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

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< cpp‎ | types

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

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 > 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

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

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