Sometimes in a simple applicattion whith a principal form and several secondary forms accesed by a TMainMenu I get the error:

Access violation.

Press Ok to ignore and risk data corruption.
Press Cancel to kill the program.

when I branch between forms. Anybody help me?

My application:

unit Unit1;
{$mode objfpc}{$H+}
interface
uses
  Classes, SysUtils, LResources, Forms, Controls, Graphics, Dialogs, Menus;
type
  { TForm1 }
  TForm1 = class(TForm)
    MainMenu1: TMainMenu;
    MenuItem1: TMenuItem;
    MenuItem2: TMenuItem;
    MenuItem3: TMenuItem;
    MenuItem4: TMenuItem;
    procedure MenuItem1Click(Sender: TObject);
    procedure MenuItem2Click(Sender: TObject);
    procedure MenuItem3Click(Sender: TObject);
  private
    { private declarations }
  public
    { public declarations }
  end;
var
  Form1: TForm1;
implementation
uses Unit2, Unit3, Unit4;

{ TForm1 }

procedure TForm1.MenuItem1Click(Sender: TObject);
begin
 Form2 := TForm2.Create (Application);
 try
  Form2.ShowModal;
 finally
  Form2.Free;
 end;
end;
procedure TForm1.MenuItem2Click(Sender: TObject);
begin
 Form3 := TForm3.Create (Application);
 try
  Form3.ShowModal;
 finally
  Form3.Free;
 end;
end;
procedure TForm1.MenuItem3Click(Sender: TObject);
begin
 Form4 := TForm4.Create (Application);
 try
  Form4.ShowModal;
 finally
  Form4.Free;
 end;
end;
initialization
  {$I unit1.lrs}
end.

Unit2.pas

unit Unit2;
{$mode objfpc}{$H+}
interface
uses
  Classes, SysUtils, LResources, Forms, Controls, Graphics, Dialogs, Menus;
type
  { TForm2 }
  TForm2 = class(TForm)
    MainMenu1: TMainMenu;
    MenuItem1: TMenuItem;
    procedure MenuItem1Click(Sender: TObject);
  private
    { private declarations }
  public
    { public declarations }
  end;
var
  Form2: TForm2;
implementation

{ TForm2 }

procedure TForm2.MenuItem1Click(Sender: TObject);
begin
  Close;
end;
initialization
  {$I unit2.lrs}
end.

Regards.
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You may need to post your application's .lpr as well... We don't know if the forms are created in there or not.

I am not sure you can assign "Appliction" as the parent of a form... isn't it just "null" or "self"? Smarter people will know that...

ie.
Form2 := TForm2.Create (self);
or
Form2 := TForm2.Create (null);
In the .lpr file the command is different too...
Application.CreateForm (TForm2, Form2);

Regards,
-Reenen

2008/10/1 Usuario An?nimo

Zitat von Reenen Laurie :

You confuse Parent and Owner.

TForm2.Create(Application) sets the Owner, not the Parent. Using Application as Owner for forms is normal and allowed. As Parent it won't even compile, because TApplication is not a TWinControl.
Owner = responsible for freeing component (can only be set at creation) Parent = visual parent (can be changed at any time)

To debug an access violation, run the program in the debugger and create a backtrace. The wiki explains how.

Mattias

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Should you free the form if you set Application as the owner? Will not then Application try to free it again if the app closes, and throw an AV at you? If you do FreeAndNil() instead of Free, does it make any diffrence?

Try running the app from gbd, then do a backtrace after the AV.

Bart
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No, when you free componentA which is owned by componentB, then componentA is removed from the childlist of componentB, avoiding a double free.

Marc

project1.lpr

program project1;
{$mode objfpc}{$H+}
uses
  {$IFDEF UNIX}{$IFDEF UseCThreads}
  cthreads,
  {$ENDIF}{$ENDIF}
  Interfaces, // this includes the LCL widgetset
  Forms
  { add your units here }, Unit1;
begin
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.
*I forgot to say that the error occurs under WinCE.*

The error not always occurs, only sometimes. Why?

2008/10/2 Marc Weustink

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I think that the problem is the TMainMenu component. Now, I use a TPopupMenu and it works correctly.

2008/10/2 Usuario An?nimo

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