[fpc-devel] suggestion: virtual method co-variance

[Sven Barth]

On 15.10.2014 04:21, Hans-Peter Diettrich wrote:
> Sven Barth schrieb:
>
>> At least at first sight there don't seem to be any real (technical)
>> reasons to not covariance for return values. Parameters would be a
>> different topic though...
>>
>> Just so I get the idea right:
>>
>> === code begin ===
>>
>> type
>>   TBar = class
>>     function Test: TObject; virtual;
>>   end;
>>
>>   TFooBar = class(TBar)
>>     function Test: TStrings; override;
>>   end;
>>
>> //...
>
> I just wonder about the purpose and use of such refinement. Should the
> compiler relate on an more specific return type, based on the *static*
> type of an object reference? I'd use different names for specialized
> methods/properties instead.

The compiler should take into account the type of the variable on which 
the method is called. See the example code which you have snipped away. ;)

And btw: the compiler even already does that! :)

> OTOH it would be nice to have specialized lists without much coding,
> i.e. without writing getters (and setters) which only do typecasts of
> their results. Something like
>
> type
>    TBar = class
>      property Test: TObject ...; virtual;
>    end;
>
>    TFooBar = class(TBar)
>      property Test: TStrings; override;
>    end;
>
> Please note that this kind of override should not require to override
> the getters/setters, it only would enforce (static) type checks/casts,
> as doable at compile time.

But here the compiler can't guarantee that the Getter behind the 
property is really a TStrings instead of a TObject. And for the setter 
we would have the same problem as for methods: if I call the property 
using a TBar variable and pass something else than a TStrings than the 
inherited Setter will be called and then (to use the example of a list) 
a foreign object will reside in the list.

Regards,
Sven