covariance, operator overloads, and interfaces

[Andrei Alexandrescu]

Michel Fortin wrote:
> On 2010-05-11 09:33:38 -0400, "Steven Schveighoffer" 
> <schveiguy at yahoo.com> said:
> 
>> interface List
>> {
>>     List concat(List rhs);
>>     List opBinary(string op)(List rhs) if (op == "~")
>>     {
>>         return concat(rhs);
>>     }
>> }
>>
>> But now I lose covariance, the above usage no longer compiles.
>>
>> I don't know how to solve this without dropping interfaces, or 
>> repeating  the template in all derived classes.
> 
> Can't you repeat the opBinary template in the ArrayList class with a 
> covariant return?
> 
> class ArrayList : List {
>     ArrayList concat(List rhs);
>     ArrayList opBinary(string op)(List rhs) if (op == "~") {
>         return concat(rhs);
>     }
> }
> 
> Not very elegant, but I think it'll work.

He already mentioned "repeating the template in all derived classes".

This is another place in which we need typeof(class) to yield the class 
being currently compiled.


Andrei