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 to true. For any other type, value is false.

The behavior of a program that adds specializations for is_const
or is_const_v
(since C++17) is undefined.

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
[static] (public static member constant)

Member functions¶

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

// Run this code

#include <iostream>
#include <type_traits>

int main()
{
std::cout << std::boolalpha
<< std::is_const_v<int> << '\n' // false
<< std::is_const_v<const int> << '\n' // true
<< std::is_const_v<const int*> // false
<< " because the pointer itself can be changed but not the int pointed at\n"
<< std::is_const_v<int* const> // true
<< " because the pointer itself can't be changed but the int pointed at can\n"
<< std::is_const_v<const int&> << '\n' // false
<< std::is_const_v<std::remove_reference_t<const int&>> << '\n' // true
;
}

Output:¶

false
true
false because the pointer itself can be changed but not the int pointed at
true because the pointer itself can't be changed but the int pointed at can
false
true

See also¶

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