GCC version bikeshedding

[Richard Biener]

On Wed, Aug 6, 2014 at 12:08 PM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> On 6 August 2014 10:06, Jakub Jelinek wrote:
>> On Wed, Aug 06, 2014 at 11:04:14AM +0200, Richard Biener wrote:
>>> > - libstdc++ ABI changes (it is a significant user visible change,
>>> >   if you rebuild everything, no extra effort is needed, but otherwise
>>> >   if you want some C++ code built with older compilers work together
>>> >   with code built with newer compilers, it might require source code
>>> >   changes (the abi_tag attribute additions where needed and warning
>>> >   suggest to put those at), at least that is my current understanding
>>> >   of the plans
>>>
>>> But that's only with -std=c++11?  Which had no compatibility
>>> guarantees before?
>>
>> No, AFAIK it is also -std=c++98.  At least my understanding was that
>> std::list and std::string are going to change ABI (and get new abi_tag)
>> in all C++ modes.  Jonathan/Jason/Paolo, is that right?
>
> Correct. We want C++03 code to continue to be able to interoperate
> with C++11 code.
>
> It's an ABI change for all modes (but not a SONAME change because the
> old and new definitions will both be present in the .so).

Ugh.  That's going to be a nightmare to support.  Is there a configure
switch to change the default ABI used?  That is, on a legacy system
can I upgrate to 5.0 and get code that interoperates fine with code
built with 4.8?  (including ABI boundaries using the affected classes?
I suspect APIs with std::string passing are _very_ common, not
sure about std::list)

What's the failure mode the user will see when linking against a
4.8 compiled library with a std::string interface using 5.0?

Thanks,
Richard.