std::add_cv, std::add_const, std::add_volatile - cppreference.com

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Defined in header `<type_traits>`
template< class T > struct add_cv;	(1)	(since C++11)
template< class T > struct add_const;	(2)	(since C++11)
template< class T > struct add_volatile;	(3)	(since C++11)

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

If the program adds specializations for any of the templates described on this page, the behavior is undefined.

Member types

Name	Definition
`type`	the type `T` with the cv-qualifier

Helper types

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

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();
}

Output: