20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.17 Polymorphic function wrappers [func.wrap]

20.14.17.3 Class template function [func.wrap.func]

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

function() noexcept;
Postconditions: !*this.
function(nullptr_t) noexcept;
Postconditions: !*this.
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.
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.
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.
template<class F> function(F&& f);
Let FD be decay_­t<F>.
Constraints:
  • is_­same_­v<remove_­cvref_­t<F>, function> is false, and
  • FD is Lvalue-Callable ([func.wrap.func]) for argument types ArgTypes... and return type R.
Mandates:
  • is_­copy_­constructible_­v<FD> is true, and
  • is_­constructible_­v<FD, F> is true.
Preconditions: FD meets the Cpp17CopyConstructible requirements.
Postconditions: !*this is true if any of the following hold:
  • f is a null function pointer value.
  • f is a null member pointer value.
  • 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.
template<class F> function(F) -> function<see below>;
Constraints: &F​::​operator() is well-formed when treated as an unevaluated operand and decltype(&F​::​operator()) is of the form R(G​::​*)(A...) cv & noexcept for a class type G.
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]
function& operator=(const function& f);
Effects: As if by function(f).swap(*this);
Returns: *this.
function& operator=(function&& f);
Effects: Replaces the target of *this with the target of f.
Returns: *this.
function& operator=(nullptr_t) noexcept;
Effects: If *this != nullptr, destroys the target of this.
Postconditions: !(*this).
Returns: *this.
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.
template<class F> function& operator=(reference_wrapper<F> f) noexcept;
Effects: As if by: function(f).swap(*this);
Returns: *this.
~function();
Effects: If *this != nullptr, destroys the target of this.