C++ Multiple Inheritance Triangle

Question

Is there an issue with such an inheritance tree:

Parent ---- Subject
|         /
|        /
Child --/

Or in code:

class Subject
{
public:
    void AddObserver( Observer *aObserver );
protected:
    ObserversList mObserversList;
};

class Parent: public Object,
              public virtual Subject
{
};

class Child: public Parent,
             public virtual Subject
{
};

iammilind · Accepted Answer · 2012-11-03 03:45:05Z

1

Compilation-wise there is no issue with this kind of inheritance. As both the bases coming to Child are virtual inherited.

The only thing one must know is that, while constructing Parent(), the order of construction will be Subject() followed by Object(). Same thing is applicable for Child(). The order will be Subject(), Object(), Parent().

answered Nov 3, 2012 at 3:45

iammilind

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2 Comments

Izhaki Over a year ago

OK, so if I understand correctly, Child will only have a 'single' Subject interface. Also, would the answer remain the same if I don't use virtual with public Subject?

iammilind Over a year ago

@Izhaki, Yes, Child will have only one Subject base inside it, which is first to be constructed. No, things won't be same if you remove virtual from either Parent or Child. Some compilers may support having just one virtual inheritance, but that's not allowed by standard. All the Subject coming towards Child must be virtual inherited.

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