Using C++, how do I correctly inherit from the same base class twice?

Question

This is our ideal inheritance hierarchy:

class Foobar;

class FoobarClient : Foobar;

class FoobarServer : Foobar;

class WindowsFoobar : Foobar;

class UnixF         


        

Answer 1

It would work, although you'd get two copies of the base Foobar class. To get a single copy, you'd need to use virtual inheritance. Read on multiple inheritance here.

class Foobar;

class FoobarClient : virtual public Foobar;

class FoobarServer : virtual public Foobar;

class WindowsFoobar : virtual public Foobar;

class UnixFoobar : virtual public Foobar;

However, there are many problems associated with multiple inheritance. If you really want to have the model presented, why not make FoobarClient and FoobarServer take a reference to Foobar at construction time, and then have Foobar& FoobarClient/Server::getFoobar ?

Composition is often a way out of multiple inheritance. Take a example now:

class WindowsFoobarClient : public WindowsFoobar 
{
    FoobarClient client;
public:
    WindowsFoobarClient() : client( this ) {}
    FoobarClient& getClient() { return client }
}

However care must be taken in using this in the constructor.