W3cubDocs

/C++

std::is_const

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

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

Inherited from std::integral_constant

Member constants

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

Member functions

operator bool
converts the object to bool, returns value
(public member function)
operator()
(C++14)
returns value
(public member function)

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

#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

(C++11)
checks if a type is volatile-qualified
(class template)
(C++17)
obtains a reference to const to its argument
(function template)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
http://en.cppreference.com/w/cpp/types/is_const