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

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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

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; };

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

(C++11)
 checks if a type is const-qualified
(class template)
(C++11)
 checks if a type is volatile-qualified
(class template)
(C++11)(C++11)(C++11)
 removes const or/and volatile specifiers from the given type
(class template)
(C++17)
 obtains a reference to const to its argument
(function template)