DOCUMENT:Q117686  07-MAY-2001  [visualc]
TITLE   :DOC: CDC::SelectObject, CDC::SelectStockObject Return
PRODUCT :Microsoft C Compiler
PROD/VER:winnt:1.0,2.0
OPER/SYS:
KEYWORDS:kbdocfix kbnokeyword kbMFC kbVC kbVC100bug kbVC150bug kbVC151bug kbVC152bug kbVC200fix

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The information in this article applies to:

 - The Microsoft Foundation Classes (MFC), used with:
    - Microsoft C/C++ for MS-DOS, version 7.0 
    - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52 
    - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0 
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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.

Additional query words: 7.00 1.00 1.50 2.00 2.10 2.50 CBrush CFont CPalette CPen

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Keywords          : kbdocfix kbnokeyword kbMFC kbVC kbVC100bug kbVC150bug kbVC151bug kbVC152bug kbVC200fix kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:1.0,2.0

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