Here's a detraction I've missed so far: using classes like this doesn't easily lend itself to more than one volitile layer at a time, and so using internal objects would have an advantage in that respect, as they can be swapped whenever, in any order. Class change seems in line with more fundamental shifts in the nature of the object. At times, perhaps farming out work to a seperate object that you work indirectly with is a little excessive when you can cut out the abstraction layer by designing the classes that fill that space to the same specs. If the classes all rely on the same parent class but represent the differences between objects with the same parent, then switching classes is exactly the sort of abstractive exploit OO is designed to allow. Perhaps my thinking is incorrect, and I welcome correction.

You need to know what type of object to plug into a given role- this doesn't require knowledge of the object's data structure, but does require setting aside a space for that object inside the larger whole and knowing how to interact with it. It needn't know how to interact with the rest of the structure outside it's own space. Any object that you plug in needs to follow the same rules- fit into the same space and allow the same manner of interaction.

You've got to design or find a class that you can assign to manage that space within your greater object, and it has to share methods of interaction with other classes that might fill the same role, and respond in the same way (or you need to put an abstracting layer between the two as glue). So far, this seems like it could be addressed with either an independant object or an additional class layer.

But the class can't safely make assumptions about what is outside it's own space unless you are able and desire to take any extra effort to make it so. Breaking encapsulation, and the pros and cons, is beaten to death elsewhere. But you needn't break encapsulation, if you restrict yourself to that space you know is assigned to you. You just need to allow, or even expect, that you might find data there you didn't create or put there- and maybe that's OK. In fact, maybe it's more than OK.

I recognize the benefit of reading up on patterns. Are these particular patterns all presented in the gang of four book, or can you suggest another?