The following are keywords and cannot be used as identifiers:
Only unsigned constants can be used
as size specifications for arrays. If an identifier is used, it must have
been declared previously as an unsigned constant in a
Constant and type identifiers within the scope of a specification are
in the same namespace and must be declared uniquely within this scope.
Similarly, variable names must be unique within the scope of
union declarations. Nested
union declarations create new scopes.
The discriminant of a
union must be of a type that evaluates
to an integer. That is, it must be an
enum type, or any
typedef that evaluates to one of these. Also, the case
values must be legal discriminant values. Finally, a case
value cannot be specified more than once within the scope of a