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

["Jan Kundrát" <jkt@gentoo.org>]

On Thursday, 19 December 2013 02:41:55 CEST, heroxbd@gentoo.org 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?

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].

Please also note that this is a very complicated problem, much more 
difficult than "code uses C++11's features -> it is incompatible". So far, 
the only known incompatibility is in the way STL is built. The impact of 
the other changes like the modified signatures of a couple of STL methods 
is not clear to me (and I did search for an ultimate answer). Even the 
document listing the breakage [4] does not provide a clear message "if you 
do $FOO, stuff breaks, but $BAR is completely safe".

For example, there is no reason for not building e.g. Qt5 with C++11 
support. In fact, limiting it to use just the C++98 features would be 
considered a regression from the perspective of a developer using Qt.

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.

Cheers,
Jan

[1] http://gcc.gnu.org/gcc-4.8/cxx0x_status.html
[2] https://lwn.net/Articles/552831/
[3] https://lwn.net/Articles/552750/
[4] http://gcc.gnu.org/wiki/Cxx11AbiCompatibility