Hi All,

I wrote a TTree generic class, the code is here:

https://github.com/xrfang/fpcollection/blob/master/src/units/tree.pas

While using this class, I encountered the following problem:

program project1;
{$mode objfpc}{$H+}
uses tree;
type
  TIntTree = class(specialize TTree<Integer>)
  end;
var
  ti : TIntTree;
begin
  ti := TIntTree.Create(1, nil);
  ti := ti.Next;  <-- error here
end.

The error message is:

Error: Incompatible types: got "TIntTree.TTree$LongInt" expected "TIntTree"

If I do not inherit TTree but just specialize it, there is no error.

How can I solve this problem so that I don' t have to use typecast everywhere?

Thanks,
Xiangrong

"TIntTree"

If I do not inherit TTree but just specialize it, there is no error. How can I solve this problem so that I don' t have to use typecast

everywhere?

Short answer: you can't. The same would happen with normal classes.

Long answer: You could add an additional method "Next" to your "TIntTree" which returns a "TIntTree" and just do "Result := TIntTree(inherited Next);" there.

Why are you subclassing in this case anyway?

Regards,
Sven

Thanks, I will inherit the method. I need to add features in the sub-class. This empty TIntTree is just to show the problem.

Hi Sven,

2013/9/23 Xiangrong Fang

program test;
{$mode objfpc}{$H+}
uses treap;
type
  TStringMapper = class(specialize TTreap<string, string>)
  end;
var
  sm1, sm2: TStringMapper;
  n: TStringMapper.PNode;
begin
  sm1 := TStringMapper.Create;
  for n in sm1.Reversed do WriteLn(n^.Key);   <-- but NOT here
  sm2 := sm1.Reversed;                        <-- error here
end.

Why is this? Also, I found that the keyword "specialize" is recognized in fpc mode, but "class" is only valid in objfpc mode, is that correct?

Regards,
Xiangrong

How is PNode declared? What is the exact error?

Why is this? Also, I found that the keyword "specialize" is recognized in fpc mode, but "class" is only valid in objfpc mode, is that correct?

"specialize" is used in all modes except mode Delphi for specializations. "class" is available in all modes except "iso", "tp" and "fpc", but can be enabled there using {$modeswitch class}.

Regards,
Sven

2013/9/24 Sven Barth

How is PNode declared? What is the exact error?

PNode is declared inside TTreap generic class, the source is here:

https://github.com/xrfang/fpcollection/blob/master/src/units/treap.pas

The exact error is:

Error: Incompatible types: got "TStringMapper.TTreap$AnsiString$AnsiString" expected "TStringMapper"

The error is exactly the same as the one in your TIntTree, because the return type of TTreap<String, String>.Reversed is TTreap<String, String> and nothing else. Thus the solution is the same as for TIntTree.Next: either none or a new "Reversed" function. And again: This problem would also occur with non generic classes.

Regards,
Sven

I know it is same but why no error on the line:

for n in sm1.reversed...

regards.
在 2013-9-24 下午11:23，"Sven Barth" 写道：

n is of type TStringMapper.PNode whereby PNode is declared inside TTreap<String, String> and thus the type is in reality a TTreap<String, String>.
sm1.reversed returns a TTreap<String, String> which has an enumerator that works on a TTreap<String, String>.PNode. So everything is well.

Regards,
Sven