Calling C++ free functions

You can call C++ functions that operate on Rust types. First, you need to add that function signature in the main.zng file inside an extern "C++" block:

extern "C++" {
    fn new_blob_store_client(crate::Flags) -> crate::Reader;
}

Then, in the generated.h file, the function signature is declared like this:

namespace rust {
namespace exported_functions {
::rust::crate::Reader new_blob_store_client(::rust::crate::Flags i0);
}
} // namespace rust

You need to implement it in a .cpp file and link it to the final binary.