std::is_const (3) - Linux Man Pages

std::is_const: std::is_const

NAME

std::is_const - std::is_const

Synopsis


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


If T is a const-qualified type (that is, const, or const volatile), provides the member constant value equal true. For any other type, value is false.

Template parameters


T - a type to check


Helper variable template


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


Inherited from std::integral_constant

Member constants


value true if T is a const-qualified 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>

Notes


If T is a reference type then is_const<T>::value is always false. The proper way to check a potentially-reference type for const-ness is to remove the reference: is_const<typename remove_reference<T>::type>.

Possible implementation


  template<class T> struct is_const : std::false_type {};
  template<class T> struct is_const<const T> : std::true_type {};

Example


// Run this code


  #include <iostream>
  #include <type_traits>


  int main()
  {
      std::cout << std::boolalpha;
      std::cout << std::is_const<int>::value << '\n'; // false
      std::cout << std::is_const<const int>::value << '\n'; // true
      std::cout << std::is_const<const int*>::value << '\n'; // false
      std::cout << std::is_const<int* const>::value << '\n'; // true
      std::cout << std::is_const<const int&>::value << '\n'; // false
      std::cout << std::is_const<typename std::remove_reference<const int&>::type>::value << '\n'; // true
  }

Output:


  false
  true
  false
  true
  false
  true

See also


is_volatile checks if a type is volatile-qualified
            (class template)
(C++11)


as_const obtains a reference to const to its argument
            (function template)
(C++17)