asked on

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

kretzschmar

guessing, its because its not overridden,
and mycar is not from the texpedition-class

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

meikl ;-)

Russell Libby

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

syloux

ASKER

Sorry, I do not understand the mechanism :(((
Know any site where I can find explanation ?

God_Ares

Procedure test ;
Var MyCar: Tcar; <<< why not TExpedition ?
Begin
MyCar := TExpedition.Create;
MyCar.drive;
End ;

ASKER CERTIFIED SOLUTION

God_Ares

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kretzschmar

yep

kretzschmar

brummel . . .