std::is_function (3) - Linux Manuals

std::is_function: std::is_function

NAME

std::is_function - std::is_function

Synopsis


Defined in header <type_traits>
template< class T > (since C++11)
struct is_function;


Checks whether T is a function type. Types like std::function, lambdas, classes with overloaded operator() and pointers to functions don't count as function types. Provides the member constant value which is equal to true, if T is a function type. Otherwise, value is equal to false.

Template parameters


T - a type to check


Helper variable template


template< class T > (since C++17)
inline constexpr bool is_function_v = is_function<T>::value;


Inherited from std::integral_constant

Member constants


value true if T is a function type , false otherwise
         (public static member constant)
[static]

Member functions


              converts the object to bool, returns value
operator bool (public member function)


operator() returns value
              (public member function)
(C++14)

Member types


Type Definition
value_type bool
type std::integral_constant<bool, value>

Possible implementation


  // primary template
  template<class>
  struct is_function : std::false_type { };


  // specialization for regular functions
  template<class Ret, class... Args>
  struct is_function<Ret(Args...)> : std::true_type {};


  // specialization for variadic functions such as std::printf
  template<class Ret, class... Args>
  struct is_function<Ret(Args......)> : std::true_type {};


  // specialization for function types that have cv-qualifiers
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) const> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) volatile> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) const volatile> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) const> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) volatile> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) const volatile> : std::true_type {};


  // specialization for function types that have ref-qualifiers
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) &> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) const &> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) volatile &> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) const volatile &> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) &> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) const &> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) volatile &> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) const volatile &> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) &&> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) const &&> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) volatile &&> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) const volatile &&> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) &&> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) const &&> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) volatile &&> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) const volatile &&> : std::true_type {};


  // specializations for noexcept versions of all the above (C++17 and later)


  template<class Ret, class... Args>
  struct is_function<Ret(Args...) noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) const noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) volatile noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) const volatile noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) const noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) volatile noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) const volatile noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) & noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) const & noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) volatile & noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) const volatile & noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) & noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) const & noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) volatile & noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) const volatile & noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) && noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) const && noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) volatile && noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args...) const volatile && noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) && noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) const && noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) volatile && noexcept> : std::true_type {};
  template<class Ret, class... Args>
  struct is_function<Ret(Args......) const volatile && noexcept> : std::true_type {};

Example


// Run this code


  #include <iostream>
  #include <type_traits>


  struct A {
      int fun() const&;
  };


  template<typename>
  struct PM_traits {};


  template<class T, class U>
  struct PM_traits<U T::*> {
      using member_type = U;
  };


  int f();


  int main()
  {
      std::cout << std::boolalpha;
      std::cout << std::is_function<A>::value << '\n';
      std::cout << std::is_function<int(int)>::value << '\n';
      std::cout << std::is_function<decltype(f)>::value << '\n';
      std::cout << std::is_function<int>::value << '\n';


      using T = PM_traits<decltype(&A::fun)>::member_type; // T is int() const&
      std::cout << std::is_function<T>::value << '\n';
  }

Output:


  false
  true
  true
  false
  true

See also


is_invocable
is_invocable_r checks if a type can be invoked (as if by std::invoke) with the given argument types
is_nothrow_invocable (class template)
is_nothrow_invocable_r


(C++17)


is_object checks if a type is an object type
                       (class template)
(C++11)


is_class checks if a type is a non-union class type
                       (class template)
(C++11)

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