I don't speak for Apple, but watching their actions, I believe that their goal is to get back to one standard library implementation for Mac OS (and iOS) - and that will be libc++. I believe that sometime in the future, libstdc++ will no longer be part of Mac OS X.
Can we mix system libc++ and manually built Clang libc++? If not, when will it cause trouble?
I do this fairly regularly - but I don't replace the one in usr/lib. Instead I run specific programs after setting the environment variable DYLD_LIBRARY_PATH to point to my newly built libc++. Replacing the one in /usr/lib has the potential to brick your system. (if you break something in the dylib - or even just change the layout of std::string
, say).