In 3.3, a C++ class defined in a header will get a typeinfo symbol
like this (e.g. in Nix's src/libutil/util.o):
(__DATA,__datacoal_nt) weak external typeinfo for nix::BaseError
But in 3.4, this has changed to:
(__DATA,__datacoal_nt) weak external automatically hidden typeinfo for nix::BaseError
This causes the linker to change the symbol to:
(__DATA,__data) non-external (was signed char private external) typeinfo for nix::BaseError
i.e. losing its weak linkage. But without weak linkage, dynamic_cast
and other RTTI-based mechanisms (such as catching an exception of a
certain type) don't work across shared libraries / executables.
The clang compiler in the SDK doesn't have this behaviour, but it's
not clear exactly which version it is (it just says "based on LLVM
3.4svn").
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