Copy a background image and save in CImage

Question

Hi,
I am attempting to emulate transparency. My background is drawn in a CView class via OnEraseBkgnd using the CImage.Draw method:

pDoc->m_imgBitmap[pDoc->m_iPage-1].Draw(pDC->m_hDC, point);

I have saved the pDC, thus:

pDoc->m_pDCBitmap[pDoc->m_iPage - 1] = pDC;

where pDC is passed as argument into OnEraseBkgnd. All this seems to work fine.

When I extract the portion of image and display it, I get a back rectangle of the correct size. I assume that the pDC I saved is incorrect. I would appreciate if someone could confirm if I have used the pDC correctly.

Many thanks

Answer 1

CView::OnEraseBkgnd() is called from  below function.  
  
BOOL CWnd::OnWndMsg(UINT message, WPARAM wParam, LPARAM lParam, LRESULT* pResult)  
{  
[...]  
    	case AfxSig_b_D_v:  
    		lResult = (this->*mmf.pfn_b_D)(CDC::FromHandle(reinterpret_cast<HDC>(wParam)));  
    		break;  
[...]  
}  

https://learn.microsoft.com/en-us/cpp/mfc/reference/cdc-class?view=msvc-160#fromhandle clearly mentions, "The pointer may be temporary and should not be stored beyond immediate use."

-SM