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

From cppreference.com
< cpp‎ | utility‎ | functional‎ | 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 function.
3-4) Copies (3) or moves (4) the target of `other` to the target of *this. If `other` is empty, *this will be empty after the call too. For (4), `other` is in a valid but unspecified state after the call.
5) Initializes the target with std::move(f). If `f` is a null pointer to function or null pointer to member, *this will be empty after the call. This constructor does not participate in overload resolution unless f is Callable for argument types `Args...` and return type `R`. (since C++14)
6-10) Same as (1-5) except that `alloc` is used to allocate memory for any internal data structures that the `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 used to initialize *this
alloc | - | an Allocator used for internal memory allocation
Type requirements
- `F` must meet the requirements of Callable and CopyConstructible.
- `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

| This section is incomplete Reason: no example

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/functional/function/function&oldid=106982"