Questions tagged [enum-class]

Enum classes address three problems with traditional C++ enumerations:

• Implicit int conversion.
• Enumerators exported to the surrounding scope.
• Their underlying type cannot be specified.

enum class are strongly typed and scoped:

// traditional enum
enum Alert { green, yellow, orange, red };

// scoped and strongly typed enum
// no export of enumerator names into enclosing scope
enum class Color { red, blue };   

// no implicit conversion to int
enum class TrafficLight { red, yellow, green };

Alert a = 7;              // error (as ever in C++)
Color c = 7;              // error: no int->Color conversion

int a2 = red;             // ok: Alert->int conversion
int a3 = Alert::red;      // error in C++98; ok in C++11
int a4 = blue;            // error: blue not in scope
int a5 = Color::blue;     // error: not Color->int conversion

Color a6 = Color::blue;   // ok

underlying type can be specified

Being able to specify the underlying type allow simpler interoperability and guaranteed sizes of enumerations:

enum class Color : char { red, blue };

// by default, the underlying type is int
enum class TrafficLight { red, yellow, green };

// how big is an E? (whatever the rules say; i.e. "implementation defined")
enum E { E1 = 1, E2 = 2, Ebig = 0xFFFFFFF0U };   

// now we can be specific
enum EE : unsigned long { EE1 = 1, EE2 = 2, EEbig = 0xFFFFFFF0U };

forward declaration enabled

enum class Color_code : char;     // (forward) declaration
void foobar(Color_code* p);       // use of forward declaration

enum class Color_code : char { red, yellow, green, blue }; // definition