mfc/vc++ subclassing

Posted on 1998-10-13
Last Modified: 2012-08-13
I am trying to subclass a CWnd class1,not just a control from another CWnd class2 to take over paint messages among others. First attempts failed because CWnd::SubclassWindow() requires the HWND be detached from the CWnd class. (can't do because then class1 will no longer have a valid HWND) Have had better success with the following:

OldProc = SetWindowLong(hwnd,GWL_WNDPROC,fn);
where hwnd= the handle of the windows to subclass
and fn = the address of the new message handler.

Now for the problem: the new message handler (fn) is
a function in a class also. (a class trying to subclass another class) class2::fn actually does get called however the "this" point is invalid, so no member variables are accessible. The "this" pointer passed to class2::fn apparently is the class1::m_hWnd of the original CWnd.

My question is 2 part; first is this a valid approch or is there an easier way of doing this ( or is this posible). Second (vc++ question) how do you obtain a "this" pointer for a class. I can think of a few other needs. mainly C/asm functions calling a C++ class function. thx!
Question by:bculver
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Expert Comment

ID: 1175050
I can't imagine under what conditions you should have HWND already attached. Probably
you need two CWND (or derived) class objects defined. In any case, detaching old one
is not bad, cause you will be re attaching next one anyway.

If if does not help, please post your code-snippet.

Expert Comment

ID: 1175051
I can't imagine under what conditions you should have HWND already attached. Probably
you need two CWND (or derived) class objects defined. In any case, detaching old one
is not bad, cause you will be re attaching next one anyway.

If if does not help, please post your code-snippet.

Author Comment

ID: 1175052
HWND is attached because CWnd::class1 is actually a dialog (CDialog) derived from
CWnd. You can't detach because the dialog class would have no HWND to work
with. I am actually tring to create a generic class to handle painting pix's on dialog
backgrounds. I don't want to take over complete control, ie. detaching HWND and
reattaching to the new class, just intercept some paint/palette messages. Class2
is actually derived from class1 and I could trap each message from class1 and make
the appropriate calls to class2, however I was hoping for something a little more elegant
like:  class2::SetBackgroundPic(picture);  in the OnInitDialog() have class2 subclass class1 functions and be done with it. :|

Expert Comment

ID: 1175053
I suggest you don't combine MFC with API that way . Create a regular ( not in the class ) function to handle the messages and store any "this" pointer to a global variable .

Accepted Solution

SamratAshok earned 200 total points
ID: 1175054
Iam slightly confused with your class1 and class2, so this is what Iam assuming them.
class1 is an instance of either a CDialog derived class or CDialog class object.
class2 contains the bitmap painting function derived from class1.

Nevertheless, I think you preety much are on the solution, just
not noticing it. All you need is this:

Derive a class from CDialog say CCustomBackgroundDialog (your generic class).
In this class, override OnInitDialog or OnPaint and just put whatever bitmap you want.

Iam mentioning a OnInitDialog because if you are creating a button with image,
you need to do it only once and then you need to override initdialog function.

If you are drawing bitmap directly on dialog device context, you need to override OnPaint

Now every class that you derive from CCustomBackgroundDialog will have
your bitmap.

Otherwise, sending or posting your code would be big help.

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