22 General utilities library [utilities]

22.10 Function objects [function.objects]

22.10.17 Polymorphic function wrappers [func.wrap]

22.10.17.3 Class template function [func.wrap.func]

22.10.17.3.2 Constructors and destructor [func.wrap.func.con]

function() noexcept;

Postconditions: !*this.

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function(nullptr_t) noexcept;

Postconditions: !*this.

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function(const function& f);

Postconditions: !*this if !f; otherwise, the target object of *this is a copy of f.target().

Throws: Nothing if f's target is a specialization of reference_wrapper or a function pointer.

Otherwise, may throw bad_alloc or any exception thrown by the copy constructor of the stored callable object.

Recommended practice: Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, where f's target is an object holding only a pointer or reference to an object and a member function pointer.

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function(function&& f) noexcept;

Postconditions: If !f, *this has no target; otherwise, the target of *this is equivalent to the target of f before the construction, and f is in a valid state with an unspecified value.

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

Let FD be decay_t<F>.

Constraints:

(9.1)
is_same_v<remove_cvref_t<F>, function> is false, and
(9.2)
FD is Lvalue-Callable ([func.wrap.func]) for argument types ArgTypes... and return type R.

Mandates:

(10.1)
is_copy_constructible_v<FD> is true, and
(10.2)
is_constructible_v<FD, F> is true.

Preconditions: FD meets the Cpp17CopyConstructible requirements.

Postconditions: !*this is true if any of the following hold:

(12.1)
f is a null function pointer value.
(12.2)
f is a null member pointer value.
(12.3)
remove_cvref_t<F> is a specialization of the function class template, and !f is true.

Otherwise, *this has a target object of type FD direct-non-list-initialized with std::forward<F>(f).

Throws: Nothing if FD is a specialization of reference_wrapper or a function pointer type.

Otherwise, may throw bad_alloc or any exception thrown by the initialization of the target object.

Recommended practice: Implementations should avoid the use of dynamically allocated memory for small callable objects, for example, where f refers to an object holding only a pointer or reference to an object and a member function pointer.

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template<class F> function(F) -> function<see below>;

Constraints: &F::operator() is well-formed when treated as an unevaluated operand and either

(16.1)
F::operator() is a non-static member function and decltype(&F::operator()) is either of the form R(G::*)(A...) cv & $_{o p t}$ noexcept $_{o p t}$ or of the form R(*)(G, A...) noexcept $_{o p t}$ for a type G, or
(16.2)
F::operator() is a static member function and decltype(&F::operator()) is of the form R(*)(A...) noexcept $_{o p t}$ .

Remarks: The deduced type is function<R(A...)>.

[Example 1: void f() { int i{5}; function g = [&](double) { return i; }; // deduces function<int(double)> } — end example]

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function& operator=(const function& f);

Effects: As if by function(f).swap(*this);

Returns: *this.

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function& operator=(function&& f);

Effects: Replaces the target of *this with the target of f.

Returns: *this.

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function& operator=(nullptr_t) noexcept;

Effects: If *this != nullptr, destroys the target of this.

Postconditions: !(*this).

Returns: *this.

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

Constraints: decay_t<F> is Lvalue-Callable ([func.wrap.func]) for argument types ArgTypes... and return type R.

Effects: As if by: function(std::forward<F>(f)).swap(*this);

Returns: *this.

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template<class F> function& operator=(reference_wrapper<F> f) noexcept;

Effects: As if by: function(f).swap(*this);

Returns: *this.

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~function();

Effects: If *this != nullptr, destroys the target of this.