Question

I have the following superclass:

unit DlgDefaultForm;

type
  TDefaultFormDlg = class(TForm)
  published
    constructor Create(AOwner: TComponent); reintroduce; virtual; 
  end;

  FormCreateFunc=function(AOwner: TComponent):TDefaultFormDlg;

which is descended by a bunch of forms as follows:

unit Form1

type
  TForm1 = class(TDefaultFormDlg)
  published
    constructor Create(AOwner: TComponent); override;
  end;

and created as follows:

unit MainForm;

procedure ShowForm(FormCreate:FormCreateFunc);
begin
  (do some stuff)
  FormCreate(ScrollBox1);
end;

When I run

ShowForm(@TForm1.Create);

two things happen:

1. When I step into TForm1.Create, AOwner=nil, even when it didn’t in ShowForm.

2. I get an EAbstractError at the following line:

   unit Forms;
   (...)
   constructor TCustomForm.Create(AOwner: TComponent);
   begin
     (...)
     InitializeNewForm; //EAbstractError
     (...)
   end;
   

What am I doing wrong?

EDIT: This of course isn’t my exact code.

Answer 1

You’re not using virtual constructors correctly. Try it like this:

type
  TDefaultFormDlgClass = class of TDefaultFormDlg;

function Show(FormClass: TDefaultFormDlgClass; AOwner: TComponent): TDefaultFormDlg;
begin
  Result := FormClass.Create(AOwner);
end;

...
var
  FormClass: TTDefaultFormDlgClass;
...
FormClass := ???;//this is where you specify the class at runtime, e.g. TForm1
MyForm := Show(FormClass, MainForm);

As an aside I do not think you need to reintroduce the constructor in the code you listed.