Abstract classes

[totya]

What does it mean, "doesn't work"?

Hi!

As I said, I create simple demo. I can't get any compiler warnings:

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, StdCtrls;
9.  
10. type
11.  
12.   { TForm1 }
13.  
14.   TForm1 = class(TForm)
15.     Button1: TButton;
16.     procedure Button1Click(Sender: TObject);
17.   end;
18.  
19. var
20.   Form1: TForm1;
21.  
22. implementation
23.  
24. {$R *.lfm}
25.  
26. uses
27.   Unit3;
28.  
29. { TForm1 }
30.  
31. procedure TForm1.Button1Click(Sender: TObject);
32. begin
33.   DataModule1.MyShow;
34. end;
35.  
36. end.
37.  

Code: Pascal  [Select][+][-]

1. unit Unit2;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, FileUtil,
9.  
10.   Dialogs;
11.  
12. type
13.   TCustomDataModule = class(TDataModule)
14.   protected
15.     procedure ImportantAbstractProcedureNeedToOverrideButNothingHappen; dynamic; abstract;
16.   public
17.     procedure MyShow;
18.   end;
19.  
20. var
21.   CustomDataModule: TCustomDataModule;
22.  
23. implementation
24.  
25. {$R *.lfm}
26.  
27. procedure TCustomDataModule.MyShow;
28. begin
29.   ShowMessage('OK');
30. end;
31.  
32. end.
33.  

Code: Pascal  [Select][+][-]

1. unit Unit3;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Sqlite3DS, db, FileUtil,
9.  
10.   Unit2;
11.  
12. type
13.  
14.   { TDataModule1 }
15.  
16.   TDataModule1 = class(TCustomDataModule)
17.     DataSource1: TDataSource;
18.     Sqlite3Dataset1: TSqlite3Dataset;
19.   end;
20.  
21. var
22.   DataModule1: TCustomDataModule;
23.  
24. implementation
25.  
26. {$R *.lfm}
27.  
28. end.
29.  

[Blaazen]

@ As I said, I create simple demo. I can't get any compiler warnings:

Where do you create DataModule1?!
No creating, no warning. 

EDIT:

This gives warnings:

Code: Pascal  [Select][+][-]

1. procedure TForm1.Button1Click(Sender: TObject);
2. begin
3.   DataModule1:=TDataModule1.Create(self);
4.   DataModule1.MyShow;
5.   DataModule1.Free;
6. end;  

[totya]

Where do you create DataModule1?!
No creating, no warning. 

OMG... Button1.Click is working, because Datamodule1 is created (with autocreate), see attached file my previous post, or see this:

Code: Pascal  [Select][+][-]

1. program project1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. uses
6.   {$IFDEF UNIX}{$IFDEF UseCThreads}
7.   cthreads,
8.   {$ENDIF}{$ENDIF}
9.   Interfaces, // this includes the LCL widgetset
10.   Forms, sqlite3laz, Unit1, Unit2, Unit3
11.   { you can add units after this };
12.  
13. {$R *.res}
14.  
15. begin
16.   RequireDerivedFormResource:=True;
17.   Application.Initialize;
18.   Application.CreateForm(TDataModule1, DataModule1);
19.   Application.CreateForm(TForm1, Form1);
20.   Application.Run;
21. end.
22.  

[Blaazen]

Ah, I see now. Coincidently, it didn't crash when DataModule1 wasn't autocreated.

So yes, in this case there are no warnings. Probably because CreateForm creates instance in unusual way.

[totya]

This gives warnings:

Code: Pascal  [Select][+][-]

1. procedure TForm1.Button1Click(Sender: TObject);
2. begin
3.   DataModule1:=TDataModule1.Create(self);
4.   DataModule1.MyShow;
5.   DataModule1.Free;
6. end;  

Yes, me too. TDatamodule isn't a really form... even hidden... Thanks for the tip!

[PascalDragon]

For those that want the compiler to stop when creating an abstract class or a class with abstract methods: use the following in an include file or so. 

Code: Pascal  [Select][+][-]

1. {$warn 4122 error} // creating an abstract class
2. {$warn 4046 error} // creating a class with an abstract method

[Thaddy]