Signedness of enum in C/C99/C++/C++x/GNU C/GNU C99

Question

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

Answer 1

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.

Answer 2

This is an old question... but I've just found out this:

typedef unsigned ENUMNAME;  // this makes it unsigned in MSVC C 2015
typedef enum {v0, v1, v2, v3} ENUMNAME;

You can use it as an 2-bit unsigned index, for example:

typedef struct {
  ENUMNAME i:2;
} STRUCTNAME;

Tried it in GCC ARM - doesn't work.
Also, WinDbg shows STRUCTNAME.i as a number, not as v0-v3.