If you have an interface hierarchy and a corresponding implementation hierarchy, making the interface base classes virtual bases is necessary.
E.g.
struct IBasicInterface
{
virtual ~IBasicInterface() {}
virtual void f() = 0;
};
struct IExtendedInterface : virtual IBasicInterface
{
virtual ~IExtendedInterface() {}
virtual void g() = 0;
};
// One possible implementation strategy
struct CBasicImpl : virtual IBasicInterface
{
virtual ~CBasicImpl() {}
virtual void f();
};
struct CExtendedImpl : virtual IExtendedInterface, CBasicImpl
{
virtual ~CExtendedImpl() {}
virtual void g();
};
Usually this only makes sense if you have a number of interfaces that extend the basic interface and more than one implementation strategy required in different situations. This way you have a clear interface hierarchy and your implementation hierarchies can use inheritance to avoid the duplication of common implementations. If you’re using Visual Studio you get a lot of warning C4250, though.
To prevent accidental slicing it is usually best if the CBasicImpl
and CExtendedImpl
classes aren’t instantiable but instead have a further level of inheritance providing no extra functionality save a constructor.