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

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On 24 March 2016 at 12:29, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> 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."

To conform to C++11 we needed new string and list implementations,
which are incompatible with the old ones.

We could have made -std=c++03 use the old ABI by default, and
-std=c++11 use the new ABI by default, but then using -std would
affect ABI, and you could not link C++03 code to C++11 code.

So instead we made the ABI that is used by default independent of the
-std option. It is controlled by a macro, which is set when GCC is
configured, and the value of the macro is independent of the -std
option.

I'm open to suggestions for how to word the documentation to make that
clearer, but it already says that AFAICT.