std::function<R(Args...)>::function
From cppreference.com
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.
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>::typeis callable for argument typesArgs...and return typeR.
|
If |
(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)
|