Re: libstdc++ c++98 & c++11 ABI incompatibility

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

On 07/02/2012 07:24 PM, Richard Guenther wrote:
> On Mon, Jul 2, 2012 at 7:00 PM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> > On 2 July 2012 17:43, Jeff Law <law@redhat.com> wrote:
> > > On 07/02/2012 10:26 AM, Michael Meeks wrote:
> > > > On Thu, 2012-06-14 at 15:14 +0200, Matthias Klose wrote:
> > > > > While PR53646 claims that c++98 and c++11 should be ABI
> > > > > compatible (modulo bugs), the addition of the _M_size member
> > > > > to std::_List_base::_List_impl makes libraries using
> > > > > std::list in headers incompatible
> > > >
> > > >          This is pretty nasty for LibreOffice (and no doubt others). We
> > > > can, and
> > > > often do depend on rather a number of system C++ libraries and at a very
> > > > minimum, having no simple way to detect which C++ ABI we have to build
> > > > against 'old' vs. 'new' - is profoundly unpleasant.
> > > >
> > > >          Is there no chance of having a bug fix that is a revision of the
> > > > (unintended?) ABI breakage in this compiler series ?
> > > That's the direction I'd prefer to see (reversion until we're ready to make
> > > the wholesale ABI changes).  Not sure what the libstdc++ maintainers are
> > > thinking right now.
> > I'm wondering why the libstdc++ list was taken out of the CC list ;-)
> >
> > I don't know what the others think but rather than just reverting it
>  From a RM point of view please go ahead and revert unintended ABI
> breakage on all affected branches.  Add an entry to the respective
> changes file to warn users about the incompatibility present on branches.
I cannot say to have followed all the details of this discussion 
(neither to fully agree with quite a few statements I read in it ;) but 
since I added the _M_size member in 4_7-branch (to fix 49561 and of 
course in order to provide C++11 conforming complexities for the various 
operations) I'm simply going to revert the change from branch and 
mainline. Consider it done. Then, it would be great if Jon could devise 
something more sophisticated, not throwing away the baby, so to speak ;)

I also want to mention (I don't think somebody did already in the 
thread) that at some point, with Jason too, we discussed the idea of 
adding to each binary a marker about the ABI version which then would be 
used by the linker to warn or error out. This vague idea goes  of course 
well beyond our specific needs for the C++98 conforming std::list vs the 
C++11 conforming version of it.

Thanks,
Paolo.