Re: [gentoo-dev] How to support C++11 in libraries?

[Michał Górny]

Dnia 2013-12-19, o godz. 09:43:40
Jan Kundrát <jkt@g.o> napisał(a):

> On Thursday, 19 December 2013 02:41:55 CEST, heroxbd@g.o wrote:
> > I'd like to make an analogy to the version bump of gcc[1]. We (gentoo)
> > decide to support c++11 officially or not. If so, open a tracker bug to
> > push it globally. If not, patch lldb to support non-c++11, or leave it
> > up to the user to fiddle with the CXXFLAGS, where we only point the user
> > by to proper docs.
>
> To be honest, I do not really see a link between the "let's bring in a new
> version of compiler, it is a bit stricter in some situations, but these
> were bugs anyway, like missing headers or unfounded assumptions about
> memcpy()" with "let's support a new version of language which produces
> object files with different ABI". Perhaps a Python 2.6 vs. Python 3.3 is a
> better analogy?

Well, it's even worse than that. I think the main difference is that
usual gcc/whatever bumps may have resulted in *one* different libstdc++
ABI. People rebuilt all their packages, world went back to normal.

The issue here is that gcc is providing two ABIs in parallel, with
a -std= switch. And this sucks pretty much...

> Anyway, GCC 4.8 is pretty clear that the C++11's support is still
> experimental [1] and subject to change [2]. The upstream developers have
> announced that they plan to break the ABI of the code using C++11 features
> in 4.9 [3].

Would it be possible to have a consistent ABI for both C++03 and C++11?
The simpler changes like adding new fields can be backported quite
easily (even if it would mean having dummy fields in C++03), I have no
idea about the more complex changes.

> Right now, it seems that we shall wait at least for GCC 4.9 to come and for
> upstream to decide how to solve this properly.

Well, if they considered the C++11 ABI in gcc-4.9 stable, we could
consider changing the default to C++11. Then, we could do our
bump/switch thing as a matter of gcc-4.9 upgrade problem.

And that brings another issue in Gentoo -- gcc-config. AFAIR this tool
is completely insane and switches libstdc++ along with gcc version.
As a result, after switching to a gcc version with different C++ ABI,
installed software gets broken. And you can't really fix it without
going through the broken-system state or some hackery.

It would be much better if the switching was done by some ebuild. We
could then use subslots to force rebuilds of stuff using libstdc++.
Well, more than that, preserved-libs would prevent disappearing old
libstdc++ from breaking stuff.

But well, that's just my wishes...

-- 
Best regards,
Michał Górny