Re: rfc: macros accepting constructors

[Bo Thorsen <bo at sonofthor dot dk>]

On Friday 28 June 2002 09:29, Andreas Schwab wrote:
> Daniel Berlin <dberlin@dberlin.org> writes:
> |> On Thu, 27 Jun 2002, Tim Hollebeek wrote:
> |> > On Thu, Jun 27, 2002 at 10:02:32AM -0700, Aldy Hernandez wrote:
> |> > > > Perhaps the best suggestion (if anything is desirable) is to
> |> > > > modify grammar so that ()'s are allowed around any grammar element
> |> > > > which is surrounded by {}'s.  Then all you have to do is remember
> |> > > > to use ({ and
> |> > >
> |> > > this already works.
> |> >
> |> > I think you misunderstood.  The part of the grammar I was proposing to
> |> > modify was the C grammar, not the preprocessor grammar.  What I
> |> > suggested most definitely does NOT work right now:
> |> >
> |> > d1.c: void foo(void) ({ int x, y; })
> |> > d2.c: int x[2] = ({ 0, 0 });
> |> >
> |> > d1.c:1: parse error before `{'
> |> > d2.c:1: braced-group within expression allowed only inside a function
> |> >
> |> > > but foo({1,2}) doesn't.
> |> >
> |> > What I'm proposing would allow foo({1,2}) to safely be rewritten as
> |> > foo(({1,2})) regardless of how the { ends up getting used.
> |>
> |> This conflicts with statement expressions (a gcc extension), probably
> |> irreconciably.
> |>
> |> ({1,2}) is an almost valid statement expression with the value "2".
> |>
> |> (It's invalid because you don't have a semicolon at the end of the 2)
> |>
> |> Thus, foo(({1,2;})) would be equivalent to foo(2).
> |> Try it, you'll get a "too many arguments" error if you have "void
> |> foo(void)", rather than some other weird error.
>
> Going back to the OP's question, it was about this:
>
> 	#define foo(x) bar((x))
>
> 	foo((vector int){0, 1, 2, 3});
>
> Since you can put parens around *every* valid expression, what is wrong
> with writing this?
>
> 	foo(((vector int){0, 1, 2, 3}));

I'm probably wrong, but to me it sounded like the guys where smoking the old 
"but macros should behave exactly like functions" pipe. Looks like the smoke 
haven't cleared yet.

Bo.

-- 

     Bo Thorsen                 |   Praestevejen 4
     Free software developer    |   5290 Marslev
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