Michel Fortin wrote:
On 2009-09-25 16:49:27 -0400, Andrei Alexandrescu <seewebsiteforem...@erdani.org> said:

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.doEquals(cast(T) this) == result);
         return result;
     }
}

[Note: I corrected the above example by replacing rhs.equals with rhs.doEquals.]

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?

I think you're writing a lot of boilerplate code for something that the compiler should be able to do by itself. I mean, it's a lot cleaner with contracts, and there is no reason the compiler couldn't generate itself that "contract-verifying" non-virtual function.

I think it would be a mistake to latch on my quick examples. It's not only about before and after checks, it's more about low-level customization points versus higher-level interfaces.

Andrei

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