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std::to_underlying(3)	       C++ Standard Libary	   std::to_underlying(3)

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(Color-
     Mask::alpha); // OK
      }

Output:
      000000FF
      0000FF00
      00FF0000
      FF000000

See also
	underlying_type obtains the underlying integer type for a given enumera-
     tion 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)

http://cppreference.com 	   2024.06.10		   std::to_underlying(3)
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