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

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( until C++17* )
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( until C++20* )
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std::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:
(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

See also

constructs a new std::move_only_function object
(public member function of std::move_only_function )
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