asked on

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.

Epsylon

I think this

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

must be

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

ivobauer

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

ivobauer

Oh, damn! Only 2 minutes delay.

Ivo

sorentop

ASKER

No I have tried it with override, but it didn't seem to help

ASKER CERTIFIED SOLUTION

ivobauer

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sorentop

ASKER

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

sorentop

ASKER

The creation look like this
Application.CreateForm(TFormX, FormX);

sorentop

ASKER

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.

ivobauer

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.