NAME¶

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

Synopsis¶

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

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

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