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]

22.10.17.3.1 General [func.wrap.func.general]

namespace std { template<class R, class... ArgTypes> class function<R(ArgTypes...)> { public: using result_type = R; // [func.wrap.func.con], construct/copy/destroy function() noexcept; function(nullptr_t) noexcept; function(const function&); function(function&&) noexcept; template<class F> function(F&&); function& operator=(const function&); function& operator=(function&&); function& operator=(nullptr_t) noexcept; template<class F> function& operator=(F&&); template<class F> function& operator=(reference_wrapper<F>) noexcept; ~function(); // [func.wrap.func.mod], function modifiers void swap(function&) noexcept; // [func.wrap.func.cap], function capacity explicit operator bool() const noexcept; // [func.wrap.func.inv], function invocation R operator()(ArgTypes...) const; // [func.wrap.func.targ], function target access const type_info& target_type() const noexcept; template<class T> T* target() noexcept; template<class T> const T* target() const noexcept; }; template<class R, class... ArgTypes> function(R(*)(ArgTypes...)) -> function<R(ArgTypes...)>; template<class F> function(F) -> function<see below>; }
The function class template provides polymorphic wrappers that generalize the notion of a function pointer.
Wrappers can store, copy, and call arbitrary callable objects ([func.def]), given a call signature ([func.def]).
A callable type ([func.def]) F is Lvalue-Callable for argument types ArgTypes and return type R if the expression INVOKE<R>(declval<F&>(), declval<ArgTypes>()...), considered as an unevaluated operand ([expr.context]), is well-formed ([func.require]).
The function class template is a call wrapper ([func.def]) whose call signature ([func.def]) is R(ArgTypes...).
[Note 1: 
The types deduced by the deduction guides for function might change in future revisions of C++.
— end note]

22.10.17.3.2 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 either
  • F​::​operator() is a non-static member function and decltype(&F​::​operator()) is either of the form R(G​::​*)(A...) cv & noexcept or of the form R(*)(G, A...) noexcept for a type G, or
  • F​::​operator() is a static member function and decltype(&F​::​operator()) is of the form R(*)(A...) noexcept.
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.

22.10.17.3.3 Modifiers [func.wrap.func.mod]

void swap(function& other) noexcept;
Effects: Interchanges the target objects of *this and other.

22.10.17.3.4 Capacity [func.wrap.func.cap]

explicit operator bool() const noexcept;
Returns: true if *this has a target, otherwise false.

22.10.17.3.5 Invocation [func.wrap.func.inv]

R operator()(ArgTypes... args) const;
Returns: INVOKE<R>(f, std​::​forward<ArgTypes>(args)...) ([func.require]), where f is the target object ([func.def]) of *this.
Throws: bad_function_call if !*this; otherwise, any exception thrown by the target object.

22.10.17.3.6 Target access [func.wrap.func.targ]

const type_info& target_type() const noexcept;
Returns: If *this has a target of type T, typeid(T); otherwise, typeid(void).
template<class T> T* target() noexcept; template<class T> const T* target() const noexcept;
Returns: If target_type() == typeid(T) a pointer to the stored function target; otherwise a null pointer.

22.10.17.3.7 Null pointer comparison operator functions [func.wrap.func.nullptr]

template<class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>& f, nullptr_t) noexcept;
Returns: !f.

22.10.17.3.8 Specialized algorithms [func.wrap.func.alg]

template<class R, class... ArgTypes> void swap(function<R(ArgTypes...)>& f1, function<R(ArgTypes...)>& f2) noexcept;
Effects: As if by: f1.swap(f2);