15 KiB
[func.wrap.move]
22 General utilities library [utilities]
22.10 Function objects [function.objects]
22.10.17 Polymorphic function wrappers [func.wrap]
22.10.17.4 Move-only wrapper [func.wrap.move]
22.10.17.4.1 General [func.wrap.move.general]
The header provides partial specializations of move_only_function for each combination of the possible replacements of the placeholders cv, ref, and noex where
cv is either const or empty,
ref is either &, &&, or empty, and
noex is either true or false.
For each of the possible combinations of the placeholders mentioned above, there is a placeholder inv-quals defined as follows:
If ref is empty, let inv-quals be cv&,
otherwise, let inv-quals be cv ref.
22.10.17.4.2 Class template move_only_function [func.wrap.move.class]
namespace std {template<class R, class... ArgTypes>class move_only_function<R(ArgTypes...) cv ref noexcept(noex)> {public:using result_type = R; // [func.wrap.move.ctor], constructors, assignments, and destructor move_only_function() noexcept; move_only_function(nullptr_t) noexcept; move_only_function(move_only_function&&) noexcept; template move_only_function(F&&); template<class T, class... Args>explicit move_only_function(in_place_type_t, Args&&...); template<class T, class U, class... Args>explicit move_only_function(in_place_type_t, initializer_list, Args&&...);
move_only_function& operator=(move_only_function&&); move_only_function& operator=(nullptr_t) noexcept; template move_only_function& operator=(F&&); ~move_only_function(); // [func.wrap.move.inv], invocationexplicit operator bool() const noexcept; R operator()(ArgTypes...) cv ref noexcept(noex); // [func.wrap.move.util], utilityvoid swap(move_only_function&) noexcept; friend void swap(move_only_function&, move_only_function&) noexcept; friend bool operator==(const move_only_function&, nullptr_t) noexcept; private:templatestatic constexpr bool is-callable-from = see below; // exposition only};}
The move_only_function class template provides polymorphic wrappers that generalize the notion of a callable object ([func.def]).
These wrappers can store, move, and call arbitrary callable objects, given a call signature.
Recommended practice: Implementations should avoid the use of dynamically allocated memory for a small contained value.
[Note 1:
Such small-object optimization can only be applied to a type T for which is_nothrow_move_constructible_v is true.
â end note]
22.10.17.4.3 Constructors, assignments, and destructor [func.wrap.move.ctor]
template<class VT> static constexpr bool is-callable-from = see below;
If noex is true,is-callable-from is equal to:is_nothrow_invocable_r_v<R, VT cv ref, ArgTypes...> && is_nothrow_invocable_r_v<R, VT inv-quals, ArgTypes...>
Otherwise, is-callable-from is equal to:is_invocable_r_v<R, VT cv ref, ArgTypes...> && is_invocable_r_v<R, VT inv-quals, ArgTypes...>
move_only_function() noexcept; move_only_function(nullptr_t) noexcept;
Postconditions: *this has no target object.
move_only_function(move_only_function&& f) noexcept;
Postconditions: The target object of *this is the target object f had before construction, andf is in a valid state with an unspecified value.
template<class F> move_only_function(F&& f);
Let VT be decay_t.
Constraints:
remove_cvref_t is not the same type as move_only_function, and
remove_cvref_t is not a specialization of in_place_type_t, and
is-callable-from is true.
Mandates: is_constructible_v<VT, F> is true.
Preconditions: VT meets the Cpp17Destructible requirements, and if is_move_constructible_v is true,VT meets the Cpp17MoveConstructible requirements.
Postconditions: *this has no target object if any of the following hold:
f is a null function pointer value, or
f is a null member pointer value, or
remove_cvref_t is a specialization of the move_only_function class template, and f has no target object.
Otherwise, *this has a target object of type VT direct-non-list-initialized with std::forward(f).
Throws: Any exception thrown by the initialization of the target object.
May throw bad_alloc unless VT is a function pointer or a specialization of reference_wrapper.
template<class T, class... Args> explicit move_only_function(in_place_type_t<T>, Args&&... args);
Let VT be decay_t.
Constraints:
is_constructible_v<VT, Args...> is true, and
is-callable-from is true.
Mandates: VT is the same type as T.
Preconditions: VT meets the Cpp17Destructible requirements, and if is_move_constructible_v is true,VT meets the Cpp17MoveConstructible requirements.
Postconditions: *this has a target object of type VT direct-non-list-initialized with std::forward(args)....
Throws: Any exception thrown by the initialization of the target object.
May throw bad_alloc unless VT is a function pointer or a specialization of reference_wrapper.
template<class T, class U, class... Args> explicit move_only_function(in_place_type_t<T>, initializer_list<U> ilist, Args&&... args);
Let VT be decay_t.
Constraints:
is_constructible_v<VT, initializer_list&, Args...> istrue, and
is-callable-from is true.
Mandates: VT is the same type as T.
Preconditions: VT meets the Cpp17Destructible requirements, and if is_move_constructible_v is true,VT meets the Cpp17MoveConstructible requirements.
Postconditions: *this has a target object of type VT direct-non-list-initialized withilist, std::forward(args)....
Throws: Any exception thrown by the initialization of the target object.
May throw bad_alloc unless VT is a function pointer or a specialization of reference_wrapper.
move_only_function& operator=(move_only_function&& f);
Effects: Equivalent to: move_only_function(std::move(f)).swap(*this);
Returns: *this.
move_only_function& operator=(nullptr_t) noexcept;
Effects: Destroys the target object of *this, if any.
Returns: *this.
template<class F> move_only_function& operator=(F&& f);
Effects: Equivalent to: move_only_function(std::forward(f)).swap(*this);
Returns: *this.
~move_only_function();
Effects: Destroys the target object of *this, if any.
22.10.17.4.4 Invocation [func.wrap.move.inv]
explicit operator bool() const noexcept;
Returns: true if *this has a target object, otherwise false.
R operator()(ArgTypes... args) cv ref noexcept(noex);
Preconditions: *this has a target object.
Effects: Equivalent to:return INVOKE(static_cast<F inv-quals>(f), std::forward(args)...); where f is an lvalue designating the target object of *this andF is the type of f.
22.10.17.4.5 Utility [func.wrap.move.util]
void swap(move_only_function& other) noexcept;
Effects: Exchanges the target objects of *this and other.
friend void swap(move_only_function& f1, move_only_function& f2) noexcept;
Effects: Equivalent to f1.swap(f2).
friend bool operator==(const move_only_function& f, nullptr_t) noexcept;
Returns: true if f has no target object, otherwise false.