I am having problems with virtual methods.

I have two classes, the second one is a descendent of the first :

1st:

	TTreeNode = Class(TObject)
	Public
		Constructor Create(Const aOwner : TTreeNode); Virtual;
2nd :
	TTreeNodeWithProperties = Class(TTreeNodeWithName)
	Public
		Constructor Create(Const aOwner : TTreeNodeWithProperties); Override;

TTreeNodeWithName is the ancestor of TTreeNodeWithProperties, and TTreeNode is the ancestor of TTreeNodeWithName.

TTreeNode -> TTreeNodeWithName -> TTreeNodeWithProperties

TTreeNodeWithName use the old constructor from the base class (IE.: It doesnt define a new constructor), so it works ok.

TTreeNodeWithProperties needs to hook the constructor/destructor to build some aditional encapsulated objects,
so i need to Override the virtual constructor.

As you can see i am using virtual constructors.

The problem is :

If i use Override it causes this output from the compiler :

{
Free Pascal Compiler version 2.4.4 [2011/04/22] for i386 Copyright (c) 1993-2010 by Florian Klaempfl
Target OS: Linux for i386
Compiling htmlnodes.pas
Compiling tree.pas
Compiling namevalue.pas
tree.pas(89,15) Error: There is no method in an ancestor class to be overridden: "constructor TTreeNodeWithProperties.Create(const TTreeNodeWithProperties);"
tree.pas(125,1) Fatal: There were 1 errors compiling module, stopping Fatal: Compilation aborted
}

If i use Reintroduce the compiler doesnt complain, but when the class is instantiated from a class factory, it builds the wrong class (the constructor called is of the base class not of the derived one).
IE.: The aditional objects i need in the derived class are not instantiated, giving a big access violation exception during runtime.

If i leave without override or reintrodce, the compiler complains :

{
Free Pascal Compiler version 2.4.4 [2011/04/22] for i386 Copyright (c) 1993-2010 by Florian Klaempfl
Target OS: Linux for i386
Compiling htmlnodes.pas
Compiling tree.pas
Compiling namevalue.pas
tree.pas(89,15) Warning: An inherited method is hidden by "constructor TTreeNodeWithProperties.Create(const TTreeNodeWithProperties);" htmlnodes.pas(34,15) Error: There is no method in an ancestor class to be overridden: "constructor
THTMLNode.Create(TTreeNodeWithProperties);"
htmlnodes.pas(80,1) Fatal: There were 1 errors compiling module, stopping Fatal: Compilation aborted
}

I think i am doing something wrong...
I'm left without options...
Pls Help.

2011/6/1 Jorge Aldo G. de F. Junior :

TTreeNode is a different type than TTreeNodeWithProperties.

I think you should override the constructor with base type as param:

   Constructor Create(Const aOwner :  TTreeNode); Override;

At runtime check with:

if not (aOwner is TTreeNodeWithProperties) then
  raise exception....
inherited Create(aOwner);
...

If you define a different constructor with the derived type as param :

   Constructor Create(Const aOwner :  TTreeNodeWithProperties); Overload;

then if you're passing TTreeNode objects to TTreeNodeWithProperties constructor it will only call the base constructor.

Yep, this situation would confuse the compiler as it's possible to assign descendant class instance to a class.
1st solution: rename one of the constructor, don't need to use virtual/override
2nd solution: try using constref instead of const (I hope constref has the same semantics as var for classes, where the assigned object must be exactly the instance of expected class, descendants not allowed)

2011/6/1 leledumbo :

Yep, this situation would confuse the compiler as it's possible to assign descendant class instance to a class. 1st solution: rename one of the constructor, don't need to use virtual/override

Its a class factory, i cannot have multiple constructors, so i need virtual constructors in order to build classes from names.

2nd solution: try using constref instead of const (I hope constref has the same semantics as var for classes, where the assigned object must be exactly the instance of expected class, descendants not allowed)

I will try that.

Cobines solution worked !

not tested constref.

2011/6/1 Jorge Aldo G. de F. Junior :