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

(1.1)
cv is either const or empty,
(1.2)
ref is either &, &&, or empty, and
(1.3)
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:

(2.1)
If ref is empty, let inv-quals be cv&,
(2.2)
otherwise, let inv-quals be cv ref.

22.10.17.4.2 Class template move_only_function [func.wrap.move.class]

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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, assignment, and destructor move_only_function() noexcept; move_only_function(nullptr_t) noexcept; move_only_function(move_only_function&&) noexcept; template<class F> move_only_function(F&&); template<class T, class... Args> explicit move_only_function(in_place_type_t<T>, Args&&...); template<class T, class U, class... Args> explicit move_only_function(in_place_type_t<T>, initializer_list<U>, Args&&...); move_only_function& operator=(move_only_function&&); move_only_function& operator=(nullptr_t) noexcept; template<class F> move_only_function& operator=(F&&); ~move_only_function(); // [func.wrap.move.inv], invocation explicit operator bool() const noexcept; R operator()(ArgTypes...) cv ref noexcept(noex); // [func.wrap.move.util], utility void 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: template<class VT> static 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<T> is true.

— end note]

22.10.17.4.3 Constructors, assignment, and destructor [func.wrap.move.ctor]

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template<class VT>
  static constexpr bool is-callable-from = see below;

If noex is true, is-callable-from<VT> 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<VT> is equal to: is_invocable_r_v<R, VT cv ref, ArgTypes...> && is_invocable_r_v<R, VT inv-quals, ArgTypes...>

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move_only_function() noexcept;
move_only_function(nullptr_t) noexcept;

Postconditions: *this has no target object.

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move_only_function(move_only_function&& f) noexcept;

Postconditions: The target object of *this is the target object f had before construction, and f is in a valid state with an unspecified value.

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template<class F> move_only_function(F&& f);

Let VT be decay_t<F>.

Constraints:

(5.1)
remove_cvref_t<F> is not the same type as move_only_function, and
(5.2)
remove_cvref_t<F> is not a specialization of in_place_type_t, and
(5.3)
is-callable-from<VT> is true.

Mandates: is_constructible_v<VT, F> is true.

Preconditions: VT meets the Cpp17Destructible requirements, and if is_move_constructible_v<VT> is true, VT meets the Cpp17MoveConstructible requirements.

Postconditions: *this has no target object if any of the following hold:

(8.1)
f is a null function pointer value, or
(8.2)
f is a null member pointer value, or
(8.3)
remove_cvref_t<F> 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>(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.

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template<class T, class... Args>
  explicit move_only_function(in_place_type_t<T>, Args&&... args);

Let VT be decay_t<T>.

Constraints:

(11.1)
is_constructible_v<VT, Args...> is true, and
(11.2)
is-callable-from<VT> is true.

Mandates: VT is the same type as T.

Preconditions: VT meets the Cpp17Destructible requirements, and if is_move_constructible_v<VT> is true, VT meets the Cpp17MoveConstructible requirements.

Postconditions: *this has a target object of type VT direct-non-list-initialized with std::forward<Args>(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.

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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<T>.

Constraints:

(17.1)
is_constructible_v<VT, initializer_list<U>&, Args...> is true, and
(17.2)
is-callable-from<VT> is true.

Mandates: VT is the same type as T.

Preconditions: VT meets the Cpp17Destructible requirements, and if is_move_constructible_v<VT> is true, VT meets the Cpp17MoveConstructible requirements.

Postconditions: *this has a target object of type VT direct-non-list-initialized with ilist, std::forward<Args>(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.

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move_only_function& operator=(move_only_function&& f);

Effects: Equivalent to: move_only_function(std::move(f)).swap(*this);

Returns: *this.

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move_only_function& operator=(nullptr_t) noexcept;

Effects: Destroys the target object of *this, if any.

Returns: *this.

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template<class F> move_only_function& operator=(F&& f);

Effects: Equivalent to: move_only_function(std::forward<F>(f)).swap(*this);

Returns: *this.

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~move_only_function();

Effects: Destroys the target object of *this, if any.

22.10.17.4.4 Invocation [func.wrap.move.inv]

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explicit operator bool() const noexcept;

Returns: true if *this has a target object, otherwise false.

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R operator()(ArgTypes... args) cv ref noexcept(noex);

Preconditions: *this has a target object.

Effects: Equivalent to: return INVOKE<R>(static_cast<F inv-quals>(f), std::forward<ArgTypes>(args)...); where f is an lvalue designating the target object of *this and F is the type of f.

22.10.17.4.5 Utility [func.wrap.move.util]

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void swap(move_only_function& other) noexcept;

Effects: Exchanges the target objects of *this and other.

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friend void swap(move_only_function& f1, move_only_function& f2) noexcept;

Effects: Equivalent to f1.swap(f2).

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friend bool operator==(const move_only_function& f, nullptr_t) noexcept;

Returns: true if f has no target object, otherwise false.