::reference_wrapper, otherwise may throw std::bad_alloc or any exception thrown by the copy constructor of the stored callable object.

Notes

std::function's allocator support was poorly specified and inconsistently implemented. Some implementations do not provide overloads (6-10) at all, some provide the overloads but ignore the supplied allocator argument, and some provide the overloads and use the supplied allocator for construction but not when the std::function is reassigned. As a result, allocator support was removed in C++17.

Example

Run this code

#include <functional>
#include <iostream>
#include <utility>
 
void print_num(int i) { std::cout << "print_num(" << i << ")\n"; }
 
int main()
{
    std::function<void(int)> func1; // (1) empty constructor
    try
    {
        func1(333 << 1);
    }
    catch (const std::bad_function_call& ex)
    {
        std::cout << "1) " << ex.what() << '\n';
    }
 
    std::function<void(int)> func2{nullptr}; // (2) empty constructor
    try
    {
        func1(222 * 3);
    }
    catch (const std::bad_function_call& ex)
    {
        std::cout << "2) " << ex.what() << '\n';
    }
 
    func1 = print_num; // initializes func1 using assignment operator
 
    std::function<void(int)> func3{func1}; // (3) copy constructor
    func3(33);
 
    std::function<void(int)> func4{std::move(func3)}; // (4) move constructor,
                                                      // func3 in unspecified state
    func4(44);
 
    std::function<void(int)> func5{print_num}; // (5) constructor with function
    func5(55);
 
    // (5) constructor with lambda
    std::function<void(int)> func6([](int i) { std::cout << "lambda(" << i << ")\n"; });
    func6(66);
}

Possible output:

1) bad_function_call
2) bad_function_call
print_num(33)
print_num(44)
print_num(55)
lambda(66)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2132	C++11	overloads (5,10) might be ambiguous	constrained
LWG 2774	C++11	(5,10) performed an additional move	eliminated

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Feature test macros (C++20)
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Metaprogramming library (C++11)
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External libraries

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!= (until C++20)
swap(std::function)
Helper classes
uses_allocator<std::function> (until C++17)
Deduction guides(C++17)