Are Singletons EVIL in GUI Programming with Qt?

I'm just starting my first fairly large Qt project which will be mostly a bunch of screens with buttons, tab widgets, and Qwt Plots. The panel stack pattern described in Qt Quarterly 27 seems pretty nice for my application. Each of my screens is a QWidget encapsulated in a Panel which is shown/hidden by a QStackedWidget. However it uses a singleton pattern for each Panel so that they aren't all created immediately as the app starts and so that more than one of each screen isn't ever created.

So I started coding. Got the panel stack working. Added some code so that dynamically updating widgets aren't dynamically updating all the time. Got my history stack/back button working for the panels. Everything seems just fine, but I've got one nagging worry:

1. My code smells.

I am in no place to argue with any of the hate posted here and on blogs about the singleton pattern. I think I get it and the code I've written does make me feel a bit dirty with all the boilerplate lines and global objects. But I do like not having to worry about whether or not I already instantiated a screen before switching to it and adding it to my history stack. I just say switch to that screen, it's added to my history stack, and the magic wo开发者_JS百科rks.

From what I've read there are also some cases where singletons can be worthwhile. Is this one of those special cases? The magic screen switching / history stack makes me think 'yes' but the sheer number of different singleton classes I'm going to have to create makes me think 'NO no NO NO NO'.

I want to just man up and figure out how to get the singleton pattern out of my code now so that I don't have to do it later. But I don't want to get rid of all my singleton classes just to get rid of my singleton classes because they're EVIL [citation needed].

Any input is much appreciated!

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I don't really hate singletons, but this sounds like a case with no use for them. I don't understand why there are so many singletons in that article.

First, the PanelStack is a singleton by itself. Why? If that's your main widget, then just create it on the stack in the main(), which is both cleaner and faster. If it is a part of a more complicated UI, then put it there as a member of that UI. A regular class is just fine here, making it singleton only limits its possible uses.

Then, each panel is also a singleton? At this point even singleton lovers should begin to feel that there are too many of them already. Which is probably why you are asking this question in the first place. Let's see what real advantages singletons give here. Well, about the only advantage I can figure out from that article is the ability of lazily creating panels on the fly as they are needed. This is actually a good thing, but in fact, lazy creation and singletons are different patterns, although one often uses the other.

Why not just put all those panels in some common container instead? In this case, the PanelStack looks like a perfect candidate for it. It is the very place where panels are stored after all. Instead of a bunch of singletons, let's create a bunch of methods in the PanelStack:

class PanelStack : public QWidget
{
  Q_OBJECT

public:
  int addPanel(AbstractPanel *);
  void showPanel(int);
  RecordingsPanel *getRecordingsPanel();
  ReecrdingDetailsPanel *getRecordingDetailsPanel();

private:
  ...
};

And so on. These get*Panel() methods can still create panels lazily as needed. Now, it's essentially the same thing as having a bunch of singletons, with some advantages added:

• If we make panels children of the stack, they are automatically deleted when the stack is deleted. No need to worry about memory management which is always a pain with singletons.
• You could even implement some sort of "garbage collector" in the PanelStack that deletes panels that haven't been used for some time. Or when some sort of "max active panels" limit is reached.

Now, the only disadvantage I can think of is that we have a dependency between the stack and the panels now. But what's worse, to store instances in one class, introducing a dependency, or to store them globally? If you think that the stack should be independent from panels, which does sound reasonable, then we probably just need another class to put all those things in. It could be a subclass of QApplication, or just some random "UI manager" class, but it is still better to store everything in one place than to store everything globally.

Using singletons here only breaks encapsulation and limits the possible uses of the whole UI. What if we want to have two windows with those panels? Or multiple tabs (think web browser)? Singletons will bite hard. And they are only really useful when the instance is accessed widely across many unrelated classes (think DB connections, loggers, pools and other typical singleton uses). They are mostly useless in an UI because with UI it is almost always obvious that "this thing belongs there, and probably nowhere else".