std::to_underlying
来自cppreference.com
定义于头文件 <utility>
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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
运行此代码
#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
(C++11) |
获取给定枚举类型的底层整数类型 (类模板) |