std::move_only_function

 

C++

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Utilities library

Language support Type support (basic types, RTTI, type traits)      Library feature-test macros (C++20) Dynamic memory management Program utilities Error handling Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) launder (C++17) initializer_list (C++11) source_location (C++20) Three-way comparison (C++20) 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_neqis_ltis_lteqis_gtis_gteq (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20) bitset hash (C++11) integer_sequence (C++14) 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) forward (C++11) move (C++11) move_if_noexcept (C++11) as_const (C++17) to_underlying (C++23) 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) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17)

 

Function objects

Function wrappers function (C++11) move_only_function (C++23) mem_fn (C++11) bad_function_call (C++11)	Partial function application bind_front (C++20) 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 (C++23)

move_only_function::~move_only_function (C++23)

move_only_function::operator= (C++23)

move_only_function::swap (C++23)

move_only_function::operator bool (C++23)

move_only_function::operator() (C++23)

Non-member functions

operator== (C++23)

swap(std::move_only_function) (C++23)

 

Defined in header <functional>
template< class... > class move_only_function; // not defined		(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...) 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>; 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>;		(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) Callable target -- 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 empty std::move_only_function results in undefined behavior.

std::move_only_functions supports every possible combination of cv-qualifiers, ref-qualifiers, and noexcept-specifiers not including volatile 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) (C++23)	constructs a new std::move_only_function object (public member function)
(destructor) (C++23)	destroys a std::move_only_function object (public member function)
operator= (C++23)	replaces or destroys the target (public member function)
swap (C++23)	swaps the targets of two std::move_only_function objects (public member function)
operator bool (C++23)	checks if the std::move_only_function has a target (public member function)
operator() (C++23)	invokes the target (public member function)

Non-member functions

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

Notes

Implementations are recommended to 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.

Like std::function, if R is a reference type that can be bound to an rvalue, std::move_only_function may wrap function objects that return by value, in which case a call to such std::move_only_function always returns a dangling reference.

std::move_only_function<const int&() const> uf([]{ return 42; });
int x = uf(); // Undefined behavior: the result of uf() is a dangling reference

Feature testing macro: __cpp_lib_move_only_function

See also

function (C++11)

wraps callable object of any copy constructible type with specified function call signature (class template)