"TCPL/A.08.04_Enumerations"版本比较

A.8.4 Enumerations

Enumerations are unique types with values ranging over a set of named constants called enumerators. The form of an enumeration specifier borrows from that of structures and unions.

• enum-specifier:
  • enum identifieropt { enumerator-list } enum identifier
  enumerator-list:
  • enumerator enumerator-list , enumerator
  enumerator:
  • identifier identifier = constant-expression

The identifiers in an enumerator list are declared as constants of type int, and may appear wherever constants are required. If no enumerations with = appear, then the values of the corresponding constants begin at 0 and increase by 1 as the declaration is read from left to right. An enumerator with = gives the associated identifier the value specified; subsequent identifiers continue the progression from the assigned value.

Enumerator names in the same scope must all be distinct from each other and from ordinary variable names, but the values need not be distinct.

The role of the identifier in the enum-specifier is analogous to that of the structure tag in a struct-specifier; it names a particular enumeration. The rules for enum-specifiers with and without tags and lists are the same as those for structure or union specifiers, except that incomplete enumeration types do not exist; the tag of an enum-specifier without an enumerator list must refer to an in-scope specifier with a list.

• Enumerations are new since the first edition of this book, but have been part of the language for some years.