Is this actually supposed to be legal?

[Sean Cavanaugh]

On 7/17/2012 12:23 PM, Jonathan M Davis wrote:
> On Tuesday, July 17, 2012 14:48:32 David Nadlinger wrote:
>> On Tuesday, 17 July 2012 at 05:24:26 UTC, Jonathan M Davis wrote:
>>> This code strikes me as being a bug:
>>>
>>> --------
>>> class MyBase(T)
>>> {}
>>>
>>> class MySubA : MyBase!MySubA
>>> {}
>>>
>>> class MySubB : MyBase!MySubB
>>> {}
>>>
>>> void main()
>>> {}
>>> --------
>>
>> This pattern is actually quite common in C++ code, and referred
>> to as CRTP (curiously recurring template pattern). If you propose
>> to kill it, Andrei is going to get mad at you. ;)
>
> Well, it certainly seems insane to me at first glance - particularly when you
> take compile time reflection into account, since the derived classes'
> definitions are now effectively recursive (so I suspect that the situation is
> worse in D, since C++ doesn't have conditional compliation like D does). But
> if it's supposed to be legal, I guess that it's suppose to be legal. I'd never
> seen the idiom before, and it seemed _really_ off to me, which is why I brought
> it up. But I'd have to study it in order to give an informed opinion on it.
>
> - Jonathan M Davis
>

A 'proper' D port of this kind design would be to use mixins instead of 
the template.  They both accomplish the same thing:

The template (or mixins) are written to call functions in the user 
defined type.  A simple example would be the C++ WTL library: A user 
defined control defines its own window style, but the template code is 
responsible for creating the window, and accesses the style and class 
flags from the user defined type.

The advantage is the same in both: you avoid making the interface 
virtual, you still get to use some generic code.