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Message-ID: <562AE5B8.6080307@gmail.com>
Date: Fri, 23 Oct 2015 19:58:16 -0600
From: Martin Sebor <msebor@gmail.com>
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To: "sc22wg14@open-std.org" <sc22wg14@open-std.org>
Subject: what's a member-designator?
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I've been spending some quality time with the offsetof expression
which is defined like so in 7.19 Common definitions <stddef.h>:

       offsetof(type, /member-designator/)

   which expands to an integer constant expression that has type
   size_t, the value of which is the offset in bytes, to the
   structure member (designated by /member-designator/), from
   the beginning of its structure (designated by type). The type
   and member designator shall be such that given

       static type t;

   then the expression &(t./member-designator/) evaluates to
   an address constant. (If the specified member is a bit-field,
   the behavior is undefined.)

/member-designator/ is italicized as a term of the grammar but
isn't defined anywhere in C or in C++ (which shares mostly the
same definition).

This is of interest because the term has been interpreted as
(IIUC) synonymous with identifier by an implementation that
diagnoses offsetof expressions where the /member-designator/
is anything else, such as b.i in

   offsetof (struct A, b.i)

where struct A is defined as

   struct A { struct B { int i; } b; };

Martin

PS This issue has been raised before, in Item 32, N847 - Issues
with CD2 of C99:

   http://www.open-std.org/jtc1/sc22/wg14/www/docs/n847.htm

But I'm not having any luck finding a response.