Re: rfc: macros accepting constructors

[Daniel Berlin <dberlin at dberlin dot org>]

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.

You can't reconcile (({})) with statement expressions, unless you are 
going to do something hackish like distinguish based on the 
presence of a semicolon.

--Dan




> 
> -Tim
> 
> 
>