BorlandTalk.com Borland discussion newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  FormResize() being called after my form's destructor Goto page 1, 2  Next          BorlandTalk.com Forum Index -> C++ Builder (VCL Components Usage) View previous topic :: View next topic   Author Message Joseph F. Muscarella Guest Posted: Thu Dec 11, 2003 1:13 pm    Post subject: FormResize() being called after my form's destructor When I close my program, I am getting an error (Access violation). When tracking it down, I discovered that the FormResize method was being called after my form's destructor had already executed. This should not be allowed, because my form is invalid at this point and any work done by FormResize() could be catastrophic (and obviously is.) I know that OnDestroy should be avoided due to problems like this, but does that apply also to FormResize? Or is this due to some obscure error in my program that I haven't yet discovered. I thought I would check here first. Thanks, Joe Back to top Vladimir Guest Posted: Thu Dec 11, 2003 3:49 pm    Post subject: Re: FormResize() being called after my form's destructor Quote: When I close my program, I am getting an error (Access violation). When tracking it down, I discovered that the FormResize method was being called after my form's destructor had already executed I also noticed that. I avoid 'Access Violations' by checking my pointers: if ( MyPointer != NULL ) DoTask(); else ViolationAvoided(); .... but be sure to initialize MyPointer to NULL after destruction: delete MyPointer; MyPointer = NULL; I would like also to hear why is this event fired. Vladimir. Back to top Joseph F. Muscarella Guest Posted: Thu Dec 11, 2003 4:00 pm    Post subject: Re: FormResize() being called after my form's destructor Quote: if ( MyPointer != NULL ) DoTask(); else ViolationAvoided(); ... but be sure to initialize MyPointer to NULL after destruction: delete MyPointer; MyPointer = NULL; Yes, but the problem here is that if MyPointer is a member of my Form, then after the form's destructor finishes, the memory used by MyPointer is invalid. Checking this memory location in the OnResize method is therefore invalid. Although it may work sometimes (if that memory hasn't been reused yet), it is not guaranteed to work and will probably result in very bad things. On the other hand, if MyPointer (or some other simple flag like a bool) is declared globally static, then I guess it would work. For what it's worth, I got around this by unhooking the OnResize event handler in my destructor: OnResize = NULL; This is a kludge, but it works. I think the VCL has a bug. Joe Back to top Vladimir Guest Posted: Thu Dec 11, 2003 4:07 pm    Post subject: Re: FormResize() being called after my form's destructor Quote: Yes, but the problem here is that if MyPointer is a member of my Form, then after the form's destructor finishes, the memory used by MyPointer is invalid. That's much bigger problem. My pointers were global, because they are defined and declared before main form construction... Quote: For what it's worth, I got around this by unhooking the OnResize event handler in my destructor: OnResize = NULL; This is a kludge, but it works. I think the VCL has a bug. I use initializing events with NULL very often to avoid some recursive calls. Back to top JD Guest Posted: Thu Dec 11, 2003 8:24 pm    Post subject: Re: FormResize() being called after my form's destructor "Joseph F. Muscarella" <joe (AT) infosight-DISCARDMUNG- (DOT) com> wrote: Quote: [...] I think the VCL has a bug. AFAIK there are no problems with the OnResize event. Please show both OnResize and the dtor. ~ JD Back to top Vladimir Stefanovic Guest Posted: Thu Dec 11, 2003 9:58 pm    Post subject: Re: FormResize() being called after my form's destructor Quote: AFAIK there are no problems with the OnResize event. Please show both OnResize and the dtor. Some problem definitely exists! The code of mine is very complex but I'll try to extract some code skeleton that reproduces the problem (tonight or maybe tomorrow). Now, just some notes... On some machines 'Access Violation' does not appear as a message (?!). But if the project is build with CodeGuard the problem can be seen very precisely in the CodeGuards Log (*.cgl) file. Beleave me I'll never use the below condition ( Cfg != 0 ) if it's not needed. void __fastcall TInitialForm::FormResize(TObject *Sender) { // ... bool result; if ( Cfg != 0 ) // { result = Cfg->Get( SomeArgs ); } else { ForgetResultAndGoHome(); return; } // ... } Vladimir. Back to top Joseph F. Muscarella Guest Posted: Fri Dec 12, 2003 2:47 pm    Post subject: Re: FormResize() being