About object and heritage


I have this :

type
TCar = class
  procedure Drive; virtual;
end;

TFord = class(TCar)
  procedure Drive; override;
end;

TExpedition = class(TFord)
  procedure Drive;
end;

Procedure TCar.Drive ;
Begin
  ShowMessage(‘From TCar’) ;
End ;

Procedure TFord.Drive ;
Begin
  ShowMessage(‘From TFord’) ;
End ;

Procedure TExpedition.Drive ;
Begin
  ShowMessage(‘From TExpedition’) ;
End ;

Prcedure test ;
Var  MyCar: TCar;
Begin
   MyCar := TExpedition.Create;
   MyCar.Drive;
End ;

If I execute the procedure Test , it is the Drive procedure from Tford that is executed.  Why ?  Why it is not the one from TExpedition ?  

Thanks

sylouxAsked:
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God_AresConnect With a Mentor Commented:
well my vague explenation will be:

when you declare mycar as tcar (1)
and initiace it as texpedition, texpedition tford and tcar will be created tcar.drive will use tfords drive since it has been overridden in other words the adres that the cpu will jump to is tford's drive. well why should it jump to tcar's driv at all if you created it wit texpedition? look at (1) you declared it as an tcar, that's why. if you would decalre mycar as texpedition then texpedition's drive will be used.
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kretzschmarCommented:
guessing, its because its not overridden,
and mycar is not from the texpedition-class

TExpedition(MyCar).Drive; //should output the texpedtion.drive

meikl ;-)

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Russell LibbySoftware Engineer, Advisory Commented:
beat me to it Meikl.... :-)

Meikl is correct on this. The TFord is overriding the TCar's drive procedure, but TExpedition has defined drive again, but without the override.

So, as Meikl said, it can be called with

TExpedition(MyCar).Drive

or if the intention was to override the base class:

TExpedition = class(TFord)
 procedure Drive; override;
end;

-------
Russell
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sylouxAuthor Commented:
Sorry, I do not understand the mechanism :(((
Know any site where I can find explanation ?
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God_AresCommented:
Procedure test ;
Var  MyCar: Tcar; <<< why not  TExpedition ?
Begin
  MyCar := TExpedition.Create;
  MyCar.drive;
End ;
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kretzschmarCommented:
yep
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kretzschmarCommented:
brummel . . .
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