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 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

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