The Non-Virtual Interface idiom in D

[Andrei Alexandrescu]

Rainer Deyke wrote:
> 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.

Contract inheritance has its place but NVI is more general than that.

>> 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'.

Oops, yah, sorry about that. Replace rhs.equals with rhs.doEquals. 
Access to doEquals was actually the point of the example.


Andrei