Many places, the C++ Driver uses deprecated libmongoc APIs, or uses lower-level APIs than it needs to. The C++ Driver implements some driver logic itself that it should delegate to libmongoc: for example, rewriter::rewrite_find_modifiers appears to duplicate logic in libmongoc's _translate_query_opt() for upgrading OP_QUERY modifiers like "$orderby" to "find" command parameters like "sort".

Note, the C++ Driver reimplements GridFS instead of using libmongoc's GridFS API; this is appropriate because libmongoc's GridFS is out of spec and not worth using.