Hello List! Another question, again looking at the Wt::Dbo tutorial:
http://www.webtoolkit.eu/wt/doc/tutorial/dbo.html and, specifically, the example code: ... /* * A unit of work happens always within a transaction. */ dbo::Transaction transaction(session); User *user = new User(); user->name = "Joe"; user->password = "Secret"; user->role = User::Visitor; user->karma = 13; dbo::ptr<User> userPtr = session.add(user); } I would like to check my understanding of Wt::Dbo::ptr. When I say "ownership" I mean, primarily, the responsibility for memory management, in particular, calling delete. As I understand it it, when the local variable "userPtr" goes out of scope (at the closing brace at the end of the example code) delete will be called on "user" (of type User*). Is this correct? (I understand that Wt::Dbo::ptr is a shared pointer, so that delete will be called when the last of multiple shared pointers is destroyed, but in this case userPtr is the only such pointer.) So, in my language, userPtr (together with any other shared pointers pointing to the same object) "owns" the instance of class User pointed to by the variable "user". Is this correct? So, in particular, something like: { User user; user-.name = "Joe"; // ... dbo::ptr<User> userPtr = session.add(&user); } would NOT be the right way to do this because when userPtr goes out of scope, it would try to delete &user, (a pointer to) an object that was not created by a call to new. (And furthermore user's destructor would be called twice -- once by the incorrect call to delete, and then again when user went out of scope.) Is this correct? I have a question about exception safety: What if the call to session.add() throws? My understanding is that userPtr will not yet have been assigned, and will therefore not be able to delete (the object. Who then would have responsibility for calling 'delete user;"? Is this issue just a simplification in the sample code and the (only?) exception-safe way to do this would be: { User *user = new User(); // this stuff can't throw -- true? user->name = "Joe"; // ... dbo::ptr<User> userPtr((user); // this could throw, but userPtr ensures that user will be deleted session.add(userPtr); } Is there ever any fully exception-safe use of the original version? dbo::ptr<User> userPtr = session.add(&user); Thanks for any clarification. K. Frank ------------------------------------------------------------------------------ _______________________________________________ witty-interest mailing list witty-interest@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/witty-interest
