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std::is_const(3) C++ Standard Libary std::is_const(3) 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) http://cppreference.com 2022.07.31 std::is_const(3)
NAME | Synopsis | Template parameters | Member constants | Member functions | Member types | Notes | Possible implementation | Example | Output: | See also
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