Is this enum declaration a standard compliant?

Question

So normally enum's are for declaring group of "constant integers" as another type, that represents something. Eg.

enum Color {RED=0, BLUE, YELLOW};

This is clear. But recently I met following in code. This was in a compiler for embedded systems.

enum State {DISABLED=0, ENABLED=!DISABLED};

And it worked just fine. It behaved as a boolean type. My question is, wheather it (this syntax) is ANSI compliant?

If it is standard compliant, then why would compilers define internally something like _Bool for boolean representation and then in stdbool.h (for C language) they do:

#define bool _Bool
... // here goes definitions of true and false

instead of

enum bool {false=0, true=!false};

Which is much cleaner?

Answer 1

Yes, this is standard-compliant.

!DISABLED is a valid constant expression, which is all that is required for an enum value.

enum State {DISABLED=0, ENABLED= (!DISABLED)};
//                               ^^^^^^^^^^^

At the point where DISABLED is referenced, the compiler knows its value, so it can compute the value of the expression derived from it, i.e. !DISABLED. It is a fancy way of writing ENABLED=1.