Destroy window ?

hi there,

I have a CEdit *m_pEdit pointer, it will be dynamically created using new and create fucntion and deleted using delete. My question is before deleting should we call the DestroyWindow function ? for ex :

m_pEdit->DestroyWindow();
delete m_pEdit;
m_pEdit = 0;

is the destroywindow necessary ?
My senior says that window will not be destroyed if not called DestroyWindow.

thanks ahead
rajs

Your senior is right.
Before calling delete you need to call DestroyWindow to destroy the window, created with Create method of CEdit class. CEdit class constructor do not do it by default.

thanks Armen for the reply

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Originally posted by Armen

CEdit class constructor do not do it by default.

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I'm sorry it should be "CEdit class destructor do not do it by default."...

Quote:

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Originally posted by brraj
My senior says that window will not be destroyed if not called DestroyWindow.

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The window will be destroyed if the parent is destroyed. It will also be destroyed if necessary by the destructor.

For example, it is common for a programmer to make the mistake of allocating a control in a function (instead of creating it with "new") and creating it in the function. Then they are confused and frustrated when they don't see the control in the dialog or other window. They don't see it because it gets destroyed when the function returns and the object goes out of scope and unallocated. The destructor destroys the window.

hi all,

thanks for the reply sam hobbs.

Armen what do u say for sam hobbs comments ?


cheers
rajs

Really I was wrong.
CEdit class destructor calls DestroyWindow of CWnd class.
You may see this with Spy++.

Thank you for the feedback. We seldom get positive feedback; we usually only get feedback when something does not work or is wrong.

hi Armen & Sam Hobbs,

I am not only talking about just CEdit. i want to know if it is common for all the other ctrls too.. for ex- ccombobox..


thanks ahead
rajs

Quote:

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Originally posted by brraj
I am not only talking about just CEdit. i want to know if it is common for all the other ctrls too.. for ex- ccombobox..

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Yes, any class derived from CWnd will call DestroyWindow() from its dtor.

hi gstercken ,

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Originally posted by gstercken
Yes, any class derived from CWnd will call DestroyWindow() from its dtor.
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If any class derived from cwnd calls DestroyWindow() from its dtor then why in CDialog modelless dialog we have call destroywindow ?

You have always been supportive to me and thanks for that.

thanks
ahead

Quote:

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Originally posted by brraj
If any class derived from cwnd calls DestroyWindow() from its dtor then why in CDialog modelless dialog we have call destroywindow ?

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You don't have to.

hi gstercken ,

Quote:

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You don't have to.

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thanks for the info
rajeev

Quote:

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Originally posted by brraj
If any class derived from cwnd calls DestroyWindow() from its dtor then why in CDialog modelless dialog we have call destroywindow ?

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You don't specify where in a modeless dialog you are refering to. I think you mean that in OnOk and/or OnCancel we must call DestroyWindow to end the dialog, and it is true that normally that is needed. Note that it is the dialog that is destroying itself, therefore we don't want to do something such as "delete this". According to the C++ standard (as best as I understand it) it is dangerous to do "delete this", but the danger is implementation-defined. Since VC is deveolped by Microsoft, who also develops Windows, and since Microsoft uses "delete this" in the PostNcDestroy virtual function, it is safe to assume we can do that. However we don't want to do "delete this" anywhere else! So how is our destructor going to be called if we want to destroy ourself? For modeless dialogs, we first destroy ourselves, and then delete ourselves from the universe when we get to our PostNcDestroy virtual function.

hi gstercken ,

what do u say about Sam Hobbs comments ?

cheers
rajs

Quote:

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Originally posted by brraj
what do u say about Sam Hobbs comments ?

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Sam is right. But as he says, you didn't specify where in a modeless dialog you mean. It depends on how the dialog is managed. Most often, with a modeless dialog, you keep a pointer to the CDialog object somewhere in your code. The dialog is instantiated, and then you call Create() and ShowWindow(). Now in OnOk / OnCancel, you have the choice of just closing the dialog (by calling ShowWindow with SW_HIDE) and keep the window object alive (for later use). In that case, the code "outside" which created the dialog would just need to delete the CDialog object later without having to call DestroyWindow() (as it is called in the dtor). However, a frequent scenario is that you want to destroy the window object in OnOk / OnCancel. Calling "delete this" would be a bad idea in this case, as it would leave the "outside" pointer to the CDialog object dangling. That's why you call DestroyWindow() in that case, leaving the CDialog object (without a valid HWND) alive. In order to redisplay the dialog, the "outside" code would have to call Create() once again, and reuse just the existing CDialog instance.

There are at least four possible scenarious for modeless dialogs, as follows:

• allocated by "new" and persists after the function that allocates it returns
• allocated by "new" but deleted prior to the function that allocates it returns
• allocated "on the stack" and therefore deleted automatically when the function that allocates it returns
• allocated as a class member variable

The corresponding actions we would typically need to do are:

• the dialog normally destroys itself (calls DestroyWindow in OnOk and OnCancel) and does the "delete this" in PostNcDestroy
• the "delete this" in PostNcDestroy is not needed if the object is deleted by the function that creates it
• no "delete this", since it would likely cause an error
• no "delete this", since it would likely cause an error

hi all,
I am calling the DestroyWindow in OnCancel.
SO i think for ctrls i will not call destroy window but only for modalless dialog i will call DestroyWindow as for ctrl it will automatically be called in destructors.


thanks samhobbs and gstercken for the comments


thanks
rajs