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