called after my form's destructor Quote: AFAIK there are no problems with the OnResize event. Please show both OnResize and the dtor. __fastcall TMainForm::~TMainForm() { DisplayPrefs dprefs = Preferences::Instance()->Get_display_prefs(); dprefs.state = WindowState; dprefs.top = Top; dprefs.left = Left; Preferences::Instance()->Set_display_prefs(dprefs); delete Preferences::Instance(); } void __fastcall TMainForm::FormResize(TObject *Sender) { term_rows = ImagePanel->Height / row_height; term_cols = (Width / col_width) - 1; display_rows = term_rows / 4; display_chars = display_rows * term_cols; Show_page(); } Using the debugger, I placed a breakpoint in both the destructor and in the FormResize function. When the form is closed, the destructor breakpoint is hit first, then the FormResize breakpoint. When my Show_page() function is called, the Access Violation happens. If I add OnResize = NULL; in my destructor, the problem goes away. BTW, I am using BCB 5 Enterprise. Regards, Joe Back to top Boba Guest Posted: Fri Dec 12, 2003 6:16 pm    Post subject: Re: FormResize() being called after my form's destructor "Joseph F. Muscarella" <joe (AT) infosight-DISCARDMUNG- (DOT) com> wrote Quote: if ( MyPointer != NULL ) DoTask(); else ViolationAvoided(); ... but be sure to initialize MyPointer to NULL after destruction: delete MyPointer; MyPointer = NULL; Yes, but the problem here is that if MyPointer is a member of my Form, then after the form's destructor finishes, the memory used by MyPointer is invalid. Checking this memory location in the OnResize method is therefore invalid. Although it may work sometimes (if that memory hasn't been reused yet), it is not guaranteed to work and will probably result in very bad things. MyPointer will become invalid only if you free it explicitly in the Form's destructor; otherwise the memory it points to will become hard to access (unless you've made a copy of that pointer) but the memory will not get freed (memory leak). If this is a case, "checking this memory location in the OnResize method is therefore invalid" because the OnResize method itself is not accessible, but not the memory location pointed by MyPointer. If MyPointer points to a memory location inside the Form, then the story is different and there will be no memory leak after Form's destruction: class TMyForm{ int I[10], *MyPointer; }; TMyForm *Form=new TMyForm; Form->MyPointer=new int[10]; delete Form;//memory leak *Form=new TMyForm; Form->MyPointer=Form->I; delete Form;//no leak //Boba Back to top Joseph F. Muscarella Guest Posted: Fri Dec 12, 2003 7:07 pm    Post subject: Re: FormResize() being called after my form's destructor Quote: MyPointer will become invalid only if you free it explicitly in the Form's destructor; otherwise the memory it points to will become hard to access (unless you've made a copy of that pointer) but the memory will not get freed (memory leak). If this is a case, "checking this memory location in the OnResize method is therefore invalid" because the OnResize method itself is not accessible, but not the memory location pointed by MyPointer. If MyPointer points to a memory location inside the Form, then the story is different and there will be no memory leak after Form's destruction: class TMyForm{ int I[10], *MyPointer; }; TMyForm *Form=new TMyForm; Form->MyPointer=new int[10]; delete Form;//memory leak *Form=new TMyForm; Form->MyPointer=Form->I; delete Form;//no leak Yes, but as I said, if MyPointer is a member of my Form (not necessarily owned by the form - perhaps a pointer to char or something, not part of the VCL), when the form's destructor executes then the memory used by the form is not valid. Therefore, you cannot access MyPointer, even from outside the class if it were public , because the Form no longer exists and its "this" is invalid. You are right about the memory leak if MyPointer is not owned by the Form and the destructor does not free it. Joe Back to top JD Guest Posted: Fri Dec 12, 2003 7:09 pm    Post subject: Re: FormResize() being called after my form's destructor "Joseph F. Muscarella" <joe (AT) infosight-DISCARDMUNG- (DOT) com> wrote: Quote: [...]When the form is closed, the destructor breakpoint is hit first, then the FormResize breakpoint. When my Show_page() function is called, the Access Violation happens. Obviousely, Show_Page is causing the AV. Without seeing the code to determine if there is a better way or place for Show_Page, you're doing the best that you can with it. The only other option is to use a global flag that you set in the dtor and test for in the FormResize. ~ JD Back to top Boba Guest Posted: Fri Dec 19, 2003 11:13 