继承C ++（钻石恐惧症）

Question

I have the following problem with inheritance (I would love to google it, but don't know the correct keywords it seems).

I have a Module class that implements IModule interface with function A() . I also have ClientModule class that inherits Module class and implements IClientModule interface with function B() that inherits IModule interface. This is where I am starting to get ambiguity and double inheritance problems .

Here is the class diagram that should tell a bit more about the problem:

I need to have separated access to Module class with IModule interface and ClientModule with IClientModule interface that offers access to IModule interface as well.

Answer 1

You are running into the "diamond inheritance" problem. Diamond inheritance has been discussed many times at this site and elsewhere. Make the inheritance of class Module and class IClientModule from class IModule virtual. This is not a perfect solution; sometimes the magic works, sometimes it doesn't. Where it doesn't work it's usually a case of demanding too much from inheritance.

Answer 2

Make IClientModule not inherit IModule (and probably rename it IClient). In ClientModule, just inherit from Module and implement IClient.

Or, an alternative solution written after reading your comments:

Make your ClientModule class inherit from IClientModule as in your example. However, instead of inheriting from Module, simply give ClientModule a member variable that is a Module, and forward the calls to this member where needed. Something like this:

class IModule { virtual void A() {} };
class Module : public IModule { ... };
class IClientModule : public IModule { virtual void B() {} };

class ClientModule : public IClientModule 
{
    // ...
    virtual void A() { _module.A(); }

private:
    Module _module;
};

However, it is likely still better to avoid mashing these together and use my original solution.