std::underlying_type

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Containers library
Iterators library
Ranges library(C++20)
Algorithms library
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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 underlying_type;		(since C++11)

IfT is a complete enumeration (enum) type, provides a member typedeftype that names the underlying type ofT.

Otherwise, the behavior is undefined.	(until C++20)
Otherwise, if`T` is not an enumeration type, there is no member`type`. Otherwise (`T` is an incomplete enumeration type), the program is ill-formed.	(since C++20)

If the program adds specializations forstd::underlying_type, the behavior is undefined.

[edit]Member types

Name	Definition
`type`	the underlying type of`T`

[edit]Helper types

template<class T> using underlying_type_t=typename underlying_type<T>::type;		(since C++14)

[edit]Notes

Eachenumeration type has anunderlying type, which can be

Specified explicitly (both scoped and unscoped enumerations);
Omitted, in which case it isint for scoped enumerations or an implementation-defined integral type capable of representing all values of the enum (for unscoped enumerations).

[edit]Example

Run this code

#include <iostream>#include <type_traits> enum e1{};enumclass e2{};enumclass e3:unsigned{};enumclass e4:int{}; int main(){constexprbool e1_t=std::is_same_v<std::underlying_type_t<e1>,int>;constexprbool e2_t=std::is_same_v<std::underlying_type_t<e2>,int>;constexprbool e3_t=std::is_same_v<std::underlying_type_t<e3>,int>;constexprbool e4_t=std::is_same_v<std::underlying_type_t<e4>,int>; std::cout<<"underlying type for 'e1' is "<<(e1_t?"int":"non-int")<<'\n'<<"underlying type for 'e2' is "<<(e2_t?"int":"non-int")<<'\n'<<"underlying type for 'e3' is "<<(e3_t?"int":"non-int")<<'\n'<<"underlying type for 'e4' is "<<(e4_t?"int":"non-int")<<'\n';}

Possible output:

underlying type for 'e1' is non-intunderlying type for 'e2' is intunderlying type for 'e3' is non-intunderlying type for 'e4' is int

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2396	C++11	incomplete enumeration types were allowed	complete enumeration type required

[edit]See also

is_enum (C++11)	checks if a type is an enumeration type (class template)[edit]
is_scoped_enum (C++23)	checks if a type is a scoped enumeration type (class template)[edit]
to_underlying (C++23)	converts an enumeration to its underlying type (function template)[edit]