std::add_cv,std::add_const,std::add_volatile(3)

NAME
std::add_cv,std::add_const,std::add_volatile - std::add_cv,std::add_const,std::add_volatile

Synopsis
   Defined in header <type_traits>
   template< class T > (1) (since C++11)
   struct add_cv;
   template< class T > (2) (since C++11)
   struct add_const;
   template< class T > (3) (since C++11)
   struct add_volatile;
   Provides the member typedef type which is the same as T, except it has a
   cv-qualifier added (unless T is a function, a reference, or already has this
   cv-qualifier)
   1) adds both const and volatile
   2) adds const
   3) adds volatile
   The behavior of a program that adds specializations for any of the templates
   described on this page is undefined.

Member types
   Name Definition
   type the type T with the cv-qualifier

Helper types
   template< class T > (since C++14)
   using add_cv_t = typename add_cv<T>::type;
   template< class T > (since C++14)
   using add_const_t = typename add_const<T>::type;
   template< class T > (since C++14)
   using add_volatile_t = typename add_volatile<T>::type;

Possible implementation
   template<class T> struct add_cv { typedef const volatile T type; };
   template<class T> struct add_const { typedef const T type; };
   template<class T> struct add_volatile { typedef volatile T type; };

Notes
   These transformation traits can be used to establish non-deduced contexts in
   template argument deduction:
 template<class T>
 void f(const T&, const T&);
 template<class T>
 void g(const T&, std::add_const_t<T>&);
 f(4.2, 0); // error, deduced conflicting types for 'T'
 g(4.2, 0); // OK, calls g<double>

Example
// Run this code
 #include <iostream>
 #include <type_traits>
 struct foo
 {
     void m() { std::cout << "Non-cv\n"; }
     void m() const { std::cout << "Const\n"; }
     void m() volatile { std::cout << "Volatile\n"; }
     void m() const volatile { std::cout << "Const-volatile\n"; }
 };
 int main()
 {
     foo{}.m();
     std::add_const<foo>::type{}.m();
     std::add_volatile<foo>::type{}.m();
     std::add_cv<foo>::type{}.m();
 }

Output:
 Non-cv
 Const
 Volatile
 Const-volatile

See also
   is_const checks if a type is const-qualified
   (C++11) (class template)
   is_volatile checks if a type is volatile-qualified
   (C++11) (class template)
   remove_cv
   remove_const
   remove_volatile removes const or/and volatile specifiers from the given type
   (C++11) (class template)
   (C++11)
   (C++11)
   as_const obtains a reference to const to its argument
   (C++17) (function template)