TTabControl leaking (?)

[chrv]

Hi all,

Just testing TTabControl. I have to create and destroy tabs and associated objects at runtime.
When i free objects

Code: Pascal  [Select][+][-]

1. for i:=TabControl1.Tabs.Count-1 downto 0 do
2.   TabControl1.Tabs.Objects[i].Free;

  it seems tabs captions are destroyed (?) and objects are not (?).
At the end : unfreed memory blocks (1 per tab created)

When i use similar code with TListBox

Code: Pascal  [Select][+][-]

1. for i := 0 to ListBox1.Items.Count-1 do
2.     ListBox1.Items.Objects[i].Free;

result is as expected.
What am i missing ?

Here is my complete exemple app.

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls, ComCtrls  ;
9.  
10. type
11.   TMyObject = class(TObject)
12.     OID : integer;
13.   end;
14.   { TForm1 }
15.   TForm1 = class(TForm)
16.     Button1: TButton;
17.     Button2: TButton;
18.     Button3: TButton;
19.     Button4: TButton;
20.     Edit1: TEdit;
21.     Label1: TLabel;
22.     Label2: TLabel;
23.     Label3: TLabel;
24.     ListBox1: TListBox;
25.     TabControl1: TTabControl;
26.     procedure Button1Click(Sender: TObject);
27.     procedure Button2Click(Sender: TObject);
28.     procedure Button3Click(Sender: TObject);
29.     procedure Button4Click(Sender: TObject);
30.   private
31.   public
32.   end;
33.  
34. var
35.   Form1: TForm1;
36. implementation
37. {$R *.lfm}
38. { TForm1 }
39. //Creates object and add it to ListBox
40. procedure TForm1.Button1Click(Sender: TObject);
41. var
42.   aMyObject : TMyObject;
43. begin
44.   aMyObject := TMyObject.Create;
45.   aMyObject.OID:=Random(20);
46.   ListBox1.AddItem(Edit1.Text,aMyObject);
47. end;
48.  
49. //Frees objects. It works OK
50. procedure TForm1.Button2Click(Sender: TObject);
51. var
52.   i : integer;
53. begin
54.   if ListBox1.Count>0 then
55.   for i := 0 to ListBox1.Items.Count-1 do
56.     ListBox1.Items.Objects[i].Free;
57.   ListBox1.Clear;
58. end;
59.  
60. //Creates object and add it to TabControl
61. procedure TForm1.Button3Click(Sender: TObject);
62. var
63.   aMyObject : TMyObject;
64. begin
65.   aMyObject := TMyObject.Create;
66.   aMyObject.OID:=Random(20);
67.   TabControl1.Tabs.AddObject(Edit1.Text,aMyObject);
68.  
69.   Edit1.Text:='';
70. end;
71.  
72. //Destroys objects. Does not work as expected : memory leaks
73. procedure TForm1.Button4Click(Sender: TObject);
74. var
75.   i : integer;
76. begin
77.   if TabControl1.Tabs.Count>0 then
78.   //for i := 0 to TabControl1.Tabs.Count-1 do
79.   //  TabControl1.Tabs.Objects[i].Free;          //<- crash
80.   for i := TabControl1.Tabs.Count-1 downto 0 do
81.     TabControl1.Tabs.Objects[i].Free;
82.   TabControl1.Tabs.Clear;  
83. end;
84. end.            

[lucamar]

As a general remark, because I don't know if it is relevant here, one shouldn't use the "objects" properties of special TStrings properties (like Tabs here) because it's very posible that they are being used internally by the propety itself.

It can be easily ascertained if that is indeed the case with Tabs: add a few in design-mode and check at run-time whether Objects[] is empty or not.

[chrv]

@lucamar
@jamie
Thanks for information.

"Objects" is indeed used for another purpose

Code: Text  [Select][+][-]

1. 44164104
2. 44165800
3. 44166824

What's the best workaround ?
- Not to use Tabs.Objects (of cours)
- Copy Tabs (without Objects) to a new TStrings (say "TabsNew")
- Store TMyObject to TabsNew.Objects
- Always synchronise Tabs with TabsNew

Is there a better solution ?

[lucamar]

The best, IMHO, is to keep your objects in a separate TStringList and keep it synchronized with Tabs, which basically means responding to adding/deleting a tab and a tab changing its position (e.g. when the user drags a tab to put it before/after another), if this last is allowed in your program.

So, yeah, what you outlined seems to be the best solution. Only, remember that TStrings is a quasi-abstract class and that its default implementation is TStringList, so use one of these.

[chrv]

That's what i thought.

Thanks a lot

(see attachment for my implementation)