Enumeration declaration

#include <cstdint>
#include <iostream>
 
// enum that takes 16 bits
enum smallenum: std::int16_t
{
 a,
 b,
 c
};
 
// color may be red (value 0), yellow (value 1), green (value 20), or blue (value 21)
enum color
{
 red,
 yellow,
 green = 20,
 blue
};
 
// altitude may be altitude::high or altitude::low
enum class altitude: char
{
 high = 'h',
 low = 'l', // trailing comma only allowed after CWG 518
}; 
 
// the constant d is 0, the constant e is 1, the constant f is 3
enum
{
 d,
 e,
 f = e + 2
};
 
// enumeration types (both scoped and unscoped) can have overloaded operators
std::ostream & operator<<(std::ostream & os, color c)
{
 switch(c)
 {
 case red : os << "red"; break;
 case yellow: os << "yellow"; break;
 case green : os << "green"; break;
 case blue : os << "blue"; break;
 default : os.setstate(std::ios_base::failbit );
 }
 return os;
}
 
std::ostream & operator<<(std::ostream & os, altitude al)
{
 return os << static_cast<char>(al);
}
 
// The scoped enum (C++11) can be partially emulated in earlier C++ revisions:
 
enum struct E11 { x, y }; // since C++11
 
struct E98 { enum { x, y }; }; // OK in pre-C++11
 
namespace N98 { enum { x, y }; } // OK in pre-C++11
 
struct S98 { static const int x = 0, y = 1; }; // OK in pre-C++11
 
void emu()
{
 std::cout << (static_cast<int>(E11::y) + E98::y + N98::y + S98::y) << '\n'; // 4
}
 
namespace cxx20
{
 enum class long_long_long_name { x, y };
 
 void using_enum_demo()
 {
 std::cout << "C++20 `using enum`: __cpp_using_enum == ";
 switch (auto rnd = []{return long_long_long_name::x;}; rnd())
 {
#if defined(__cpp_using_enum)
 using enum long_long_long_name;
 case x: std::cout << __cpp_using_enum << "; x\n"; break;
 case y: std::cout << __cpp_using_enum << "; y\n"; break;
#else
 case long_long_long_name::x: std::cout << "?; x\n"; break;
 case long_long_long_name::y: std::cout << "?; y\n"; break;
#endif
 }
 }
}
 
int main()
{
 color col = red;
 altitude a;
 a = altitude::low;
 
 std::cout << "col = " << col << '\n'
 << "a = " << a << '\n'
 << "f = " << f << '\n';
 
 cxx20::using_enum_demo();
}

col = red
a = l
f = 3
C++20 `using enum`: __cpp_using_enum == 201907; x