std::to_underlying

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison (C++20) three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)    is_eqis_neqis_ltis_lteqis_gtis_gteq (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator> rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)    cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17)

 

Defined in header <utility>
template< class Enum > constexpr std::underlying_type_t<Enum> to_underlying( Enum e ) noexcept;		(since C++23)

Converts an enumeration to its underlying type. Equivalent to return static_cast<std::underlying_type_t<Enum>>(e);.

Parameters

e

-

enumeration value to convert

Return value

The integer value of the underlying type of Enum, converted from e.

Notes

std::to_underlying can be used to avoid converting an enumeration to an integer type other than its underlying type.

Feature-test macro
__cpp_lib_to_underlying

Example

Run this code

#include <cstdint>
#include <iomanip>
#include <iostream>
#include <type_traits>
#include <utility>
 
int main()
{
    enum class E1 : char { e };
    static_assert(std::is_same_v<char, decltype(std::to_underlying(E1::e))>);
    enum struct E2 : long { e };
    static_assert(std::is_same_v<long, decltype(std::to_underlying(E2::e))>);
    enum E3 : unsigned { e };
    static_assert(std::is_same_v<unsigned, decltype(std::to_underlying(e))>);
 
    enum class ColorMask : std::uint32_t {
        red = 0xFF, green = (red << 8), blue = (green << 8), alpha = (blue << 8)
    };
    std::cout << std::hex << std::uppercase << std::setfill('0')
        << std::setw(8) << std::to_underlying(ColorMask::red) << '\n'
        << std::setw(8) << std::to_underlying(ColorMask::green) << '\n'
        << std::setw(8) << std::to_underlying(ColorMask::blue) << '\n'
        << std::setw(8) << std::to_underlying(ColorMask::alpha) << '\n';
 
//  std::underlying_type_t<ColorMask> x = ColorMask::alpha; // Error: no known conversion
    [[maybe_unused]]
    std::underlying_type_t<ColorMask> y = std::to_underlying(ColorMask::alpha); // OK
}

Output:

000000FF
0000FF00
00FF0000
FF000000

See also

underlying_type (C++11)	obtains the underlying integer type for a given enumeration type (class template)
is_enum (C++11)	checks if a type is an enumeration type (class template)
is_scoped_enum (C++23)	checks if a type is a scoped enumeration type (class template)