Qt C++ destructor taking a long time to return

Question

I'm working on a pretty standard Qt mobile app (written in C++, targeted at Symbian devices), and am finding that sometimes when the app is closed (i.e. via a call to QApplication::quit), the final destructor in the app can take a long time to return (30 seconds plus). By this I mean, all clean up operations in the destructor have completed (quickly, all well within a second) and we've reached the point where execution is leaving the destructor and returning to the code that implicitly called it (i.e. when we delete the object).

Obviously at that point I'd expect execution to return to just after the call to delete the object, virtually instantly, but as I say sometimes this is taking an age!

This long closure time happens both in debug and release builds, with logging enabled or disabled, so I don't think that's a factor here. When we reach the end of the destructor I'm pretty certain no file handles are left open, or any other open resources (network connections etc.)...though even if they where surely this wouldn't present itself as a problem on exiting the destructor (?).

This is on deleting the application's QMainWindow object. Currently the call to do this is in a slot connected to QApplication::aboutToQuit, though I've tried deleting that object in the apps "main" function too.

The length of delay we experience seems proportional to the amount of activity in the app before we exit. This sort of makes me think memory leaks may be a problem here, however we're not aware of any (doesn't mean there aren't any of course), and also I've never seen this behaviour before with leaked memory.

Has anyone any ideas what might be going on here?

Cheers

Answer 1

If your final destructor is for a class than inherits QObject then the QObject destructor will be called immediately following the destructor of your final object. Presumably this object is the root of a possibly large object tree which will trigger a number of actions to occur including calling the destructor of all the child QObjects. Since you state that the problem is componded by the amount of activity, there are likely a very large number of children being added to the object tree that are deleted at this time, perhaps more than you intended. Instead of adding all the objects to one giant tree to be deleted all at once. Identify objects that are being created often that don't need to persist through the entire execution. Instead of creating those objects with a parent, start a new tree that can be deleted earlier (parent =0). Look at QObject::deleteLater() which will wait until there is no user interaction to delete the objects in these independant trees.