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

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

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