Possible bug? (4.2)

Chris Scott -- cscott@aceltd.com
Thursday, August 01, 1996

Enviroment: Visual C++ 4.2, NT 4.0 Beta 2

I recently recompiled my application under VC++ 4.2 and found that it was 
suddenly leaking ImageLists.

Here's the error:
Detected memory leaks!
Dumping objects ->
winctrl2.cpp(515) : {8195} client block at 0x0054AEA0, subtype 0, 8 bytes 
long.
a CImageList object at $0054AEA0, 8 bytes long

More specifically, CListCtrl::CreateDragImage() and 
CTreeCtrl::CreateDragImage() are the culprits.  The functions return a 
pointer to an ImageList, but nowhere in the 4.2 docs does it say that I 
need to delete the memory.  Besides, this worked fine under 4.1.

Is anyone else running into this?

- Chris Scott

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Mike Blaszczak -- mikeblas@nwlink.com
Monday, August 05, 1996

[Mini-digest: 2 responses]

At 06:20 AM 8/1/96 -0400, you wrote:
>Enviroment: Visual C++ 4.2, NT 4.0 Beta 2

>I recently recompiled my application under VC++ 4.2 and found that it was 
>suddenly leaking ImageLists.

>Here's the error:
>Detected memory leaks!
>Dumping objects ->
>winctrl2.cpp(515) : {8195} client block at 0x0054AEA0, subtype 0, 8 bytes 
>long.
>a CImageList object at $0054AEA0, 8 bytes long

>More specifically, CListCtrl::CreateDragImage() and 
>CTreeCtrl::CreateDragImage() are the culprits.  The functions return a 
>pointer to an ImageList, but nowhere in the 4.2 docs does it say that I 
>need to delete the memory.  Besides, this worked fine under 4.1.

MFC 4.1 had a bug because it returned a temporary object when you called
CListCtrl::CreateDragImage().  It turns out that you can (and just might)
hold that object for longer than it takes to process the current message,
and MFC might delete the temporary object in that time.

MFC 4.2 fixes that problem by returning a permanent object.  You're right:
you do need to delete the returned object when you're done with it by
yourself.  The documentation doesn't say that, and that's a doc bug.


.B ekiM
http://www.nwlink.com/~mikeblas/
These words are my own. I do not speak on behalf of Microsoft.

-----From: Gonzalo Isaza 

A bug in version 4.1 has been fixed.   In VC++ 4.1 the image list was
created as a temporary object.  The problem was, the object was
destroyed during idle time.  A problematic situation could be the
following :  if a user implemented dragging without using the mouse
buttons (just by moving the mouse), it was possible for OnIdle to
execute at the same time the object was being dragged, destroying the
CImageList object.

The bug was fixed in 4.2 and it is necessary now to call delete for
those image lists.

Gonzalo.
I speak for myself.  I do not speak for Microsoft.

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