Hello
Is enum type signed or unsigned? Is the Signedness of enums differ in C/C99/ANSI C/C++/C++x/GNU C/ GNU C99?
Thanks
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A:
An enum is guaranteed to be represented by an integer, but the actual type (and its signedness) is implementation-dependent.
You can force an enumeration to be represented by a signed type by giving one of the enumerators a negative value:
enum SignedEnum { a = -1 };
In C++0x, the underlying type of an enumeration can be explicitly specified:
enum ShortEnum : short { a };
(C++0x also adds support for scoped enumerations)
For completeness, I'll add that in The C Programming Language, 2nd ed., enumerators are specified as having type int (p. 215). K&R is not the C standard, so that's not normative for ISO C compilers, but it does predate the ISO C standard, so it's at least interesting from a historical standpoint.
James McNellis
2010-04-05 15:38:09
What signedness is actually used by gcc?
osgx
2010-04-05 15:46:23
@osgx: I would guess that it would depend on the number of enumerators and the range of their values. I really don't know.
James McNellis
2010-04-05 16:02:54
The C standard also specifies that each *enumeration constant* has type `int`. Howeve, the term “enumeration constant” refers to the value constants declared inside the `enum { }` block. A variable with an `enum` type may have any integer type in C, e.g. it might be a shorter type than `int` if one can represent all the values. (GCC, for example, has an option `-fshort-enums` for doing precisely this.)
Arkku
2010-04-05 16:14:22
The term "enumerator" is commonly understood as the enumeration constant, and AFAIK inside the `enum{}` they never have a type smaller than `int` because the compiler doesn't know yet what values will follow.
MSalters
2010-04-06 15:39:44
@MSalters @Arkku: That's true; in C, enumeration constants are always of type `int`. In C++, they are of the same type as their initializer, so they could be of any integral type.
James McNellis
2010-04-06 16:03:36