std::is_const (3) - Linux Manuals

std::is_const: std::is_const


std::is_const - std::is_const


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)

Member functions

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

operator() returns value
              (public member function)

Member types

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


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 {};


// 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



See also

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

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