On inheritance and polymorphism in cats and dogs (and other beasts too)

[tcak via Digitalmars-d-learn]

On Saturday, 20 December 2014 at 15:40:32 UTC, Derix wrote:
> So, I have this pet project where classes Cat and Dog inherit
> from the more generic Beast class.
>
> All beasts prosper and multiply and so do cats and dogs. The
> breeding routine is fairly constant across species, with minor
> variations. So I'd like to define the "breed" method in the 
> Beast
> class and overload it in the Cat and Dog classes.
>
> Something like :
>
> Cat rita = new Cat(female);
> Cat ringo= new Cat(male);
> Cat junior=rita.breed(ringo);
>
> with
>
> Class Cat:Beast{
> ...
> 	Cat breed(Cat sire){
>   		// do what all beasts do
>
>   		//then add cat-specific genetics
>   	}
> ...
> } 	
>
> Now, what I can't seem to figure out is the "// do what all 
> beast
> do" part. "this" current object is obviously an instance of the
> Cat class. How do I de-specialize it so it can behave as an
> instance of the more generic Beast class ? Then, the offspring
> will in turn be an instance of Beast : how to cast it as a Cat ?
>
> So far, all I've been able to do is to dance around the issue by
> writting ad-hoc constructors like
>
> Beast b=new Beast( someCat )
>
> or
>
> Cat c=new Cat(someBeast)
>
> but this seems awkward and inefficient.
>
> There must be some more clever and straightforward way to do
> this, surely ?
>
> Also : D is not my first OO language, but polymorphism and
> inheritance still are advanced concepts for me.

What you do is to implement the "breed" method in Beast class, 
then override it in both Cat and Dog, but then call 
"super.breed()" method which belongs to Beast.