Forward Declarations for Enumerations

Under the new standard, we can forward declare an enum. An enum forward declaration must specify (implicitly or explicitly) the underlying size of the enum:

// forward declaration of unscoped enum named intValues
enum intValues : unsigned long long; // unscoped, must specify a type
enum class open_modes;  // scoped enums can use int by default

Because there is no default size for an unscoped enum, every declaration must include the size of that enum. We can declare a scoped enum without specifying a size, in which case the size is implicitly defined as int.

As with any declaration, all the declarations and the definition of a given enum must match one another. In the case of enums, this requirement means that the size of the enum must be the same across all declarations and the enum definition. Moreover, we cannot declare a name as an unscoped enum in one context and redeclare it as a scoped enum later:

// error: declarations and definition must agree whether the enum is scoped or unscoped
enum class intValues;
enum intValues;  // error: intValues previously declared as scoped enum
enum intValues : long; // error: intValues previously declared as int