NAME¶

std::to_underlying - std::to_underlying

Synopsis¶

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

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