[cfe-dev] How to control C++0x adoption in a large codebase?

[Howard Hinnant]

On Oct 5, 2011, at 8:05 PM, John McCall wrote:

> 
> On Oct 5, 2011, at 4:44 PM, Jeffrey Yasskin wrote:
> 
>> On Wed, Oct 5, 2011 at 4:30 PM, John McCall <rjmccall at apple.com> wrote:
>>> On Oct 5, 2011, at 3:35 PM, Jeffrey Yasskin wrote:
>>>> I'm working on migrating a certain large codebase to C++0x, and we're
>>>> looking for a way to turn on C++0x mode but make sure people don't use
>>>> C++0x features before we're ready for them.
>>>> 
>>>> * We need to turn on -std=gnu++0x for the whole codebase at once
>>>> because C++98 and C++0x have different ABIs.
>>> 
>>> As far as I'm aware, this is not really true, and I actually put a lot of
>>> effort into fighting a few committee proposals that would have forced
>>> ABI changes.  There are a few things like name mangling which have been
>>> changed/clarified in the Itanium ABI, and obviously the standard library
>>> has to export more symbols in '0x, but I don't know of well-formed '03
>>> programs that have changed.
>> 
>> http://gcc.gnu.org/PR45093 concludes that the libstdc++ maintainers
>> don't intend to maintain binary compatibility between the two language
>> versions. It certainly may be that it would have been possible to
>> maintain compatibility, but they didn't, so that's what we have to
>> live with. libc++ probably does this better, but we don't really want
>> to block constrained C++0x support on switching to that either.
> 
> Okay.  Just wanted to make sure you weren't talking about some
> core-language thing I'd missed.  I believe Howard has, in fact, designed
> libc++ with cross-dialect binary compatibility in mind, although obviously
> he's been assisted in that by being able to design with foreknowledge
> of C++0x's requirements.
> 
> John.

libc++ uses C++11's inline namespace to version the ABI between gcc's C++03 std::lib and libc++.  This translates to a /noisy/ ABI incompatibility as gcc::std::vector and libc++::std::vector have different manglings (you get your errors at link time instead of run time).  Although some ABI's have been maintained as compatible:  std-defined exception classes, operator new/delete, handler functions.

Howard