Re: Using C++11

[Murray Cumming <murrayc murrayc com>]

On Tue, 2015-06-30 at 12:06 +0200, Murray Cumming wrote:
> > > And would you agree that Ubuntu (for instance) will have to do the
> > > same thing when C++11 (--std=c++11) becomes the default in g++ 6,
> > > when they use g++ 6?
> >
> > No.  Assuming gcc-6 still provides libstdc++ with both ABIs (my
> guess
> > is that gcc will do so for a considerable number of releases), then
> it
> > would depend on whether ubuntu chooses to use the new ABI instead of
> > the old one for its C++ binaries.
>
> Isn't Ubuntu likely to make the same choice that Fedora has made, at
> least at some point?
>
> Or is Fedora just doing this (using the C++11 libstdc++ API) because
> they really like C++11 and want to make the C++11 experience as good
> as
> possible?

It looks like Ubuntu does plan to use gcc 5 in Ubuntu 15.10 (in
development now):
http://summit.ubuntu.com/uos-1505/meeting/22506/gcc-5-update-for-1510/
though it's currently still using gcc 4.9.2:
http://packages.ubuntu.com/search?suite=wily&keywords=gcc

The plan isn't clear to me, but they are maybe going to try doing
parallel-installs of all the affected libraries, though that seems
unmanageable to me:
http://pad.ubuntu.com/uos-1505-gcc-5-update-for-1510
These are the meeting notes. If you can't log in to see that, here's a
copy/paste of part of the current contents:

"
== libstdc++ ABI transition ===

 * libstdc++ doesn't change the soname, provides a dual ABI
 * having objects with both ABIs in a process is doomed to fail.
 * libraries depending on libstdc++ may need to change their ABI
   How to see:
     * build the library with GCC 5
     * if the library exports some __cxx11 symbols, it may be
incompatible,
        if these are symbols which form part of the public API
     * To be conservative, you want to assume that the library is
        incompatible.
 * If the library is incompatible, rename the package, append "c++11" to
the name of the package.
 * Instead of renaming, support ABI coexistence like libstdc++ (maybe
some upstreams will do that)
 * To build code with gcc-5 which is compatible with the old ABI, define
the macro
  _GLIBCXX_USE_CXX11_ABI to 0 before including any C++ standard library
headers.
   Should only be used for leaf packages, not for libraries as a last
resort.
"



-- 
Murray Cumming
murrayc murrayc com
www.murrayc.com