Mixin and interface implementation

Question

According to http://www.thinkbottomup.com.au/site/blog/C%20%20_Mixins_-_Reuse_through_inheritance_is_good

But hang on a minute, none of this helps us plug into our Task Manager framework as the classes do not implement the ITask interface. This is where one final Mixin helps - a Mixin which introduces the ITask interface into the inheritance hierarchy, acting as an adapter between some type T and the ITask interface:

template< class T >
class TaskAdapter : public ITask, public T
{
public:
    virtual void Execute()
    {
        T::Execute();
    }

    virtual std::string GetName()
    {
        return T::GetName();
    }
};

Using the TaskAdapter is simple - it's just another link in the chain of mixins.

// typedef for our final class, inlcuding the TaskAdapter<> mixin
typedef public TaskAdapter< 
                    LoggingTask< 
                        TimingTask< 
                            MyTask > > > task;

// instance of our task - note that we are not forced into any heap allocations!
task t;

// implicit conversion to ITask* thanks to the TaskAdapter<>
ITask* it = &t;
it->Execute();

Why is TaskAdapter needed, when ITask is implemented by MyTask? Also if ITask is not abstract, it may lead to diamond problem.

Answer 1

That's a very cool and interesting article.

In the final Mixin example, the MyTask class is not derived from ITask. Which means it can't be cast to an ITask pointer which is being done at the very end.

In that example, I believe you could derive MyTask from ITask. But I think the author wanted to illustrate that you can even decouple the MyTask class by using the TaskAdapter.