NAME¶

std::function::function - std::function::function

Synopsis¶

function() noexcept; (1) (since C++11)
function( std::nullptr_t ) noexcept; (2) (since C++11)
function( const function& other ); (3) (since C++11)
function( function&& other ); (since C++11)
(until C++20)
function( function&& other ) noexcept; (since C++20)
template< class F > (5) (since C++11)
function( F&& f );
template< class Alloc > (since C++11)
function( std::allocator_arg_t, const Alloc& (6) (removed in C++17)
alloc ) noexcept;
template< class Alloc >

function( std::allocator_arg_t, const Alloc& (7) (since C++11)
alloc, (removed in C++17)

std::nullptr_t ) noexcept;
template< class Alloc > (4)

function( std::allocator_arg_t, const Alloc& (8) (since C++11)
alloc, (removed in C++17)

const function& other );
template< class Alloc >

function( std::allocator_arg_t, const Alloc& (9) (since C++11)
alloc, (removed in C++17)

function&& other );
template< class F, class Alloc > (since C++11)
function( std::allocator_arg_t, const Alloc& (10) (removed in C++17)
alloc, F f );

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:

* std::is_same_v<std::remove_cvref_t<F>, std::function<R(Args...)> (since C++23)
is false.

* 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 (since C++23)
ill-formed.

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 (until C++20)
exception thrown by the constructor used to copy or move the stored
callable object.

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

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¶

constructor constructs a new std::move_only_function object
(C++23) (public member function of std::move_only_function)

Category:¶

* Todo no example