Re: libstdc++ and dual ABI

[Lars Reichardt <lars%paradoxon.info@localhost>]

> Joerg Sonnenberger <joerg%bec.de@localhost> hat am 28. September 2017 um 15:10 geschrieben:
> 
> 
> Hello everyone,
> during the developer summit, I was made aware that we got the fancy dual
> ABI for libstdc++ with the GCC 5 import. I thought that $#%#$% was
> disabled from the start, but alas, it isn't.
> 
> What does dual ABI mean? Long story. std::string in libstdc++ had
> a performance "feature" for ages where it avoided string copying by
> using reference counting. This has a number of implications, one of them
> is that in-place modifications will invalidate iterators. Since this
> makes the behavior highly unpredictable especially in multithreaded
> programs, C++11 forbids it. Since changing central parts of std::string
> requires an ABI change, GCC decided to provide both old and new
> interface in the same code and introduce a fancy hack in the name
> mangler. Inline namespaces as used by libc++ wouldn't allow both ABIs to
> coexist, since functions using std::string only in the return type
> wouldn't be mangled different as that case is not allowed in C++.
> 
> Why is this bad? It means that if libfoo is compiled with -std=c++11 and
> foobar is compiled with -std=c++03, they can't be linked together and
> vice versa. While many libraries are starting to adopt C++11 features,
> not everyone does. The dual ABI makes interaction between the standards
> *worse*, just to avoid a major bump.
> 
> What should we do? I want to take the time before the 8.0 release and
> switch GCC over to always use the new ABI. This means recompiling all
> C++ code, but I consider it the lesser evil of having to deal with the
> fallout in pkgsrc for years to come.
> 
> Joerg

Yes, please.
This dual ABI is annoying and I've got hit by it (under Linux)

Lars