DOC: CDC::SelectObject, CDC::SelectStockObject Return (117686)

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.

MORE INFORMATION

If you want to store the object that was previously selected in the DC, then you should store the Windows handle to the GDI object (HGDIOBJ), not a pointer to the CGdiObject. The handle can be obtained from the temporary file by using the function CGdiObject::GetSafeHandle(). You can later use the function CGdiObject::FromHandle() to obtain a pointer to a CGdiObject that encapsulates the HGDIOBJ. For example:

   ...
   // 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);
   ...

REFERENCES

For further information on mapping C++ objects to handles in the MFC, see MFC TechNote #3, "Mapping of Windows Handles to Objects."

NOTE: The documentation for Visual C++ 2.0 (MFC 3.0) has been corrected.