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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: enums in structs
Date: Wed, 14 Mar 2018 17:57:07 -0700
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bartc <bc@freeuk.com> writes:
> I thought I knew most of the C quirks, but here's another one which I 
> don't think I've seen before (perhaps with enum tags, not names):
>
>    typedef struct {enum {red,green,blue} x;} S;

The typedef is irrelevant.  This:

    struct S {
        enum { red, green, blue } x;
    };

illustrates the same thing.

> Despite being declared inside the {...} of the struct, the scope of red, 
> green and blue belong to the outer scope (the one containing the typedef).

Not all curly braces define scopes.  Take a look at N1570 6.2.1 "Scopes
of identifiers".  There's no such thing as "struct scope".  The only
scopes are:

- Function scope (applies only to labels)
- File scope
- Block scope (refers to a block / compound statement)
- Function prototype scope (not often used)

Note that the member name "x" also has a wider scope than the struct.
But member names always have to be prefixed with "s." or "p->".

> Which also means that:
>
>    typedef struct {enum {red,green,blue} x;} S;
>    typedef struct {enum {red,green,blue} x;} T;
>
> won't work, as the names will clash.

Right.

If "red", "green", and "blue" had scopes restricted to the struct
definition, you wouldn't be able to refer to their names outside the
struct definition.  C has no mechanism for prefixing an enumeration
constant with the name of a type.

(You'd usually want to define the enum type separately -- or at least I
would.)

[...]

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"