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 哋它亢++11)
template< class T >
struct add_const;
 (2) (since 哋它亢++11)
template< class T >
struct add_volatile;
 (3) (since 哋它亢++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 哋它亢++14)
template< class T >
using add_const_t    = typename add_const<T>::type;
 (since 哋它亢++14)
template< class T >
using add_volatile_t = typename add_volatile<T>::type;
 (since 哋它亢++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

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