[fpc-pascal] class inheritance and type incompatibility

[Xiangrong Fang]

2013/9/28 Sven Barth <pascaldragon at googlemail.com>

> On second sight your solution is not correct, because you are using Clone
> inside your parent class which would not use the (non-virtual) Clone you
> created.
>
> I've now played around with 2.6.2 myself and this should solve this
> problem:
>
> === code begin ===
>
> type
>   generic TTree<T> = class
>   private type
>     TSelfType = TTree;
>     TSelfClass = class of TSelfType; // earlier I suggested TTree which
> 2.6.2 does not support=== code end ===
>
>
​I think this solution only fixed the problem with 2.6.2 that you have to
use TSelfType, but the logic is still not correct.

Using your modified version, I try to achieve some complex Clone​

​strategy, for example, either one of the following:

1) the descendant has added a field other than Data that needs to be cloned;
2) the clone is "non-verbatim", it will multiple Data by 2 when cloning.

Let's use the 2nd one as an example, the following does not work:

type
  TIntTree = class(specialize TTree<Integer>)
  public
    function Clone: TIntTree;
  end;

function TIntTree.Clone: TIntTree;
begin
  Result := TIntTree(inherited Clone);
  Result.Data *= 2;
end;

It only multiple the ROOT node of the cloned tree, not every node.

​
Regards,
Xiangrong
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freepascal.org/pipermail/fpc-pascal/attachments/20130929/c2525dd2/attachment.html>