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

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< cpp‎ | utility‎ | functional‎ | function
 
 
 
Function objects
Function wrappers
(C++11)
(C++11)
Partial function application
(C++20)
(C++11)
Function invocation
(C++17)
Identity function object
(C++20)
Reference wrappers
(C++11)(C++11)
Operator wrappers
Negators
(C++17)
Searchers
Constrained comparators
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++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 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