Re: C++98/C++11 ABI compatibility for gcc-4.7

["James Y Knight" <foom at fuhm dot net>]

> It seems to be an inadvertent incompatibility caused by the
> interaction of a libstdc++ workaround for a bug and g++ behaviour that
> may not have been known to the libstdc++ devs, so not something that
> could have been prevented by making it a linker error, because noone
> knew it was even broken.  Testing and reporting bugs and analysing the
> problem should lead to it being fixed.  I think the problem was GCC
> devs not knowing the problem existed, rather than not taking it
> seriously.

Whether or not this particular incompatibility was intended or not, the
point remains. You cannot say that GCC devs are taking the C++11 binary
incompatibility issue seriously while:
a) there exist serious ABI incompatibilities between the modes.
b) there is essentially no notice to users about the problem (and lots of
users already brokenly compiling in C++11 mode!)
c) there are no recommendations or plans for how users and distros are
expected to deal with the issue.

I am happy to see that Matthias has also recently noted this issue from
the point of view of a distribution. But I'm *quite* surprised to see that
even now it is apparently a surprise to some on this very list that there
is such an incompatibility!

When I noted the std::list incompatibility back last October when it was
introduced (on http://gcc.gnu.org/PR49561), the response was quite clear
that there is no expectation of compatibility between the modes, and never
was.

When I tried to ask some of these same questions last November (in the
"Long-term plan for C++98/C++11 incompatibility" thread), the answer I
heard there seemed to me to be ~"distros should just provide two sets of
libraries, not our problem".

IMO, at the /very least/, libstdc++ should go ahead and change std::string
to be the new implementation. Once std::string is ABI-incompatible between
the modes, there's basically no chance that anyone would think that
linking things together from the two modes is a good thing to try, for
more than a couple minutes.