The Non-Virtual Interface idiom in D

[Jeremie Pelletier]

Andrei Alexandrescu wrote:
> In this article:
> 
> http://www.gotw.ca/publications/mill18.htm
> 
> Herb Sutter makes a powerful argument that overridable functions 
> (customization points) should actually not be the same as the publically 
> available interface. This view rhymes with the Template Method pattern 
> as well.
> 
> This leads to the interesting setup in which an interface should ideally 
> define some signatures of to-be-defined functions, but disallow client 
> code from calling them. For the clients, the same interface should 
> expose some higher-level final functions.
> 
> Ignoring for the moment access protection semantics in D (which are 
> broken anyway), let's say this would work:
> 
> 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;
>     }
> }
> 
> So clients must implement doClone, but nobody can ever call it except 
> Cloneable's module. This ensures that no cloning ever gets away with 
> returning the wrong object.
> 
> Pretty powerful, eh? Now, sometimes you do want to allow a derived class 
> to call the base class implementation. In that case, the interface 
> function must be protected:
> 
> 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;
>     }
> }
> 
> The difference is that now a derived class could call super.doEquals.
> 
> This feature would require changing some protection rules, I think for 
> the better. What do you think?
> 
> 
> Andrei

What about:

interface ComparableForEquality(T) {
	bool equals(T rhs)
	out(result) {
		assert(rhs.equals(cast(T)this) == result);
	}
}

Getting instead a contract that gets added to implementations, or at 
least only the base implementation, and letting the actual method 
unimplemented.

However having code for interfaces as well as protection would be neat. 
They could prevent a lot of template mixins within every implementation 
to get a common feature.