Language: C C++ D Haskell Lua OCaml PHP Perl Plain Text Python Ruby Scheme Tcl #ifndef CONNECTIONDB_H #define CONNECTIONDB_H #include "dbclient.h" #include "connectiondb.h" class ConnectionDb { public: static ConnectionDb& instance(); //create instance virtual void connectDb() = 0; protected: ConnectionDb(); virtual ~ConnectionDb(); private: mongo::DBClientConnection connection; static ConnectionDb *instance_; // sub class friend class DisConnection; }; #endif // CONNECTIONDB_H Private [?] Run code