Notes

Named integer constants, grouped under one type.

Plain enum — leaks its names, implicitly converts to int

enum Color { Red, Green, Blue };
Color c = Red;
int x = c;          // implicit conversion to int - usually not what you want

Red, Green, Blue all leak into the surrounding scope — a second enum with a member also named Red in the same scope won’t compile.

enum class — scoped, no implicit conversion

enum class Color { Red, Green, Blue };
Color c = Color::Red;   // must qualify with Color::
int x = c;               // error - no implicit conversion
int y = static_cast<int>(c);   // fine, if you actually want the int

Names are scoped under Color::, so two different enums can both have a Red with zero conflict. No accidental int conversion means no accidentally comparing a Color to an unrelated int and having it silently compile.

Default: use enum class

Plain enum mostly exists for backward compatibility with C. New code should reach for enum class unless there’s a specific reason to want the implicit int conversion.

Related

static