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

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Metaprogramming library

Type traits

Type categories

is_void

(C++11)

is_null_pointer

(C++11)(DR*)

is_array (C++11)
is_pointer (C++11)
is_enum (C++11)
is_union (C++11)
is_class (C++11)
is_function (C++11)
is_reference (C++11)

is_object (C++11)
is_scalar (C++11)
is_compound (C++11)
is_integral (C++11)
is_floating_point (C++11)
is_fundamental (C++11)
is_arithmetic (C++11)

is_lvalue_reference

(C++11)

is_rvalue_reference

(C++11)

is_member_pointer

(C++11)

is_member_object_pointer

(C++11)

is_member_function_pointer

(C++11)

Type properties

(C++11)

(C++11)

(C++11)

(C++11)

(C++14)

(C++11)

(C++17)

(C++23)

(C++11)(deprecated in C++26)

is_trivially_copyable

(C++11)

is_standard_layout

(C++11)

is_literal_type

(C++11)(until C++20*)

is_pod

(C++11)(deprecated in C++20)

(C++11)

(C++11)

(C++20)

(C++20)

(C++23)

has_unique_object_representations

(C++17)

Type trait constants

integral_constantbool_constanttrue_typefalse_type

(C++11)(C++17)(C++11)(C++11)

Metafunctions

conjunction

(C++17)

disjunction

(C++17)

negation

(C++17)

Supported operations

is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11)
is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible (C++11)(C++11)(C++11)
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)
is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable (C++17)(C++17)(C++17)(C++17)

Relationships and property queries

is_same (C++11)
is_convertibleis_nothrow_convertible (C++11)(C++20)
is_layout_compatible (C++20)
is_pointer_interconvertible_base_of (C++20)
is_pointer_interconvertible_with_class (C++20)
is_corresponding_member (C++20)
reference_constructs_from_temporary (C++23)
reference_converts_from_temporary (C++23)

is_base_of (C++11)
is_virtual_base_of (C++26)
alignment_of (C++11)
rank (C++11)
extent (C++11)
is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)(C++17)(C++17)(C++17)

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)(deprecated in C++23)
aligned_union (C++11)(deprecated in C++23)
decay (C++11)
remove_cvref (C++20)
result_ofinvoke_result (C++11)(until C++20*)(C++17)

conditional (C++11)
common_type (C++11)
common_reference (C++20)
underlying_type (C++11)
type_identity (C++20)
enable_if (C++11)
void_t (C++17)

Compile-time rational arithmetic

Compile-time integer sequences

integer_sequence

(C++14)

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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 typedeftype which is the same asT, except it has a cv-qualifier added (unlessT is a function, a reference, or already has this cv-qualifier)

1) adds bothconst andvolatile

2) addsconst

3) addsvolatile

If the program adds specializations for any of the templates described on this page, the behavior is undefined.

[edit]Member types

Name	Definition
`type`	the type`T` with the cv-qualifier

[edit]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)

[edit]Possible implementation

template<class T>struct add_cv{typedefconstvolatile T type;}; template<class T>struct add_const{typedefconst T type;}; template<class T>struct add_volatile{typedefvolatile T type;};

[edit]Notes

These transformation traits can be used to establishnon-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

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()constvolatile{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-cvConstVolatileConst-volatile

[edit]See also

is_const (C++11)	checks if a type is const-qualified (class template)[edit]
is_volatile (C++11)	checks if a type is volatile-qualified (class template)[edit]
remove_cvremove_constremove_volatile (C++11)(C++11)(C++11)	removesconst and/orvolatile specifiers from the given type (class template)[edit]
as_const (C++17)	obtains a reference toconst to its argument (function template)[edit]