>I wish that when GC encountered a cycle it would just tell the programmer that they now have a leak, maybe even suggest where it could be fixed with one of those neat little IDE quick fixes, and continue chugging along.
Are you saying that cyclic data structures are always a bad idea? So no one should ever make a doubly-linked list?
Or, to take a case from a game I'm working on: I'm writing a set of GUI controls for the game's UI. There will almost always be at least a few controls on screen, so I have a WindowManager object that lives for the entire duration of the application and does, among other things, tasks like telling all of the controls to draw themselves, determining which control has focus, etc. The controls all have a reference to the WindowManager object (or whatever their direct parent is, once I start supporting more complex controls) so they can implement methods like onClick() { parent->removeGroupOfControls(); }
Even though the WindowManager is persistent, the individual controls come and go quite often. So if I was using a GC (I happen to be doing this in C++) that simply threw up its hands whenever it encountered a cycle, none of the controls would ever get garbage collected, leading to a huge memory leak over time.
Is this a case where you think cycles are appropriate? Or is there a better way to refactor this so it doesn't use cycles? (If someone has ideas for a better design, I would absolutely love to know.)
I cannot imagine creating any sort of complex system that does not, at some conceptual level, contain cycles. What I am suggesting is that the author of a complex system should, whenever possible, untangle as many of these cycles as possible by differentiating between the different classes of references. Think of it like the difference between your directory tree and your symbolic links in unix.
Reg. your example, I have generally found it effective to model UI hierarchies where the top level components refer to their children 'strongly' while the children refer 'weakly' to their parents. I am not familiar with the recent smart pointer stuff in c++ but I believe that these concepts have been cleanly modeled with abstractions such as this:
N.B. A more difficult conundrum that I have encountered when dealing with UI hierarchies is whether or not child components should require a constant reference to their parent passed upon creation or you should be able to 'reparent' or even be in an initial parentless case. My current thinking is that this is a mistake and interactions such as drag and drop are better handled by sending a gesture up to your data layer which then regenerates the component in its new place. This might get ugly, however, if you have a heavily direct manipulation based UI which doesn't really need a data layer on top.
By the way, what has lead you down the road of UI toolkits? They are a fetish of mine. Feel free to shoot me an email if you'd like to connect.
Are you saying that cyclic data structures are always a bad idea? So no one should ever make a doubly-linked list?
Or, to take a case from a game I'm working on: I'm writing a set of GUI controls for the game's UI. There will almost always be at least a few controls on screen, so I have a WindowManager object that lives for the entire duration of the application and does, among other things, tasks like telling all of the controls to draw themselves, determining which control has focus, etc. The controls all have a reference to the WindowManager object (or whatever their direct parent is, once I start supporting more complex controls) so they can implement methods like onClick() { parent->removeGroupOfControls(); }
Even though the WindowManager is persistent, the individual controls come and go quite often. So if I was using a GC (I happen to be doing this in C++) that simply threw up its hands whenever it encountered a cycle, none of the controls would ever get garbage collected, leading to a huge memory leak over time.
Is this a case where you think cycles are appropriate? Or is there a better way to refactor this so it doesn't use cycles? (If someone has ideas for a better design, I would absolutely love to know.)