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

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< cpp‎ | utility‎ | functional‎ | function
 
 
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Old binders and adaptors
(until C++17*)
(until C++17*)
(until C++17*)
(until C++17*)  
(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
(until C++20*)
(until C++20*)
(until C++17*)(until C++17*)
(until C++17*)(until C++17*)

(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
(until C++20*)
(until C++20*)
 
 
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.

Contents

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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)

[edit] 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

[edit] See also

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

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