20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.16 Polymorphic function wrappers [func.wrap]

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This subclause describes a polymorphic wrapper class that encapsulates arbitrary callable objects.

20.14.16.1 Class bad_­function_­call [func.wrap.badcall]

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An exception of type bad_­function_­call is thrown by function​::​operator() ([func.wrap.func.inv]) when the function wrapper object has no target.
namespace std {
  class bad_function_call : public exception {
  public:
    // see [exception] for the specification of the special member functions
    const char* what() const noexcept override;
  };
}
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const char* what() const noexcept override;
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Returns: An implementation-defined ntbs.

20.14.16.2 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&&) 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>;

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

  // [func.wrap.func.alg], specialized algorithms
  template<class R, class... ArgTypes>
    void swap(function<R(ArgTypes...)>&, function<R(ArgTypes...)>&) noexcept;
}
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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.
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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]).
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The function class template is a call wrapper whose call signature is R(ArgTypes...).
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[ Note
:
The types deduced by the deduction guides for function may change in future versions of this International Standard.
— end note
 ]

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

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function() noexcept;
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Postconditions: !*this.
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function(nullptr_t) noexcept;
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Postconditions: !*this.
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function(const function& f);
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Postconditions: !*this if !f; otherwise, *this targets a copy of f.target().
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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.
[ Note
:
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.
— end note
 ]
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function(function&& f) noexcept;
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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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[ Note
:
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.
— end note
 ]
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template<class F> function(F f);
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Constraints: F is Lvalue-Callable ([func.wrap.func]) for argument types ArgTypes... and return type R.
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Preconditions: F meets the Cpp17CopyConstructible requirements.
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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.
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Otherwise, *this targets a copy of f initialized with std​::​move(f).
[ Note
:
Implementations should 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
 ]
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#
Throws: Nothing if f is a specialization of reference_­wrapper or a function pointer.
Otherwise, may throw bad_­alloc or any exception thrown by F's copy or move constructor.
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template<class F> function(F) -> function<see below>;
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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.
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Remarks: The deduced type is function<R(A...)>.
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[ Example
: 
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);
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Effects: As if by function(f).swap(*this);
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Returns: *this.
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function& operator=(function&& f);
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Effects: Replaces the target of *this with the target of f.
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Returns: *this.
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function& operator=(nullptr_t) noexcept;
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Effects: If *this != nullptr, destroys the target of this.
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Postconditions: !(*this).
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Returns: *this.
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template<class F> function& operator=(F&& f);
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Constraints: decay_­t<F> is Lvalue-Callable ([func.wrap.func]) for argument types ArgTypes... and return type R.
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Effects: As if by: function(std​::​forward<F>(f)).swap(*this);
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Returns: *this.
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template<class F> function& operator=(reference_wrapper<F> f) noexcept;
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Effects: As if by: function(f).swap(*this);
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Returns: *this.
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~function();
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Effects: If *this != nullptr, destroys the target of this.

20.14.16.2.2 Modifiers [func.wrap.func.mod]

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

20.14.16.2.3 Capacity [func.wrap.func.cap]

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

20.14.16.2.4 Invocation [func.wrap.func.inv]

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

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

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

20.14.16.2.6 Null pointer comparison functions [func.wrap.func.nullptr]

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

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

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