std::to_underlying

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定义于头文件 <utility>
template< class Enum >
constexpr std::underlying_type_t<Enum> to_underlying( Enum e ) noexcept;
(C++23 起)

转换枚举到其底层类型。等价于 return static_cast<std::underlying_type_t<Enum>>(e);

参数

e - 要转换的枚举值

返回值

Enum 的底层类型的整数值,从 e 转换得到。

注解

std::to_underlying 能用于避免将枚举类型转换成其底层类型以外的整数类型。

示例

#include <cstdint>
#include <iomanip>
#include <iostream>
#include <type_traits>
#include <utility>
 
namespace cxx {
#if defined(__cpp_lib_to_underlying)
    using std::to_underlying;
#else
    template <class Enum>
    constexpr std::underlying_type_t<Enum>
    to_underlying(Enum e) noexcept {
        return static_cast<std::underlying_type_t<Enum>>(e);
    }
#endif
}
 
int main()
{
    enum class E1 : char { e };
    static_assert(std::is_same_v<char, decltype(cxx::to_underlying(E1::e))>);
    enum struct E2 : long { e };
    static_assert(std::is_same_v<long, decltype(cxx::to_underlying(E2::e))>);
    enum E3 : unsigned { e };
    static_assert(std::is_same_v<unsigned, decltype(cxx::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) << cxx::to_underlying(ColorMask::red) << '\n'
        << std::setw(8) << cxx::to_underlying(ColorMask::green) << '\n'
        << std::setw(8) << cxx::to_underlying(ColorMask::blue) << '\n'
        << std::setw(8) << cxx::to_underlying(ColorMask::alpha) << '\n';
 
//  std::underlying_type_t<ColorMask> x = ColorMask::alpha; // 错误:无已知转换
    [[maybe_unused]]
    std::underlying_type_t<ColorMask> y = cxx::to_underlying(ColorMask::alpha); // OK
}

输出:

000000FF
0000FF00
00FF0000
FF000000

参阅

获取给定枚举类型的底层整数类型
(类模板)