< cpp‎ | types
Utilities library
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types

Elementary string conversions
Type support
Basic types
Fundamental types
Fixed width integer types (C++11)
Numeric limits
C numeric limits interface
Runtime type information
Type traits
Type categories
Type properties
(C++11)(until C++20)
(C++11)(deprecated in C++20)
Type trait constants
Constant evaluation context
Supported operations
Relationships and property queries
Type modifications
Type transformations
(C++11)(until C++20)(C++17)
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 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.


[edit] Template parameters

T - a type to check

[edit] 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

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)
returns value
(public member function)

Member types

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

[edit] 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>.

[edit] Possible implementation

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

[edit] Example

#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


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

[edit] See also

checks if a type is volatile-qualified
(class template) [edit]
obtains a reference to const to its argument
(function template) [edit]