[fpc-pascal] class inheritance and type incompatibility

[Xiangrong Fang]

2013/9/29 Sven Barth <pascaldragon at googlemail.com>

>
> If you want to override the virtual Clone method of TTree<LongInt> in
> TIntTree you need to use the same result type + override or otherwise you
> gain nothing, because the compiler will not consider the method as
> overloaded and use the Clone method of TTree<LongInt> inside
> TTree<LongInt>'s methods instead.


​Yes, you are right, for virtual method, you need have exactly same type...
But this leads me to think how the generics are implemented? For example:

=== snippet 1 ===
type
  TIntTree = class(specialize TTree<Integer>)
  public
    function Clone: TTree; override;
  end;
=== end of snippet 1 ===

This will leads to a compile error: Generics without specialization cannot
be used as a type for a variable.

If so, why in the code TTree can be used everywhere, for example:

=== snippet 2 ===
function TTree.Level: Cardinal;
var
  n: TTree;  <-- here TTree is not specialized.
begin
  ... ...
end;
=== end of snippet 2 ===

In snippet 1, why the compiler don't treat the TTree same as its own type
i.e. TIntTree? I imagine that TTree in snippet 1 is logically similar to
TObject. I mean, a class's method is of course possible to return a value
of its parent's type or any other class's instance?

Would you please show how exactly you changed the code? With all those
> methods calling each other it's a bit hard to imagine in my head what you
> changed. ;)
>

​With your TSelfClass solution in TTree, I mean either:

=== solution 1 ===
type
  TIntTree = class(specialize TTree<Integer>
  public
    function Clone: TIntTree;
  end;
  function TIntTree.Clone: TIntTree;
  begin
    Result := TIntTree(inherited Clone); //typecast in TIntTree
  end;
​=== end of solution 1===
or:
=== solution 2 ===
type
  TIntTree = class(specialize TTree<Integer>)
  end;
begin //main
  it2 := TIntTree(it1.Clone);  //typecast in main program
end.
=== end of solution 2 ===​
​
Are both solutions correct?

Regards,
Xiangrong​
​
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