[Lazarus] object type: strange behaviour

[Birger Jansen]

Forget my last message I upgraded FPC 2.4.1 to 2.5.1 and now this works like it should!

-----Oorspronkelijk bericht-----
Van: Birger Jansen <birger at cnoc.nl>
Verzonden: vr 22-10-2010 11:38
Aan: Lazarus at lists.lazarus.freepascal.org; 
Onderwerp: [Lazarus] object type: strange behaviour

> Hi all,
> 
> I'm converting a project from Delphi to FreePascal. It is a mathematical 
> application that uses records with methods. Since this is not possible in FPC I 
> am trying to convert this base type from record to object:
> 
>   TPoint = object
>   public
>     X,Y,Z: Double;
>     class function Create(const Ax, Ay, Az: Double): TPoint;static;
>     class function Distance(const P1, P2: TPoint): Double;static;
>   end; 
> 
>   class function TPoint.Create(const Ax, Ay, Az: Double): TPoint;static;
>   begin
>     Result.X := Ax;
>     Result.Y := Ay;
>     Result.Z := Az;
>   end;
> 
>   class function TPoint.Distance(const P1, P2: TPoint): Double;static;
>   var
>     aX: Double;
>   begin
>     aX := Sqr(P1.X - P2.X) + Sqr(P1.Y - P2.Y) + Sqr(P1.Z - P2.Z);
>     Result := Sqrt(aX);
>   end; 
> 
> So far so good, this works (compiles and produces the desired result):
> 
>   var
>     P1, P2: TPoint;
>     d: Double;
>   begin
>     P1 := TPoint.Create(0,1,1);
>     P2 := TPoint.Create(1,0,0);
>     d := TPoint.Distance(P1, P2);
>     WriteLn(d);
>     ReadLn;
>   end.
> 
> Now there is a class type that has a TPoint field:
> 
>   TNode = class(TObject)
>   public
>     FCoord: TPoint; { coords }
>     constructor Create(const X, Y, Z: Double);
>   end; 
> 
> And a constructor that sets this point:
> 
>   constructor TNode.Create(const X, Y, Z: Double);
>   begin
>     FCoord.X := X;
>     FCoord.Y := Y;
>     FCoord.Z := Z;
>   end;    
> 
> This works, however this doesn't:
> 
>   constructor TNode.Create(const X, Y, Z: Double);
>   begin
>     FCoord := TPoint.Create(X,Y,Z);
>   end;    
> 
> It fails with an error: Class isn't a parent class of the current class. If I 
> set TNode.FCoord outside the constructor there is no problem at all. Also, when 
> I try to access other methods of TPoint this fails (see UpdateCoord in the full 
> source below).
> 
> Any idea what causes this behaviour? I included the complete code below for 
> your reference.
> 
> Kind regards,
>   Birger Jansen
> 
> 
> 
> 
> program TestProject;
> 
> {$mode delphi}{$H+}
> 
> uses
>   {$IFDEF UNIX}{$IFDEF UseCThreads}
>   cthreads,
>   {$ENDIF}{$ENDIF}
>   Classes;
> type
> 
>   { TPoint }
> 
>   TPoint = object
>   public
>     X,Y,Z: Double;
>     class function Create(const Ax, Ay, Az: Double): TPoint;
>     class function Distance(const P1, P2: TPoint): Double;
>   end;
> 
>   { TNode }
> 
>   TNode = class(TObject)
>   public
>     FCoord: TPoint; { coords }
>     constructor Create(const X, Y, Z: Double);
>     procedure UpdateCoord(const Value: TPoint);
>   end;
> 
> { TPoint }
> 
> class function TPoint.Create(const Ax, Ay, Az: Double): TPoint;
> begin
>   Result.X := Ax;
>   Result.Y := Ay;
>   Result.Z := Az;
> end;
> 
> class function TPoint.Distance(const P1, P2: TPoint): Double;
> var
>   aX: Double;
> begin
>   aX := Sqr(P1.X - P2.X) + Sqr(P1.Y - P2.Y) + Sqr(P1.Z - P2.Z);
>   Result := Sqrt(aX);
> end;
> 
> constructor TNode.Create(const X, Y, Z: Double);
> begin
>   FCoord.X := X;
>   FCoord.Y := Y;
>   FCoord.Z := Z;
> // this does not work:
>   FCoord := TPoint.Create(X,Y,Z);
> end;
> 
> procedure TNode.UpdateCoord(const Value: TPoint);
> begin
> // this does not work:
>   if TPoint.Distance(FCoord, Value) < 1E-10 then
>     FCoord := Value;
>   end;
> end;
> 
> var
>     P1, P2: TPoint;
>     N: TNode;
>     d: Double;
> 
> begin
>   N := TNode.Create(1,2,3);
> // this works:
>   N.FCoord := TPoint.Create(1,2,3);
>   P1 := TPoint.Create(0,1,1);
>   P2 := TPoint.Create(1,0,0);
>   d := TPoint.Distance(P1, P2);
>   WriteLn(d);
>   ReadLn;
> end.
> 
> --
> _______________________________________________
> Lazarus mailing list
> Lazarus at lists.lazarus.freepascal.org
> http://lists.lazarus.freepascal.org/mailman/listinfo/lazarus
>