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Exactly - the C interop is used almost exclusively for connecting to existing libraries.

Can you also call C++ libraries directly from Julia?

Name mangling, as always, is a mess. If they export extern "C" headers, then of course there's no trouble.

There's work on interfacing with the C++ ABI, though. See Cpp.jl[0] and Clang.jl (whose C++ support is still a WIP)[1].