Using CDC::InvertRgn

Peter Moss -- pmoss@bbn.com
Sunday, November 24, 1996

Environment: MSVC 4.2-flat, NT 4.0, Win 95

I am creating a CRgn and then calling CDC::InvertRgn. I haven't played with 
this before, but it worked the first time. My question is that I am not 
explicitly selecting the CRgn into the DC before calling CDC::InvertRgn, nor 
am I calling DeleteObject on it. I saw some note in the MFC doc on CRgn that 
implies that I should call CGDIObject::DeleteObject().  Am I causing resource 
leaks by not doing this? It makes me a little nervous cause I'm doing this in 
a OnMouseMove() response method, so it will get called a lot.

Here is my code currently:

CRgn Rgn;
..
Rgn.CreatePolygonRgn(...);

CClientDC dc(this);
dc.InvertRgn(&Rgn);
__________________________________________________

i.e., do I need to do something like the following:

CRgn Rgn;
..
Rgn.CreatePolygonRgn(...);

CClientDC dc(this);
dc.SelectObject(&Rgn);
dc.InvertRgn(&Rgn);

Rgn.DeleteObject();
_________________________________

If I do need to do the above, how do I "deselect" the CRgn since the 
CGDIObject::DeleteObject() doc says that 
"An application should not call DeleteObject on a CGdiObject object that is 
currently selected into a device context. "
The CDC::SelectObject() call for CRgns does not return any "previously 
selected" region.  

Any comments?

Thanks,
Pete Moss
pmoss@bbn.com

---

Jim Lawson Williams -- jimlw@mail.ccur.com.au
Tuesday, November 26, 1996

[Mini-digest: 3 responses]

At 10:38 PM 24-11-96 -0500, you wrote:
>Environment: MSVC 4.2-flat, NT 4.0, Win 95
>
>I am creating a CRgn and then calling CDC::InvertRgn.  I saw some note in the MFC doc on CRgn that implies that I should call CGDIObject::DeleteObject().  Am I causing resource 
leaks by not doing this?

If "Debug Mode" reports no leaks, believe it.  From the little code you supply I infer that you are creating your CRgn on the stack, and it disappears at function-end.
 

>i.e., do I need to do something like the following:
>
>CRgn Rgn;
>..
>Rgn.CreatePolygonRgn(...);
>
>CClientDC dc(this);
>dc.SelectObject(&Rgn);
>dc.InvertRgn(&Rgn);
>
>Rgn.DeleteObject();
>

  Try that and see what happens.....  :-}

>The CDC::SelectObject() call for CRgns does not return any "previously 
>selected" region.  

No, it simply tells you what kind of CRgn you're attempting to deal with  --  you missed this part of the on-line documentation:

The version of the member function that takes a region parameter performs the same task as the SelectClipRgn member function. Its return value can ........
    ^^^^^^^^^^^^^

Regards,
Jim LW
-----From: "L.P. Fu" 

 Since you are using CDC and CRgn as stack variables.  MFC will clean up
for you.
 One of the thing you have remember is you can't delete an object when it
is still selected in the DC.

-----From: Mike Blaszczak 

At 22:38 11/24/96 -0500, Peter Moss wrote:
>Environment: MSVC 4.2-flat, NT 4.0, Win 95

>I am creating a CRgn and then calling CDC::InvertRgn. I haven't played with 
>this before, but it worked the first time. My question is that I am not 
>explicitly selecting the CRgn into the DC before calling CDC::InvertRgn, nor 
>am I calling DeleteObject on it.

The CRgn destructor will destroy the region for you.

You don't need to select a region to use InvertRgn().  InvertRgn() uses the
region you pass as a paremeter.

>If I do need to do the above, how do I "deselect" the CRgn since the 
>CGDIObject::DeleteObject() doc says that 
>"An application should not call DeleteObject on a CGdiObject object that is 
>currently selected into a device context. "

Again, you don't.  But you do need to do it for things like brushes, which
you do need to select into the DC to use, you might code something like
this:

void CYourView::OnDraw(CDC* pDC)
{
   CBrush brush(RGB(83, 83, 83));
   CBrush* pOldBrush;

   pOldBrush = pDC->SelectObject(&brush);

   CRect rect(0, 0, 100, 100);
   pDC->Rectangle(&rect);

   pDC->SelectObject(pOldBrush);
}

The CBrush destrctor destroys the "brush" object for you.  You
don't need to destroy pOldBrush--it isn't yours. You unselect
"brush" from the  DC by selecting pOldBrush back in there again.

.B ekiM
http://www.nwlink.com/~mikeblas/
I'm afraid I've become some sort of speed freak.
These words are my own. I do not speak on behalf of Microsoft.

---

Mike Blaszczak -- mikeblas@nwlink.com
Wednesday, November 27, 1996

At 23:19 11/26/96 +1100, Jim Lawson Williams wrote:

>>I am creating a CRgn and then calling CDC::InvertRgn.  I saw
>>some note in the MFC doc on CRgn that implies that I should call
>>CGDIObject::DeleteObject().  Am I causing resource 
>>leaks by not doing this?

>If "Debug Mode" reports no leaks, believe it. 

MFC and the C Runtimes only track memory leaks, not resource leaks.

.B ekiM
http://www.nwlink.com/~mikeblas/
I'm afraid I've become some sort of speed freak.
These words are my own. I do not speak on behalf of Microsoft.

---

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