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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Debugging support is_debugger_present (C++26) breakpoint_if_debugging (C++26) breakpoint (C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)    is_eqis_ltis_lteq (C++20)(C++20)(C++20) is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>     rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)    cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

Function objects

Function wrappers function (C++11) move_only_function (C++23) copyable_function (C++26) function_ref (C++26) mem_fn (C++11) bad_function_call (C++11)	Partial function application bind_frontbind_back (C++20)(C++23) bind (C++11) is_bind_expression (C++11) is_placeholder (C++11) _1, _2, _3, ... (C++11)
Function invocation invokeinvoke_r (C++17)(C++23) Identity function object identity (C++20)	Reference wrappers reference_wrapper (C++11) refcref (C++11)(C++11) unwrap_referenceunwrap_ref_decay (C++20)(C++20)
Operator wrappers plus minus negate multiplies divides modulus bit_and bit_or bit_not (C++14)   bit_xor equal_to not_equal_to greater less greater_equal less_equal logical_and logical_or logical_not	Transparent operator wrappers plus<> (C++14) minus<> (C++14) negate<> (C++14) multiplies<> (C++14)   divides<> (C++14) modulus<> (C++14) bit_and<> (C++14) bit_or<> (C++14) bit_not<> (C++14) bit_xor<> (C++14) equal_to<> (C++14) not_equal_to<> (C++14) greater<> (C++14) less<> (C++14) greater_equal<> (C++14) less_equal<> (C++14) logical_and<> (C++14) logical_or<> (C++14) logical_not<> (C++14)
Negators not_fn (C++17) Searchers default_searcher (C++17) boyer_moore_searcher (C++17) boyer_moore_horspool_searcher (C++17)	Constrained comparators ranges::equal_to (C++20) ranges::not_equal_to (C++20) ranges::greater (C++20) ranges::less (C++20) ranges::greater_equal (C++20) ranges::less_equal (C++20) compare_three_way (C++20)
Old binders and adaptors
unary_function (until C++17*) binary_function (until C++17*) ptr_fun (until C++17*) pointer_to_unary_function (until C++17*) pointer_to_binary_function (until C++17*)   mem_fun (until C++17*) mem_fun_tmem_fun1_tconst_mem_fun_tconst_mem_fun1_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) not1 (until C++20*) not2 (until C++20*)	binder1stbinder2nd (until C++17*)(until C++17*) bind1stbind2nd (until C++17*)(until C++17*) mem_fun_ref (until C++17*) mem_fun_ref_tmem_fun1_ref_tconst_mem_fun_ref_tconst_mem_fun1_ref_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) unary_negate (until C++20*) binary_negate (until C++20*)

 

std::function

Member functions

function::function

function::~function

function::operator=

function::swap

function::assign (until C++17)

function::operator bool

function::operator()

function::target_type

function::target

Non-member functions

operator==operator!= (until C++20)

swap(std::function)

Helper classes

uses_allocator<std::function> (until C++17)

Deduction guides(C++17)

 

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 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...)> is false.

(since C++23)

• 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 ill-formed.

(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.

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 (public member function of std::move_only_function)