[Bug c++/53673] Add magic weak symbol to indicate C++ standard setting (C++03, C++11 etc) to help debug ABI problems

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53673

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-06-15 17:46:51 UTC ---
(In reply to comment #6)
> Technically, you could add it to the top of <stddef.h> or whatever is a
> guaranteed included library header:

libstdc++'s <bits/c++config.h> would be the right place and as part of the std
lib the symbol should probably be named __glibcxx_blah

I think you'd also need an actual definition or nothing will be emitted for the
declaration alone:

#if __GXX_WEAK__
#if __cplusplus == 201103L
extern "C" void __glibcxx_std_cxx11() __attribute__((weak));
extern "C" void __glibcxx_std_cxx11() { }
#else if __cplusplus == 199711L
extern "C" void __glibcxx_std_cxx98() __attribute__((weak));
extern "C" void __glibcxx_std_cxx98() { }
#else
#warning Unknown C++ standard version
#endif
#endif


> No, it's fair enough, I only know that from watching the discussions on ISO and
> I have no idea if it's actually written in the final published standard. It is
> however written in Nicolai Josuttis' updated C++11 "The C++ standard library"
> in the chapter on C++11 core language changes. And if you think it through,
> there has to be in practice ABI breakage in 03 ABIs because no one could have
> anticipated during their design of what 11 would require [1].
> 
> [1]: This may not apply to GCC as it revised its ABI quite recently, and I'm
> sure its designers took into account likely future 11 requirements.

The current G++ ABI is eight years old and (modulo bugs) the same for c++98 and
c++11, see Bug 53646 comment 17

Again, the incompatibilities are in the library not the core language.
Whether that's generally true for other compilers is irrelevant here.