The Non-Virtual Interface idiom in D

[Rainer Deyke]

Andrei Alexandrescu wrote:
> interface Cloneable(T) if (is(T == class))
> {
>     private T doClone(); // must implement but can't call
>     T clone()            // this is what everybody can call
>     {
>         auto result = doClone();
>         assert(typeof(result) == typeof(this));
>         assert(this.equals(result));
>         return result;
>     }
> }

This sounds like a case for contract inheritance rather than two layers
of functions.

> interface ComparableForEquality(T)
> {
>     protected bool doEquals(T);
>     final bool equals(T rhs)
>     {
>         auto result = doEquals(rhs);
>         assert(rhs.equals(cast(T) this) == result);
>         return result;
>     }
> }

This, on the other hand, does require two layers of functions if you
want to remove the infinite recursion by replacing the 'equals' in the
assertion with 'doEquals'.


-- 
Rainer Deyke - rainerd at eldwood.com