Lazarus

Programming => General => Topic started by: J-23 on October 28, 2020, 10:14:44 pm

Title: Bug TFPGInterfacedObjectList??
Post by: J-23 on October 28, 2020, 10:14:44 pm

Hello,

I haven't used TFPGInterfacedObjectList for a long time and now I need such a list.

Everything works fine theoretically, but I think I ran into a bug in the FPC.

I have a code: (Work OK)

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, fgl;
9.  
10. type
11.   ITest = interface(IUnknown)
12.   ['{1416EB7E-39B0-4134-BA20-F363D0B8C476}']
13.    procedure Test;
14.   end;
15.  
16. type
17.   TListInterface = specialize TFPGInterfacedObjectList<ITest>;
18.  
19. type
20.  
21.   { TTestClass }
22.  
23.   TTestClass = class(TInterfacedObject, ITest)
24.     procedure Test;
25.   end;
26.  
27. type
28.  
29.   { TForm1 }
30.  
31.   TForm1 = class(TForm)
32.     procedure FormCreate(Sender: TObject);
33.     procedure FormDestroy(Sender: TObject);
34.   private
35.     FListInterface: TListInterface;
36.     FTestClass: TTestClass;
37.   public
38.      procedure ExecuteInterface(Test: ITest);
39.   end;
40.  
41. var
42.   Form1: TForm1;
43.  
44. implementation
45.  
46. {$R *.lfm}
47.  
48. { TTestClass }
49.  
50. procedure TTestClass.Test;
51. begin
52.  
53. end;
54.  
55. { TForm1 }
56.  
57. procedure TForm1.FormCreate(Sender: TObject);
58. begin
59.   FListInterface := TListInterface.Create;
60.  
61.   FTestClass := TTestClass.Create;
62.   FListInterface.Add(FTestClass);
63. end;
64.  
65. procedure TForm1.FormDestroy(Sender: TObject);
66. begin
67.   FListInterface.Free;
68. end;
69.  
70. procedure TForm1.ExecuteInterface(Test: ITest);
71. begin
72.  
73. end;
74.  
75. end.                                                          
76.  

But the code below causes an error. (I think he shouldn't)

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, fgl;
9.  
10. type
11.   ITest = interface(IUnknown)
12.   ['{1416EB7E-39B0-4134-BA20-F363D0B8C476}']
13.    procedure Test;
14.   end;
15.  
16. type
17.   TListInterface = specialize TFPGInterfacedObjectList<ITest>;
18.  
19. type
20.  
21.   { TTestClass }
22.  
23.   TTestClass = class(TInterfacedObject, ITest)
24.     procedure Test;
25.   end;
26.  
27. type
28.  
29.   { TForm1 }
30.  
31.   TForm1 = class(TForm)
32.     procedure FormCreate(Sender: TObject);
33.     procedure FormDestroy(Sender: TObject);
34.   private
35.     FListInterface: TListInterface;
36.     FTestClass: TTestClass;
37.   public
38.      procedure ExecuteInterface(Test: ITest);
39.   end;
40.  
41. var
42.   Form1: TForm1;
43.  
44. implementation
45.  
46. {$R *.lfm}
47.  
48. { TTestClass }
49.  
50. procedure TTestClass.Test;
51. begin
52.  
53. end;
54.  
55. { TForm1 }
56.  
57. procedure TForm1.FormCreate(Sender: TObject);
58. begin
59.   FListInterface := TListInterface.Create;
60.  
61.   FTestClass := TTestClass.Create;
62.   ExecuteInterface(FTestClass);
63.   FListInterface.Add(FTestClass);
64. end;
65.  
66. procedure TForm1.FormDestroy(Sender: TObject);
67. begin
68.   FListInterface.Free;
69. end;
70.  
71. procedure TForm1.ExecuteInterface(Test: ITest);
72. begin
73.  
74. end;
75.  
76. end.                                                                
77.  

The only difference is this line

Code: Pascal  [Select][+][-]

1. ExecuteInterface(FTestClass);

Title: Re: Bug TFPGInterfacedObjectList??
Post by: J-23 on October 29, 2020, 03:08:00 am

Eh ... memory plays tricks :)

Code: Pascal  [Select][+][-]

1. TInterfaceList

Title: Re: Bug TFPGInterfacedObjectList??
Post by: ASerge on October 29, 2020, 09:56:09 pm

Quote from: J-23 on October 28, 2020, 10:14:44 pm

The only difference is this line

Code: Pascal  [Select][+][-]

1. ExecuteInterface(FTestClass);

The "ExecuteInterface" and "FListInterface.Add" methods are expected an interface parameter, but you pass the class parameter. An implicit conversion is made, but you do not control the lifetime, so the object (this is the interface) by the reference counter "dies" on the first call.
There are different solutions.
1. Skip a reference counter in methods:
  procedure ExecuteInterface(const Test: ITest);
2. Explicit add reference:
  ExecuteInterface(FTestClass as ITest);
3. Use a variable of the desired type (the safest solution):

Code: Pascal  [Select][+][-]

1. procedure TForm1.FormCreate(Sender: TObject);
2. var
3.   LTest: ITest;
4. begin
5.   FListInterface := TListInterface.Create;
6.   FTestClass := TTestClass.Create;
7.   LTest := FTestClass;
8.   ExecuteInterface(LTest);
9.   FListInterface.Add(LTest);
10. end;