template< class T >
struct add_cv;
template< class T >
struct add_const;
template< class T >
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
If the program adds specializations for any of the templates described on this page, the behavior is undefined.
[edit] Member types Name Definitiontype the type T with the cv-qualifier [edit] Helper types
template< class T >
using add_cv_t = typename add_cv<T>::type;
template< class T >
using add_const_t = typename add_const<T>::type;
template< class T >
using add_volatile_t = typename add_volatile<T>::type;
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; };[edit] 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>[edit] 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:
Non-cv Const Volatile Const-volatile[edit] See also
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