pm    Post subject: Re: FormResize() being called after my form's destructor "Joseph F. Muscarella" <joe (AT) infosight-DISCARDMUNG- (DOT) com> wrote Quote: ... Yes, but as I said, if MyPointer is a member of my Form (not necessarily owned by the form - perhaps a pointer to char or something, not part of the I don't understand what you mean by 'a member pointer not owned by a Form'. Quote: VCL), when the form's destructor executes then the memory used by the form is not valid. correct. Quote: Therefore, you cannot access MyPointer, even from outside the class if it were public , because the Form no longer exists and its "this" is invalid. ... wrong. I can: even when 'this'prt is invalid, that memory location is accessible until freed. Boba Back to top JD Guest Posted: Sat Dec 20, 2003 12:16 pm    Post subject: Re: FormResize() being called after my form's destructor "Vladimir Stefanovic" <spam (AT) not (DOT) needed> wrote: Quote: AFAIK there are no problems with the OnResize event. Please show both OnResize and the dtor. Some problem definitely exists! I tested the OnResize event. It does not get called when the form is destroyed unless you explicitly do something from within the dtor to cause it to be called. In that case, the problem would be with your logic, not the VCL. ~ JD Back to top Vladimir Stefanovic Guest Posted: Sat Dec 20, 2003 4:18 pm    Post subject: Re: FormResize() being called after my form's destructor Quote: "Vladimir Stefanovic" <spam (AT) not (DOT) needed> wrote: AFAIK there are no problems with the OnResize event. Please show both OnResize and the dtor. Some problem definitely exists! I tested the OnResize event. It does not get called when the form is destroyed unless you explicitly do something from I tested it too, and couldn't reproduce the problem in the simple example. I'll try to reduce my code step by step and find what's the problematic part of the code. I haven't noticed the problem on my machine, but it crached on some others. Then I included CodeGuard in the app. build and find invalid pointers access. I'm not sure what can cause the OnResize after destruction, but it realy exists. Till then I have to include the aditioonal condition to avoid that... Thanks for not forgetting this problem. Vladimir. Back to top Joseph F. Muscarella Guest Posted: Mon Dec 22, 2003 3:59 pm    Post subject: Re: FormResize() being called after my form's destructor I Googled the problem, and one thing came up: It appears to have something to do with the MainMenu when it is destroyed. It apparently causes OnResize to fire regardless of whether the MainForm has been destroyed. I haven't tested this, but who knows... Joe Quote: AFAIK there are no problems with the OnResize event. Please show both OnResize and the dtor. Some problem definitely exists! I tested the OnResize event. It does not get called when the form is destroyed unless you explicitly do something from I tested it too, and couldn't reproduce the problem in the simple example. I'll try to reduce my code step by step and find what's the problematic part of the code. I haven't noticed the problem on my machine, but it crached on some others. Then I included CodeGuard in the app. build and find invalid pointers access. I'm not sure what can cause the OnResize after destruction, but it realy exists. Till then I have to include the aditioonal condition to avoid that... Thanks for not forgetting this problem. Vladimir. Back to top Joseph F. Muscarella Guest Posted: Mon Dec 22, 2003 4:05 pm    Post subject: Re: FormResize() being called after my form's destructor Quote: I don't understand what you mean by 'a member pointer not owned by a Form'. I should have been more specific, I guess. A class member that is part of TForm but is not part of the VCL, so TForm knows nothing of it. It doesn't own it, so the VCL will not destroy it. My own destructor is responsible for destroying it. Quote: Therefore, you cannot access MyPointer, even from outside the class if it were public , because the Form no longer exists and its "this" is invalid. ... wrong. I can: even when 'this'prt is invalid, that memory location is accessible until freed. Boba Not if you try to access it by MainForm->MyPointer This is dangerous since the memory formerly owned by MainForm **MAY** have been overwritten and you don't know it. Thus, MyPointer may still be valid somewhere in the heap (abandoned, and thus a leak) but you can't get to it through MainForm. Only if you had made a copy of MyPointer *outside* of MainForm would you still be able to access it (and this would be bad practice anyway.) 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