[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.
>

​What If I make Clone virtual? If by adding "virtual" to Clone, the problem
is solved, then what's the difference between virtual Clone and the method
you provided?
​


>  === 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


​This compiles, however, if I remove my version of TIntTree (i.e. implement
a Clone in it and call parent's DoClone), then the main program does not
compile, it says:

demo2.lpr(20,13) Error: Incompatible types: got "TIntTree.TTree$LongInt"
expected "TIntTree"

I then can either:

1) add a Clone method to TIntTree and do a typecast inside it, as you said.
2) do a typecast in the main program, i.e. it2 := it1.Clone as TIntTree.

So, I guess the problem is solved?  I have two further questions:

1) Why class of TTree does not work in 2.6.2 but class of TSelfType worked?
Is it a problem of generics in 2.6.2, or it is *not* related to generics?
I feel that TSelfType is same as TTree?

2) What is the difference between the following:

- it2 := TIntTree(it1.Clone);
- it2 := it1.Clone as TIntTree;

Where in the official document can I find the behavior/definition of "as"?

Thanks a lot.

Regards,
Xiangrong
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