Hello,
On Fri, 4 Apr 2025, Qing Zhao wrote:
> So, a different attribute name “counted_by_exp” might be better?
I would prefer Martins empty-decl idea to that: "counted_by(;len+0)"
(looks up 'len' normally, i.e. doesn't look into current struct). It
would naturally fit the either decl+expr or lone-ident parse.
It may look weird but empty declarations are okayish IMHO.
But overall: I just don't know, it all looks a bit unsexy, there only seem
to be rocks and hard places :)
Ciao,
Michael.
>
> Qing
>
> > On Apr 4, 2025, at 11:55, Michael Matz <m...@suse.de> wrote:
> >
> > Hello,
> >
> > On Fri, 4 Apr 2025, Qing Zhao wrote:
> >
> >> A:
> >> constexpr int len = 20;
> >> struct s {
> >> int len;
> >> int *buf __attribute__ ((counted_by (len))); // this continues to be
> >> member ‘len’, not global ‘len'
> >> };
> >>
> >> B:
> >> constexpr int len = 20;
> >> struct s {
> >> int len;
> >> int *buf __attribute__ ((counted_by (len+0))); //this is an expression ,
> >> ‘len' refers to the global;
> >> };
> >>
> >> When the parser is parsing the first identifier “len” inside the
> >> counted_by attribute, it cannot decide which syntax to use yet, it must
> >> look ahead at least one more word to decide, is this okay for the
> >> current C parser?
> >
> > As I understood Bills proposal (but see below) a full expression that
> > isn't a lone identifier would always require the decl+expression syntax,
> > so the above would lead to a syntax error and wouldn't require further
> > look-ahead. ('len' doesn't introduce a type, hence it can't be decl+expr,
> > hence it must be lone-ident, which then generates the syntax error on
> > seeing '+', after having successfully looked up 'len' among
> > the struct members).
> >
> > But I now realize that I may have misunderstood the proposal in the cace
> > that the expression does not in fact contain references to any struct
> > members, e.g.
> >
> > enum {FOO=42};
> > struct s {
> > int len;
> > int *buf __attribute__((counted_by( /*???*/ FOO + 0))); // no use of len
> > };
> >
> > The proposal doesn't specifically talk about this case. Clearly there is
> > no need to locally declare anything (at ???), but that would have been the
> > syntactic hint to differentiate between both branches in the proposal.
> > So, ... hmm, that would seem to again introduce the ambiguity between
> > 'lone-ident' and 'expression'. I'm not sure how Bill wants to handle
> > that. One could requre a useless dummy declaration, but that would be
> > meh.
> >
> >
> > Ciao,
> > Michael.
>
>
>
