23 General utilities library [utilities]

23.14 Function objects [function.objects]

23.14.13 Polymorphic function wrappers [func.wrap]

23.14.13.2 Class template function [func.wrap.func]

namespace std {
  template<class> class function; // not defined

  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&&);
    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>;

  // [func.wrap.func.nullptr], Null pointer comparisons
  template <class R, class... ArgTypes>
    bool operator==(const function<R(ArgTypes...)>&, nullptr_t) noexcept;

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

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

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

  // [func.wrap.func.alg], specialized algorithms
  template <class R, class... ArgTypes>
    void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept;
}

The function class template provides polymorphic wrappers that generalize the notion of a function pointer. Wrappers can store, copy, and call arbitrary callable objects, given a call signature, allowing functions to be first-class objects.

A callable type 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, is well formed ([func.require]).

The function class template is a call wrapper whose call signature is R(ArgTypes...).

[ Note: The types deduced by the deduction guides for function may change in future versions of this International Standard. — end note ]

23.14.13.2.1 function construct/copy/destroy [func.wrap.func.con]

function() noexcept;

Postconditions: !*this.

function(nullptr_t) noexcept;

Postconditions: !*this.

function(const function& f);

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

Throws: shall not throw exceptions 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. [ Note: Implementations are encouraged to 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. — end note ]

function(function&& f);

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.

Throws: shall not throw exceptions 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 or move constructor of the stored callable object. [ Note: Implementations are encouraged to 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. — end note ]

template<class F> function(F f);

Requires: F shall be CopyConstructible.

Remarks: This constructor template shall not participate in overload resolution unless F is Lvalue-Callable for argument types ArgTypes... and return type R.

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

(9.1)
f is a null function pointer value.
(9.2)
f is a null member pointer value.
(9.3)
F is an instance of the function class template, and !f.

Otherwise, *this targets a copy of f initialized with std::move(f). [ Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, where f is an object holding only a pointer or reference to an object and a member function pointer. — end note ]

Throws: shall not throw exceptions when f is a function pointer or a reference_wrapper<T> for some T. Otherwise, may throw bad_alloc or any exception thrown by F's copy or move constructor.

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

Remarks: This deduction guide participates in overload resolution only if &F::operator() is well-formed when treated as an unevaluated operand. In that case, if decltype(&F::operator()) is of the form R(G::*)(A...) cv & $_{o p t}$ noexcept $_{o p t}$ for a class type G, then the deduced type is function<R(A...)>.

[ Example:

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);

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

Returns: *this.

Remarks: This assignment operator shall not participate in overload resolution unless decay_t<F> is Lvalue-Callable for argument types ArgTypes... and return type R.

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.

23.14.13.2.2 function modifiers [func.wrap.func.mod]

void swap(function& other) noexcept;

Effects: interchanges the targets of *this and other.

23.14.13.2.3 function capacity [func.wrap.func.cap]

explicit operator bool() const noexcept;

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

23.14.13.2.4 function 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 of *this.

Throws: bad_function_call if !*this; otherwise, any exception thrown by the wrapped callable object.

23.14.13.2.5 function 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.

23.14.13.2.6 null pointer comparison functions [func.wrap.func.nullptr]

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

Returns: !f.

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

Returns: (bool)f.

23.14.13.2.7 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);