Re: generalized lvalues

[Michael Matz <matz@suse.de>]

Hi,

On Thu, 18 Nov 2004, Matt Austern wrote:

> But I'll give two examples (details changed partly to protect the guilty and
> partly because I'm doing this by memory):
> (1) #define foo(x) *(a, b, x)
> (2) ((long*) p)++

Actually, from my experience looking at assorted packages which are tried
to be build with 4.0, the latter use is most common.  I.e. casting a
pointer to some type to implicitely specify how much it is incremented.  
Some software is very much known for using such constructs (wine, rpm,
elfutils, dietlibs, many others) .  I even tend to say, that such usage is
well defined (within certain limits), although I admit that it's not ISO
C99.

<rant>
All the whining about how unspecified this is, and how the writer of the
code deserve to be bitten, is just language lawyer masturbation (which I
also very much enjoy when closing bugreports about this or that compiler
bug with INVALID), but it doesn't change the fact, that there are quite
some packages out there actually using such extensions.  This 
cast-as-lvalue extension removal is one of the bigger items which result 
in much work for package maintainers (not so much for C++, as there anyway 
this extension wasn't used much).
</rant>

So, if anyone wants to bring it back, with defined semantics of course
(like for instance memory accessed through a casted pointer is well
defined (not outside the memory for the object accessed), and only for C)  
I'm all ears.


Ciao,
Michael.