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Let's say I have BaseA and BaseB classes. BaseB can have to several references of BaseA instances. Now, we have two other classes, DerivedA and DerivedB that respectively inherit from BaseA and BaseB.

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How can I show in such a diagram that DerivedB should only store instances of DerivedA and not instances of BaseA? It seemed pretty simple before I tried to draw it, but now I'm confused. One obvious answer would be "redesign your code"; if there is no alternative, I will probably try to do so.

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make the type in derived B as has many of DerivedA ? does not matter that Derived A's base class is BaseA. As far as DerivedB is concrned it has variables of type 'DerivedA' –  tgkprog Apr 19 '13 at 12:49
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@tgkprog Well, since I am using Python, If I feed DerivedB some BaseA, those instances will not automagically be converted to DerivedA. When I think about it again, it may be more a "contract" with the programmer than a strict design... –  Morwenn Apr 19 '13 at 12:55
    
@Morwenn: I am not a Python export, but in other languages you would just create BaseA as an abstract base class, so you cannot have any instances of BaseA which are not also a "DerivedA". Perhaps this concept docs.python.org/2/library/abc.html is similar? –  Doc Brown Apr 19 '13 at 13:13
    
@DocBrown Hum, I never had to use this module before, I will have a look :) –  Morwenn Apr 19 '13 at 13:18

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