On Mon, Aug 3, 2015 at 2:56 PM, K. Frank <kfrank2...@gmail.com> wrote:
> Hello List! > > I am 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 clarify a few points: > > First, as I understand it, the constructor of Wt::Dbo::Transaction > (called when the local variable "transaction" is declared) causes > the "begin transaction" SQL statement to be called. Is this correct? > yup. To see evidence of this (and much more), use Wt::Dbo::backend::Sqlite3::setProperty("show-queries", "true"); > > Then, when the destructor is called (when "transaction" goes out > of scope at the closing brace in the example), the transaction is > committed, i.e., the "commit transaction" SQL statement is called. > Is this also correct? > As long as no exception is being thrown, yes. > > I'm a little confused about what happens if an exception is thrown. > Presumably an SQL "rollback transaction" will be executed. But > how? Destructors will be called for local variable within the try block. > So, tweaking the example code a little: > > try { > 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); > } > catch (...) { > // ... > } > > If an exception is thrown in the try block, the destructor for > "transaction" > will be called. How will this destructor know to call "rollback > transaction", > rather than "commit transaction"? One possibility is that if the call to > "session.add" fails and throws an exception, it could first set some > "dirty transaction" flag that gets inspection by Wt::Dbo::Transaction's > destructor. But what if it's not a Wt::Dbo exception? Suppose I add > the line "if (42) throw 17;" to the try block. How would the destructor > then know to roll back the SQL transaction? > > (Or is this some kind of c++ thing where the destructor "knows" that > it's being called in an exception-triggered stack unwind?) > It's some kind of C++ thing: http://en.cppreference.com/w/cpp/error/uncaught_exception Happy coding! In Christ, Aaron Laws
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