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

 

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strong_order weak_order partial_order strong_equal weak_equal	strong_ordering weak_ordering partial_ordering strong_equality weak_equality	is_eqis_neqis_ltis_lteqis_gtis_gteq
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is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible (C++11)(C++11)(C++11)
is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible (C++11)(C++11)(C++11)
is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11)
is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11)
is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11)
is_destructibleis_trivially_destructibleis_nothrow_destructible (C++11)(C++11)(C++11)
has_virtual_destructor (C++11)
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Relationships and property queries
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Type modifications
remove_cvremove_constremove_volatile (C++11)(C++11)(C++11) add_cvadd_constadd_volatile (C++11)(C++11)(C++11) make_signed (C++11) make_unsigned (C++11)	remove_reference (C++11) add_lvalue_referenceadd_rvalue_reference (C++11)(C++11) remove_pointer (C++11) add_pointer (C++11) remove_extent (C++11) remove_all_extents (C++11)
Type transformations
aligned_storage (C++11) aligned_union (C++11) decay (C++11) remove_cvref (C++20) enable_if (C++11) void_t (C++17)	conditional (C++11) common_type (C++11) common_reference (C++20) underlying_type (C++11) result_ofinvoke_result (C++11)(until C++20)(C++17) type_identity (C++20)

 

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

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_cvremove_constremove_volatile (C++11)(C++11)(C++11)	removes const or/and volatile specifiers from the given type (class template)
as_const (C++17)	obtains a reference to const to its argument (function template)