calling delphi procedure as method

Question

Here is my simple code which compiles well, but raise Access Violation. It enters MD procedure and debugger displays some X and Y value, but after exiting procedure AV happens. Hope someone can help.

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Controls, Forms,  ExtCtrls;

type
  TForm1 = class(TForm)
    Panel1: TPanel;
    procedure FormCreate(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

    {$R *.dfm}

procedure MD(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
  form1.caption:= inttostr(x)+ ' '+ inttostr(y);
end;

procedure TForm1.FormCreate(Sender: TObject);

function MakeMethod(data, code: pointer): TMethod;
begin
  result.Data:= data;
  result.Code:= code;
end;

begin
  panel1.OnMouseDown:= TMouseEvent(MakeMethod(nil, @MD));
end;

end.

Thanks

Are you surprised by the AV? - You passing Nil to MakeMethod. — MartynA
– MartynA, Commented Jul 12, 2016 at 8:27
@MartynA, your comments are completely misleading, MD never references data pointer, the actual reason of failure is parameter mismatch. — Free Consulting
– Free Consulting, Commented Jul 12, 2016 at 9:23
If you don't want to make MD a method of TForm1 you could try the technique I described here: stackoverflow.com/questions/11083336/…. — Uli Gerhardt
– Uli Gerhardt, Commented Jul 12, 2016 at 9:37
@Uli Gerhardt - Yes, I was aware of that solution and it works OK. However solution with class procedures raises compile problems with Lazarus (it is my prime interest) which I didn't understand. However this should be for some other thread. — Djole
– Djole, Commented Jul 12, 2016 at 10:04

kludg · Accepted Answer · 2016-07-12 09:28:49Z

5

MD signature should include additional hidden parameter; it solves AV issue.

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls;


type
  TForm1 = class(TForm)
    Panel1: TPanel;
    procedure FormCreate(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure MD(Instance, Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
  form1.caption:= inttostr(x)+ ' '+ inttostr(y);
end;

procedure TForm1.FormCreate(Sender: TObject);

function MakeMethod(data, code: pointer): TMethod;
begin
  result.Data:= data;
  result.Code:= code;
end;

begin
  panel1.OnMouseDown:= TMouseEvent(MakeMethod(nil, @MD));
end;

end.

edited Jul 12, 2016 at 9:28

answered Jul 12, 2016 at 9:22

kludg

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8 Comments

Free Consulting Over a year ago

Care to explain why OnCreate isn't good place? I never have any problem with that. VCL/FMX also does that as the step of Create sequence.

kludg Over a year ago

@FreeConsulting - my fault, works with OnCreate too. Edited.

Free Consulting Over a year ago

@Djole, it worked because not all TMouseEvent parameters can fit into registers, so uses stack. And so causes misaligned stack when you mismatch the signature.

Uli Gerhardt Over a year ago

This technique is basically the same without the hackery. :-)

David Heffernan Over a year ago

Using a class method is without doubt a better solution.

|

Sherlock70 · Accepted Answer · 2016-07-12 08:29:01Z

1

Try making MD a member of your form class. Edit your example the following way: In the class definition:

type
TForm1 = class(TForm)
  Panel1: TPanel;
  procedure FormCreate(Sender: TObject);
  procedure MD(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
end;

Later in your code make sure to enter the owner of the MD method like so:

procedure TForm1.MD(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);

Finally, when assigning this eventhandler to your component, all you have to do is:

panel1.OnMouseDown:= MD;

I don't know what your intention was, but that is how you take care of eventhandlers at run time.

answered Jul 12, 2016 at 8:29

Sherlock70

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2 Comments

Djole Over a year ago

I know that, but I intentionally try to use global function, rather then method. It was just the matter of curiosity.

Sherlock70 Over a year ago

Perhaps clarify your intention. I simply don't see the need for doing things the way your example suggests. Perhaps try your method with something other than an event handler. Try something simple first, maybe a method without parameters, then work yourself up to methods with parameters.

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