Why does -Wabi-tag complain when -std=c++03?

[Jonathan Wakely]

On 24 March 2016 at 12:20, Jeffrey Walton <noloader@gmail.com> wrote:
> On Thu, Mar 24, 2016 at 8:10 AM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>> On 24 March 2016 at 11:55, Jeffrey Walton wrote:
>>> In the test program below, why does -Wabi-tag trigger a warning with
>>> the std::string when using -std=c++03?
>>
>> "Although the changes were made for C++11 conformance, the choice of
>> ABI to use is independent of the -std option used to compile your
>> code, i.e. for a given GCC build the default value of the
>> _GLIBCXX_USE_CXX11_ABI macro is the same for all dialects. This
>> ensures that the -std does not change the ABI, so that it is
>> straightforward to link C++03 and C++11 code together. "
>>
>> https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html
>
> Thanks. I recall reading that a couple of days ago. I still don't
> quite understand what it means. I'm obviously missing something.
>
> I guess my knowledge gap is due to _GLIBCXX_USE_CXX11_ABI should apply
> to C++11 and above to comply with the newer C++ rules, wrote-on-copy
> strings, etc. I don't understand how it effects C++03 and below since
> the rules that required the change don't apply.

The new std::string and std::list are also valid implementations for C++03.

Try reading this bit again: "This ensures that the -std does not
change the ABI, so that it is straightforward to link C++03 and C++11
code together."