Hi Aaron! Thanks again.
On Mon, Aug 3, 2015 at 4:30 PM, Aaron Laws <dartm...@gmail.com> wrote: > On Mon, Aug 3, 2015 at 3:40 PM, K. Frank <kfrank2...@gmail.com> wrote: >> >> Hello List! >> >> Another question, again looking at the Wt::Dbo tutorial: >> >> http://www.webtoolkit.eu/wt/doc/tutorial/dbo.html >> ... > ... >> 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); (Oops ... Typo on my part. I meant to write "session.add(user)", where user is of type User*.) > Excellent question. Based on the current source, it should be okay to pass a > pointer to a business object even in an exceptional situation. The source > is: > > 228 template <class C> > 229 ptr<C> Session::add(C *obj) > 230 { > 231 ptr<C> result(obj); > 232 return add(result); > 233 } > > If Wt::Dbo::Session::add(Wt::Dbo::ptr<C>&) throws, the original object will > be deleted. Ah, okay. The overload that takes a raw pointer, Session::add (C*), creates a Wt::Dbo::ptr smart pointer owning the C* before it does anything that might throw, so were safe. Or to say it another way, both: Wt::Dbo::ptr<User> userPtr (user); and: Wt::Dbo::Session::add (user); take ownership of (the object pointed to by the raw pointer) user, and become responsible for deleting user, even if an exception is thrown. > Of course, I can't think of many ways that session.add(&user); > could be used, excepting something like (Yes, of course. The use of "&user" in this case was a typo on my part.) > User * ptr_user = new User(); > User & user {*ptr_user}; > session.add(&user); > /*We don't own ptr_user anymore, so don't delete it!*/ > > Happy coding! > > In Christ, > Aaron Laws Best regards. K. Frank ------------------------------------------------------------------------------ _______________________________________________ witty-interest mailing list witty-interest@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/witty-interest
