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Virtual inheritence (I guess)

I have two form which must have the same public procedure, so I decide to create an abstract base class for them to inheritence like this.

TAbstractForm = class(TForm)
public
  function ShowX: integer; virtual; abstract;
end;

The two subclass forms look like this

TFormX = class(TAbstractForm)
public
  function ShowX: integer; virtual;
end;

function TFormX.ShowX: integer;
begin
  ShowModal;
  result := 217;
end;



I then want to call a procedure with either of the two forms like this

procedure TForm1.ShowMyForm( Form: TAbstractForm );
begin
  Form.ShowX;
end;

the actual call
..
begin
  ShowMyForm( FormX );
end;

I get this strange memory error when performing the last call. I guess because Delphi tries to execute ShowX of the abstract base class.
How do I handle this problem ?

BTW this was just a quick example I put together, so perhaps some details is forgotten or left out.
0
sorentop
Asked:
sorentop
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1 Solution
 
EpsylonCommented:
I think this

TFormX = class(TAbstractForm)
public
 function ShowX: integer; virtual;
end;

must be

TFormX = class(TAbstractForm)
public
 function ShowX: integer; override;
end;
0
 
ivobauerCommented:
Hi sorentop!

Just first look but,

instead

TFormX = class(TAbstractForm)
public
  function ShowX: integer; virtual;
end;

try

TFormX = class(TAbstractForm)
public
  function ShowX: integer; override;
end;

Hope this helps, Ivo
0
 
ivobauerCommented:
Oh, damn! Only 2 minutes delay.

Ivo
0
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sorentopAuthor Commented:
No I have tried it with override, but it didn't seem to help
0
 
ivobauerCommented:
sorentop:

> the actual call
>..
>begin
>  ShowMyForm( FormX );
>end;

Where does the FormX variable come from? Please post a code snippet where you get/create this var.

Ivo
0
 
sorentopAuthor Commented:
FormX is a form just like when you have Delphi create a default one. With the var in the interface section.

Instead of TForm1 = class(TForm) I have changed it to TFormX = class(TAbastractForm)

FormX is also created automatic by Delphi
0
 
sorentopAuthor Commented:
The creation look like this
Application.CreateForm(TFormX, FormX);
0
 
sorentopAuthor Commented:
Wait a minute. I have found the error, I turned out I assigned the variable of FormX in the Form1's OnCreation, before! FormX was created.
0
 
ivobauerCommented:
OK. I have also tried to run the project and it worked well. But remember to always OVERRIDE the virtual (abstract) method otherwise when you declare the method with same name with virtual directive in descendant class, you simply declare another method (and hide ancestor's one) and in case the original method was abstract, you get EAbstractError exception when attempting to call an abstract method.

Best regards, Ivo.
0

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