Re: [Boost-users] shared_ptr problems

From: Oliver Schoenborn (oliver.schoenborn_at_[hidden])
Date: 2005-11-03 13:27:01

> class Connection {
>   boost::shared_ptr<MYSQL> m_mysql;
> public:
>   void connect() {
>     m_mysql.reset(mysql_init(), mysql_close);
>   }
>   bool isConnected() const { return m_mysql.get() != 0; } };
> 
> class ConnectionManager {
>   std::map<std::string, Connection> m_connections;
> public:
>   static ConnectionManager* instance() {
>     static ConnectionManager cm;
>     return &cm;
>   }
>   Connection connection(const std::string& name = "default")
>   { return m_connections[name]; }
> };
> 
> int main()
> {
>   {
>     Connection c = ConnectionManager::instance()->connection();
>     std::clog << "isConnected " << c.isConnected() << std::endl;
>     c.connect();
>     std::clog << "isConnected " << c.isConnected() << std::endl;
>   }
>   Connection c = ConnectionManager::instance()->connection();
>   std::clog << "isConnected " << c.isConnected() << std::endl;
> 
>   return 0;
> }
> 
> And the output is:
> 
> isConnected 0
> MYSQL* mysql_init()
> isConnected 1
> void mysql_close(MYSQL*)
> isConnected 0
> 
> What I don't understand is why shared_ptr calls mysql_close too soon.
> After all still exists a reference to Connection. I mean, 
> mysql_close should by called when the ConnectionManager 
> singleton is destroyed or am I doing something wrong.

Both c and the map item in ConnectionManager have a shared pointer that
points to the "same" MYSQL object, a null. Then you reset c to point to
something else. This will not reset the shared pointer in ConnectionManager.
Your MYSQL obj gets killed as soon as c goes out of scope. 

Oliver

> - -- Filipe Sousa
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.4.2 (GNU/Linux)
> 
> iD8DBQFDalOEbQdNYqwwwCwRAsNLAJ43BSZTCuraPQ2ouFqGoW/2+JDPDgCeOoQ7
> 8ZoF4LDO7vQyrPM/Whtx/O8=
> =iil3
> -----END PGP SIGNATURE-----
> _______________________________________________
> Boost-users mailing list
> Boost-users_at_[hidden]
> http://lists.boost.org/mailman/listinfo.cgi/boost-users
>