std::to_underlying

 

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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	Value	Std	Feature
__cpp_lib_to_underlying	202102L	(C++23)	std::to_underlying

Example

Run this code

#include <cstdint>
#include <iomanip>
#include <iostream>
#include <type_traits>
#include <utility>
 
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))>);
 
int main()
{
    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)