When using multiple inheritance with diamond-shaped structures, you should use virtual inheritance.
I assume your code looks a bit like this:
class List {
...
};
class DoublyLinkedList: public List {
...
};
class CircularlyLinkedList: public List {
...
};
class CircularlyDoublyLinkedList: public DoublyLinkedList, public CircularlyLinkedList {
...
};
void doStuff() {
List* aList = new CircularlyDoublyLinkedList();
...
}
which produces the following error:
ambiguous conversion from derived class 'CircularlyDoublyLinkedList' to base class 'List':
class CircularlyDoublyLinkedList -> class DoublyLinkedList -> class List
class CircularlyDoublyLinkedList -> class CircularlyLinkedList -> class List
If you change the inheritance of DoublyLinkedList and CircularlyLinkedList to virtual public like follows:
class DoublyLinkedList: virtual public List {
...
};
class CircularlyLinkedList: virtual public List {
...
};
class CircularlyDoublyLinkedList: public DoublyLinkedList, public CircularlyLinkedList {
...
};
everything should compile properly. However, there is an additional performance cost. I would suggest using a fully abstract list interface that would be inherited by all your list classes, and composition to allow implementation reuse.