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Re: reusing identifier in two enums

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ImpalerCore <jadill33@gmail.com> writes:
> I have two related but separate enumerations where I'd like to use the
> same identifier to represent an invalid type.
>
> enum typeA
> {
>   invalid_type = -1,
>   typeA = 0,
>   typeB = 1,
>   ...
> };
>
> enum type0
> {
>   invalid_type = -1,
>   type0 = 0,
>   type1 = 1,
>   ...
> };
>
> Is it okay to for these enumerations to share the same identifier as
> long as they have the same value in all enumerations that use them?
> From my limited experience with the standard, I couldn't pull out
> whether this was defined behavior.

It's a constraint violation, assuming both types are declared in the
same scope.

Quoting N1570:

6.7.2.2:
    The identifiers in an enumerator list are declared as constants that
    have type int and may appear wherever such are permitted.

So your declaration of invalid_type is equivalent to:

    const int invalid_type = -1;

A footnote makes it even clearer:

    Thus, the identifiers of enumeration constants declared in the same
    scope shall all be distinct from each other and from other
    identifiers declared in ordinary declarators.

And 6.7p3 says:

    If an identifier has no linkage, there shall be no more than one
    declaration of the identifier (in a declarator or type specifier)
    with the same scope and in the same name space, except that:
    [...]
(The exceptions don't apply in this case.)

This is a constraint, so a diagnostic is required.  A compiler *could*
issue a warning and accept the declaration anyway, but I don't know of
any that do so.

If you had tried to compile your example, the compiler would have
rejected it; for example, gcc gives me:

    c.c:10:18: error: redeclaration of enumerator ‘invalid_type’
    c.c:3:18: note: previous definition of ‘invalid_type’ was here

This doesn't prove that it's invalid (it *could* be a compiler bug), but
it's a strong indication.

You could declare the "invalid_type" enumeration constant with a
different name in each enum type, then declare a single:

    enum { invalid_type = -1 };

and use that identifier everywhere.  Enumeration constants aren't as
strongly associated with their types as you might expect; they're all of
type int, so there's no real type checking.  (C++ has different rules,
so if you want your code to be compatible with C++ for some reason
you'll have to be more careful.)

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
    Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Thread

reusing identifier in two enums ImpalerCore <jadill33@gmail.com> - 2012-03-19 08:04 -0700
  Re: reusing identifier in two enums "Morris Keesan" <mkeesan@post.harvard.edu> - 2012-03-19 12:19 -0400
    Re: reusing identifier in two enums ImpalerCore <jadill33@gmail.com> - 2012-03-19 10:43 -0700
  Re: reusing identifier in two enums Keith Thompson <kst-u@mib.org> - 2012-03-19 10:43 -0700
    Re: reusing identifier in two enums ImpalerCore <jadill33@gmail.com> - 2012-03-19 11:42 -0700
  Re: reusing identifier in two enums jgk@panix.com (Joe keane) - 2012-03-21 02:45 +0000
    Re: reusing identifier in two enums Jens Gustedt <jens.gustedt@loria.fr> - 2012-03-21 17:04 +0100

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