mfc, GDI OnPaint DCs and memory leaks

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mfc >> GDI OnPaint DCs and memory leaks

by Torsten Curdt » Fri, 28 Nov 2003 05:01:46 GMT

I am tracking down a memory leak. So I wanna make sure
I got everything right...

I have an AbstractDialog class with

void AbstractDialog::OnPaint()
{
CPaintDC dc(this);
OnDraw(dc);
}

void AbstractDialog::OnDraw( CPaintDC& dc )
{
}

and each Dialog can now easily override the OnDraw method.
AFAIU overriding OnPaint() is not a good idea if you have
a deeper inheritance because the paint message should only
be sent once. Now in my dialog class I have e.g.:

void MyDialog::OnDraw( CPaintDC& dc )
{
CDC memDC;
memDC.CreateCompatibleDC(&dc);
...
memDC.DeleteDC();
}

IIUC omitting the DeleteDC() call would give a GDI memory
leak, right? ...but the CPaintDC will be deleted from it's
destructor, right?

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mfc >> GDI OnPaint DCs and memory leaks

by Jeff Partch [MVP] » Fri, 28 Nov 2003 07:06:58 GMT






The dtors for both objects should do the right thing: CPaintDC should call
EndPaint and CDC should DeleteDC.
--
Jeff Partch [VC++ MVP]




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mfc >> GDI OnPaint DCs and memory leaks

by Torsten Curdt » Fri, 28 Nov 2003 07:23:11 GMT

>>IIUC omitting the DeleteDC() call would give a GDI memory

That's strange :-/ ...I was came across a resource leak in
my application because DeleteDC calls were missing.

I added them and then found a 1:1 ratio of create/delete
calls. I am currently running a test in release mode.
But that will take a few hours... tomorrow we'll see

Talk to you tomorrow. Thanks!

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GDI OnPaint DCs and memory leaks

by aman_zafar » Fri, 28 Nov 2003 14:02:57 GMT

why do you want to do like that since ur OnPaint Handler will call the
Destuctor so what the need to do it, don't delete it.

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GDI OnPaint DCs and memory leaks

by Torsten Curdt » Fri, 28 Nov 2003 17:18:24 GMT





Ok .. you were right that wasn't the problem :-/
Seems like the problem only appears in the release
version. But I cannot see why. Here is the code
that must do something terribly wrong:

void AbstractDialog::DrawBitmap( CDC* pDc, int pX, int pY, CBitmap*
pBitmap )
{
CDC memDC;
memDC.CreateCompatibleDC(pDc);

CBitmap* pOld = memDC.SelectObject(pBitmap);
BITMAP bm;
pBitmap->GetObject(sizeof(bm), &bm);
CSize size = pBitmap->SetBitmapDimension(bm.bmWidth, bm.bmHeight);
pDc->BitBlt(pX, pY, bm.bmWidth, bm.bmHeight, &memDC, 0, 0, SRCCOPY);
pDc->SelectObject(pOld);
}

Can someone explain?

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