[Solved] Defining multiple classes in TCustomApplication

[nikel]

Hello, I'm trying to learn oop and publish some programs that can be easily developed. I tried the code below but got error: External Sigsegv at Application.Run. Here's my code.

Code: Pascal  [Select][+][-]

1. program project1;
2.  
3. {$mode objfpc}{$H+}{$J-}
4. {$typedaddress on}
5. {$m+}
6.  
7. uses
8.   {$IFDEF UNIX}
9.   cthreads,
10.   {$ENDIF}
11.   Classes, SysUtils, CustApp
12.   { you can add units after this };
13.  
14. type
15.  
16.   { TMain }
17.  
18.   TMain = class(TCustomApplication)
19.   protected
20.     procedure DoRun; override;
21.   public
22.     constructor Create(TheOwner: TComponent); override;
23.     destructor Destroy; override;
24.     procedure WriteHelp; virtual;
25.   end;
26.  
27.   type
28.     TSong = class(TMain)
29.     private
30.       FPath: string;
31.     protected
32.       procedure DoRun; override;
33.     public
34.       property Path: string read FPath write FPath;
35.     end;
36.  
37.   { TMain }
38.  
39. procedure TMain.DoRun;
40. var
41.   ErrorMsg  : string;
42.  
43.   S1        : TSong;
44. begin
45.   // quick check parameters
46.   ErrorMsg:=CheckOptions('h', 'help');
47.   if ErrorMsg<>'' then begin
48.     ShowException(Exception.Create(ErrorMsg));
49.     Terminate;
50.     Exit;
51.   end;
52.  
53.   // parse parameters
54.   if HasOption('h', 'help') then begin
55.     WriteHelp;
56.     Terminate;
57.     Exit;
58.   end;
59.  
60.   { add your program here }
61.  
62.   WriteLn(S1.Path);
63.  
64.   // stop program loop
65.   Terminate;
66. end;
67.  
68. constructor TMain.Create(TheOwner: TComponent);
69. begin
70.   inherited Create(TheOwner);
71.   StopOnException:=True;
72. end;
73.  
74. destructor TMain.Destroy;
75. begin
76.   inherited Destroy;
77. end;
78.  
79. procedure TMain.WriteHelp;
80. begin
81.   { add your help code here }
82.   writeln('Usage: ', ExeName, ' -h');
83. end;
84.  
85. procedure TSong.DoRun;
86. begin
87.   FPath:='/home/user/Desktop/song.mp3';
88. end;
89.  
90. var
91.   Application: TMain;
92.  
93. {$R *.res}
94.  
95. begin
96.   Application:=TMain.Create(nil);
97.   Application.Title:='project1';
98.   Application.Run;
99.   Application.Free;
100. end.

How can I fix the error?

[Nimbus]

Hello,
You've declared S1 as TSong instance in a var section, but you never actually created an instance (by calling the constructor) before use. This is essential when working with classes - just declaring a variable is not enough. Normally the relevant code might be:

Code: Pascal  [Select][+][-]

1. var
2.   S1: TSong;
3. begin
4.   { ... }
5.  
6.   { add your program here }
7.  
8.   S1 := TSong.Create(Nil);  { constructor - always call before use }
9.   S1.DoRun;
10.   WriteLn(S1.Path);
11.   S1.Free;  { destroy the instance when no longer needed }
12.  

However, I'm not sure I fully understand what you are actually trying to achieve. For instance, I don't know why you'd want to inherit TSong class from TMain.

[nikel]

Thank you for your reply. That helped a lot.