My helper removes original helpers

[Artur2024]

This work

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4. {$modeswitch TypeHelpers}
5.  
6. interface
7.  
8. uses
9.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs;
10.  
11. type
12.   //HString = type helper for string
13.   //  function MyClear(): String;
14.   //end;
15.  
16.   { TForm1 }
17.  
18.   TForm1 = class(TForm)
19.     procedure FormClick(Sender: TObject);
20.   private
21.  
22.   public
23.  
24.   end;
25.  
26. var
27.   Form1: TForm1;
28.  
29. implementation
30.  
31. {$R *.lfm}
32.  
33. //function HString.MyClear(): String;
34. //begin
35. //  Result := '';
36. //end;
37.  
38.  
39. { TForm1 }
40.  
41. procedure TForm1.FormClick(Sender: TObject);
42. var
43.   s: string;
44. begin
45.   s := 'aaa';
46.   ShowMessage(s.Replace('a', 'A'));
47. end;
48.  
49. end.
50.  

When I add my helpers, this code not work
"Replace" is: unit1.pas(46,17) Error: Illegal qualifier

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4. {$modeswitch TypeHelpers}
5.  
6. interface
7.  
8. uses
9.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs;
10.  
11. type
12.   HString = type helper for string
13.     function MyClear(): String;
14.   end;
15.  
16.   { TForm1 }
17.  
18.   TForm1 = class(TForm)
19.     procedure FormClick(Sender: TObject);
20.   private
21.  
22.   public
23.  
24.   end;
25.  
26. var
27.   Form1: TForm1;
28.  
29. implementation
30.  
31. {$R *.lfm}
32.  
33. function HString.MyClear(): String;
34. begin
35.   Result := '';
36. end;
37.  
38.  
39. { TForm1 }
40.  
41. procedure TForm1.FormClick(Sender: TObject);
42. var
43.   s: string;
44. begin
45.   s := 'aaa';
46.   ShowMessage(s.Replace('a', 'A'));
47. end;
48.  
49. end.
50.  

How can I add my own helpers?

[jamie]

you can inherit from TStringHelper.

If memory serves, I think you specify TStringHelper instead of string.


I would need to look that up.

[Blaazen]

https://wiki.freepascal.org/Helper_types

By default, only one helper type can be active for a type at the same time. This helper is always the last one that is in scope (which follows the usual rules of Pascal).

[jamie]

Code: Pascal  [Select][+][-]

1.  TMyStringHelper = type helper(TStringHelper) for Ansistring
2.    //
3.   end;                                                          
4.  
5.  

[Artur2024]

Thank you, now the code works 

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4. {$modeswitch TypeHelpers}
5.  
6. interface
7.  
8. uses
9.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs;
10.  
11. type
12.   TMyStringHelper = type helper(TStringHelper) for Ansistring
13.     function MyClear(): String;
14.   end;
15.  
16.   { TForm1 }
17.  
18.   TForm1 = class(TForm)
19.     procedure FormClick(Sender: TObject);
20.   private
21.  
22.   public
23.  
24.   end;
25.  
26. var
27.   Form1: TForm1;
28.  
29. implementation
30.  
31. {$R *.lfm}
32.  
33. function TMyStringHelper.MyClear(): String;
34. begin
35.   Result := '';
36. end;
37.  
38.  
39. { TForm1 }
40.  
41. procedure TForm1.FormClick(Sender: TObject);
42. var
43.   s: string;
44. begin
45.   s := 'aaa';
46.   ShowMessage(s.Replace('a', 'A'));
47.   s := s.MyClear();
48. end;
49.  
50. end.
51.  
52.  

[Zoran]

https://wiki.freepascal.org/Helper_types

By default, only one helper type can be active for a type at the same time. This helper is always the last one that is in scope (which follows the usual rules of Pascal).

Yes. In next fpc version this limitation will be removed though. https://wiki.freepascal.org/FPC_New_Features_Trunk#Support_for_multiple_active_helpers_per_type

[Thaddy]

Correct, but beware: the concept suffers from the same pitfalls as multiple inheritance in C++ classes, it is the programmer's responsability to keep and maintain scope. So use with care.
Also note it needs a modeswitch, possibly for that reason.

The suggested inheritance is often the better option.

[Artur2024]

In my "MyClear" function, I lost the ability to use the "Copy" function. How can I deal with it?

Code: Pascal  [Select][+][-]

1. function TMyStringHelper.MyClear(): String;
2. begin
3.   Result := Copy(Self, 1, 0);
4. end;

unit1.pas(37,29) Error: Wrong number of parameters specified for call to "Copy"

[Zoran]

In my "MyClear" function, I lost the ability to use the "Copy" function. How can I deal with it?

Code: Pascal  [Select][+][-]

1. function TMyStringHelper.MyClear(): String;
2. begin
3.   Result := Copy(Self, 1, 0);
4. end;

unit1.pas(37,29) Error: Wrong number of parameters specified for call to "Copy"

The problem is with this method in TStringHelper, which hides System.Copy function.

Try

Code: Pascal  [Select][+][-]

1. Result := System.Copy(Self, 1, 0);

[Thaddy]

Posts almost crossed, but indeed, specify system.copy.

[Artur2024]

Everything works perfectly.
Thank you very much for your very quick help