Move the struct adb definition from the auidioDB_API.h into the audioDB-internals.h header file, leaving only the typedef behind. Thus a user of the API sees only an incomplete type, which cannot be instantiated (but /pointers/ to it can); there's then less temptation to break the abstraction barrier by using structure fields in client code. Not only that, but we can now safely put C++ stuff in the structure. Take advantage of this by putting a std::set<std::string> in there, to hold all the keys currently in the database; populate this field on audiodb_open() (and delete it on audiodb_close). This will be useful when we come to implement variants of audiodb_insert().