StackOverflow2013

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copied!<p>I have three classes interacting in an interesting way. One is a model class, and it has to be accessed by both of the other classes, so a single instance of it is kept as a member of each. Both of these classes interact with the model in different ways.</p> <p>There are a couple of instances where the model object has to be completely thrown away and replaced with a new instance, and this complicates things. And these occasions arise in both of the viewing/controlling classes. So, either one of those classes has to be able to send a signal to the other saying "We need to coordinate and facilitate the replacement of our Model object with a new Model object." Right now I have code in class B to tell class A to construct a new Model and send it back, but now I need to handle the opposite situation, where the event arises in class A, and unfortunately class A does not have a reference to class B and probably shouldn't.</p> <p>What's a good way to handle this?</p> <p><strong>Update</strong>: Sorry, folks, this can't be a singleton. Singletons are when you need to guarantee there's only one of something. That has nothing to do with any of the requirements I expressed above. This class is not a singleton and shouldn't be.</p> <p><strong>Update</strong>: Up till now, there has actually only been one instance of this Model class, but I had a vague suspicion I needed to allow for more, and I didn't want to limit myself by using the Singleton design pattern when that actually addresses different concerns from what I have. Turns out I was right: yesterday I received a new requirement and now I need support an arbitrary number of these. :) Don't limit yourself when you don't have to, and don't misuse design patterns for situations where they were not intended!</p>

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