override(T)

[Andrei Alexandrescu]

Steven Schveighoffer wrote:
> On Thu, 24 Sep 2009 09:30:46 -0400, Andrei Alexandrescu 
> <SeeWebsiteForEmail at erdani.org> wrote:
> 
>> Walter and I discussed last night about contravariance and all and 
>> could not find a compelling argument in favor of implementing 
>> contravariant arguments right now. The feature is nontrivial to 
>> implement, potentially surprising, and has a number of odd corner cases.
>>
>> One feature that does get requested often in C++ and Java is the 
>> ability to choose which class/interface contains the method you want 
>> to override. Consider:
>>
>> interface Lottery {  void draw();  }
>> interface Figure {  void draw();  }
>>
>> class LotterySimulation : Lottery, Figure {
>>      override void draw();
>> }
>>
>> Right now draw() overrides both methods, but you'd want to override 
>> them separately. You could do so through an intermediate interface:
>>
>> class Figure2 : Figure {  void draw2() { return draw(); }  }
>> class LotterySimulation : Lottery, Figure2 {
>>      override void draw();
>>      override void draw2();
>> }
>>
>> There are a few problems with this, among which the fact that 
>> LotterySimulation now cannot inherit another class; the one class slot 
>> was occupied by Figure2.
>>
>> So I was thinking of this:
>>
>> class LotterySimulation : Lottery, Figure {
>>      override(Lottery) void draw();
>>      override(Figure) void draw();
>> }
>>
>> This is easy to implement, scales well, and has good real world uses. 
>> What say you?
> 
> In fact, your example *still* does not work, since draw2 calls draw :)  
> I think you meant:
> 
> class Figure2 : Figure { abstract void draw2(); void draw() { return 
> draw2();} }
> 
> And I think actually, this still wouldn't work because LotterySimulation 
> is overriding draw from Figure2.  Maybe if you make it final in Figure2?
> 
> In the override(T) world, What does this do?
> 
> auto ls = new LotterySimulation();
> ls.draw();
> 
> ???
> 
> I'm not sure this works...
> 
> -Steve

That would be a compile-time error. You'd need to extract the interface 
you want to work with, and then call against it. In addition, 
LotterySimulation could offer convenience functions that do that.

Andrei