DOC: CDC::SelectObject, CDC::SelectStockObject Return

SUMMARY

The pointer to a CGdiObject that is returned from CDC::SelectObject() or CDC::SelectStockObject() is stored for use. When the pointer is used later, it causes unexpected behavior [for example, a general protection fault (GPF) or heap corruption].

The documentation for the SelectObject() member does not make clear that the pointer returned from the call might point to a temporary object that is only valid during processing of one Windows message. This is an error in the documentation.

The CDC::SelectObject() and CDC::SelectStockObject() functions make a call to CGdiObject::FromHandle() to determine which CGdiObject object to return. If there is no CGdiObject object attached to the Windows GDI object that is currently selected in the DC, then a temporary object is created. See the documentation for CGdiObject::FromHandle() for further information.

   ...
   // Select a pen in the DC.
   CPen *ppenOld = pDC->SelectObject(ppenNew);
   m_hpenOld = (HPEN) ppenOld->GetSafeHandle();
   ...
   ...
   // In some other message handler we wish to
   // restore the old pen.
   CPen *ppenOld = CPen::FromHandle(m_hpenOld);
   pDC->SelectObject(ppenOld);
   ...