std::move_only_function

 

C++

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Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros(C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support(C++20) Contract support(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 Function objects Bit manipulation(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)

 

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::move_only_function

Member functions

move_only_function::move_only_function

move_only_function::~move_only_function

move_only_function::operator=

move_only_function::swap

move_only_function::operator bool

move_only_function::operator()

Non-member functions

operator==

swap(std::move_only_function)

 

Defined in header <functional>
template<class... > class move_only_function;// not defined	(1)	(since C++23)
template<class R, class... Args> class move_only_function<R(Args...)>; template<class R, class... Args> class move_only_function<R(Args...)noexcept>; template<class R, class... Args> class move_only_function<R(Args...)&>; template<class R, class... Args> class move_only_function<R(Args...)&noexcept>; template<class R, class... Args> class move_only_function<R(Args...)&&>; template<class R, class... Args> class move_only_function<R(Args...)&&noexcept>; template<class R, class... Args> class move_only_function<R(Args...)const>; template<class R, class... Args> class move_only_function<R(Args...)constnoexcept>; template<class R, class... Args> class move_only_function<R(Args...)const&>; template<class R, class... Args> class move_only_function<R(Args...)const&noexcept>; template<class R, class... Args> class move_only_function<R(Args...)const&&>; template<class R, class... Args> class move_only_function<R(Args...)const&&noexcept>;	(2)	(since C++23)

Class template std::move_only_function is a general-purpose polymorphic function wrapper. std::move_only_function objects can store and invoke any constructible (not required to be move constructible) Callabletarget — functions, lambda expressions, bind expressions, or other function objects, as well as pointers to member functions and pointers to member objects.

The stored callable object is called the target of std::move_only_function. If a std::move_only_function contains no target, it is called empty. Unlike std::function, invoking an emptystd::move_only_function results in undefined behavior.

std::move_only_functions supports every possible combination of cv-qualifiers (not including volatile), ref-qualifiers, and noexcept-specifiers provided in its template parameter. These qualifiers and specifier (if any) are added to its operator().

std::move_only_function satisfies the requirements of MoveConstructible and MoveAssignable, but does not satisfy CopyConstructible or CopyAssignable.

Member types

Type	Definition
result_type	R

Member functions

(constructor)	constructs a new std::move_only_function object (public member function)
(destructor)	destroys a std::move_only_function object (public member function)
operator=	replaces or destroys the target (public member function)
swap	swaps the targets of two std::move_only_function objects (public member function)
operator bool	checks if the std::move_only_function has a target (public member function)
operator()	invokes the target (public member function)

Non-member functions

swap(std::move_only_function) (C++23)	specializes the std::swap algorithm (function)
operator== (C++23)	compares a std::move_only_function with nullptr (function)

Notes

Implementations may store a callable object of small size within the std::move_only_function object. Such small object optimization is effectively required for function pointers and std::reference_wrapper specializations, and can only be applied to types T for which std::is_nothrow_move_constructible_v<T> is true.

If a std::move_only_function returning a reference is initialized from a function or function object returning a prvalue (including a lambda expression without a trailing-return-type), the program is ill-formed because binding the returned reference to a temporary object is forbidden. See also std::function Notes.

Feature-test macro	Value	Std	Feature
__cpp_lib_move_only_function	202110L	(C++23)	std::move_only_function

Example

Run this code

#include <functional>#include <future>#include <iostream>   int main(){std::packaged_task<double()> packaged_task([](){return3.14159;});   std::future<double> future = packaged_task.get_future();   auto lambda =[task = std::move(packaged_task)]() mutable { task();};   // std::function<void()> function = std::move(lambda); // Error std::move_only_function<void()> function = std::move(lambda);// OK   function();   std::cout<< future.get();}

Output:

3.14159

See also

function (C++11)	copyable wrapper of any copy constructible callable object (class template)
function_ref (C++26)	non-owning wrapper of any callable object (class template)
copyable_function (C++26)	copyable wrapper of any copy constructible callable object that supports qualifiers in a given call signature (class template)