Re: -Wconversion versus libstdc++

[Paolo Carlini <pcarlini at suse dot de>]

Hi Manuel,

> > in a nutshell, what is happening is that a difference of two pointers is
> > assigned to an unsigned variable. First blush, the warning seems to me a
> > bit overzealous, but if we concur that this is what we *really* want for
> > 4.3, we can change the affected (very old, indeed) lines of code, no
> > problem at all.
>
> Hi Paolo,
>
> Wconversion is supposed to warn for conversions that may change a
> value. Can the resulting value change when converted to unsigned? If
> so, the warning is warranted (I think).
>
> Anyhow, Wconversion needs fine-tuning and I prefer for it to be useful
> rather than to literally follow its definition, so if this operation
> is quite frequent and safe, I could detect it and avoid the warning.
>
> I am no GCC or C++ expert, so please give me more information on the
> issue and feedback from others would be appreciated.

Well, just have a look to Gerald' examples: it's old HP / SGI STL code, 
we are assigning / initializing a size_t from a pointer difference, or 
arithmetic, more generally. There are quite a few instances of the 
issue, beyond that pointed out by Gerald. Certainly, the HP / SGI 
engineers considered those assignment harmless, and I respect that kind 
of assessment, but on the other hand, at the time the compilers were 
probably weaker at warning, I'm not sure. All in all, I think we can 
definitely add casts to the library, would be only a few tens of lines 
worth of patch, I think. Whether the warning is useful to the entire GCC 
community, I cannot say... But I hope we can resolve the issue rather 
quickly, because, in case, I'd like to start the audit of the library as 
soon as possible and be done with the issue as far as we are concerned...

Thanks,
Paolo.

PS: well, the issue reminds me the hyper-super-long-standing issue that 
pragma system headers doesn't suppress warnings from templates: as we 
can see, this behavior can be really annoying... probably also very hard 
to change, if I remember correctly... ;-/