接口的钻石继承（C ++）

Question

I've stumbled upon a diamond inheritance problem, and I am not sure of the best solution. The following code works and has no diamond problem:

class Element { /* pure virtual functions */ };
class Diode : public Element {};
class Thyristor : public Diode {};

I don't like the public inheritance though, because a Thyristor is not a Diode, it just acts like a Diode often enough that I want to use a lot of the Diode code. I can make it work by using composition rather than inheritance, but that results in duplication of internal data structures between Diode and Thyristor which I don't like. What I would like to do is use private inheritance. If I do that, then Thyristor would also need to inherit publicly from Element:

class Thyristor : public Element, private Diode {};

The potential problem is that I have now created a diamond, as Element is inherited directly and through Diode. Is this a problem if Element is a pure virtual function? If it is, what is the proper way to solve this problem, making changes only to the Thyristor class?

Answer 1

You should surely use aggregation instead of inheritance this case.

Just trace back and ask yourself: why I'm inheriting here? How the thyristors and diodes could be used in the outer code? Quite soon you'll find you don't need inheritance for elements, just aggregate necessary behaviors (number of contacts, conduction behavior, etc.)