On Wed, 12 Mar 2025, Martin Uecker wrote:
> For a designator
>
> struct foo { int n; } a = { .n = 1 };
>
> we also refer to a member 'n' of an instance 'a' of a structure type.
> The instance is simply implied by the context.
>
> For
>
> struct foo { int n; char *x __counted_by(.n) };
>
> is also refers to a member of an instance of the struct. The
> instance is the 'a' which is later used in an expression 'a.x'
> So the instance would again be implied by the context.
>
> So for me this makes perfect sense in both cases (and
> for both C and C++)
The main concern with the designator syntax is if you try to embed it in
arbitrary expressions (that is, say that __counted_by takes an expression,
but with an additional kind of primary-expression .IDENTIFIER that can be
used as a sub-expression therein). The above is fine, but
struct foo { int n; char *x __counted_by((struct bar){.n = 1}.n };
leaves an ambiguity of whether ".n = 1" is a designated initializer in the
struct bar compound literal, or an assignment expression where .n refers
to the member of the struct foo for which the number of elements of x is
being counted. Note that N3188 definitely does not allow .IDENTIFIER as
part of an expression, only as an alternative to an expression in an array
declarator.
--
Joseph S. Myers
[email protected]
