Re: g++ and aliasing bools

[Daniel Berlin <dan at dberlin dot org>]

On Mon, 28 Jan 2002, Mark Mitchell wrote:

> 
> [ zero-sized based class example elided ]
> 
> >
> > But the C aliasing rules completely ignore the question of size, so you're
> > bringing up a red herring.  The rules have *nothing* to do with whether we
> > think that two objects have the same address!  It doesn't even come up.
> > For that reason, you don't have to waste any time thinking about class
> > layout.
> >
> 
> Joe's argument is a reduction from C++ to C; the claim is that (basically)
> the C++ alias rules are the same as C and that certain C++ classes are
> laid out like certain C structs and that there are no additional
> "dangerous" operations and therefore correctness for C implies correctness
> for C++.

Um, that's Joe talking, i think we are getting attribution messed up. 
Unless you really meant to talk about Joe as "Joe's argument", rather than 
"His argument".


> 
> The argument doesn't work in this case -- the proposed isomorphism from
> C++ classes to C structs is not correct.

> 
> Joe's argument either needs to handle zero-sized classes in some other
> way (arguing that they're not dangerous for some other reason), or it
> needs to not apply to class hierarchives that involve zero-sized bases,
> treating such things as not simple_enough.

*I* already argued that they aren't dangerous for another reason.  In C, 
since they would have the same address, they would alias.  In reality, 
in C++, they *don't* alias because they *don't* have the same address.  
However, it's perfectly fine to say things alias that don't.
Thus, it's not dangerous.

--Dan