How to properly extend interface?

Question

I have interface based on another:

class IDrawable {
public:
   virtual ~IDrawable();
};

class IExtendedDrawable: public IDrawable {
public:
   virtual ~IExtendedDrawable();
};

class DrawableImplementation: public IDrawable {
public:
   virtual ~DrawableImplementation();
};

class ExtendedDrawableImplementation: 
   public DrawableImplementation, public IExtendedDrawable
{
public:
   virtual ~ExtendedDrawableImplementation();
};

Then ExtendedDrawableImplementation = DrawableImplementation (+IDrawable) + IExtendedDrawable (+IDrawable)

Is it right to have IDrawable twice in same class?

Answer 1

I'll give the benefit of the doubt that you DO indeed need/want multiple inheritance. I see it as good in only limited situations, and interfaces is one of them (even Java allows this).

As said above, use virtual inheritance and be sure to only use pure virtual methods in the interface classes.

class IDrawable {
public:
   virtual ~IDrawable();
   virtual void doSomething() = 0;
};

class IExtendedDrawable: virtual public IDrawable {
public:
   virtual ~IExtendedDrawable();
   virtual void doSomethingElse() = 0;
};

class DrawableImplementation: virtual public IDrawable {
public:
   virtual ~DrawableImplementation();
   virtual void doSomething() {/*code here*/}
};

class ExtendedDrawableImplementation: 
   public DrawableImplementation, public IExtendedDrawable
{
public:
   virtual ~ExtendedDrawableImplementation();
   virtual void doSomething() {/*code here*/}
   virtual void doSomethingElse() {/*code here*/}
};

Answer 2

It largely depends on the logic inside the classes is, but IMO you're better off using composition instead:

class ExtendedDrawableImplementation : public IExtendedDrawable
{
   IDrawable* drawableImplementation; //points to a DrawableImplementation
public:
   virtual ~ExtendedDrawableImplementation();
};

Multiple inheritance is rarely the answer.