std::add_cv, std::add_const, std::add_volatile

template< class T >
struct add_cv;

template< class T >
struct add_const;

template< class T >
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)

The behavior of a program that adds specializations for any of the templates described on this page is undefined.

Member types

Helper types

template< class T >
using add_cv_t       = typename add_cv<T>::type;

template< class T >
using add_const_t    = typename add_const<T>::type;

template< class T >
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

#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();
}

Non-cv
Const
Volatile
Const-volatile

See also