std::is_const
From cppreference.com
| Defined in header <type_traits>
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||
| template< class T > struct is_const; |
(since C++11) | |
std::is_const is an UnaryTypeTrait.
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 std::is_const or std::is_const_v (since C++17) is undefined.
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
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
| (C++11) |
checks if a type is volatile-qualified (class template) |
| (C++17) |
obtains a reference to const to its argument (function template) |