Q117686: DOC: CDC::SelectObject, CDC::SelectStockObject Return

Article: Q117686
Product(s): Microsoft C Compiler
Version(s): winnt:1.0,2.0
Operating System(s): 
Keyword(s): kbdocfix kbnokeyword kbMFC kbVC kbVC100bug kbVC150bug kbVC151bug kbVC152bug kbVC200fix
Last Modified: 07-MAY-2001

-------------------------------------------------------------------------------
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 
-------------------------------------------------------------------------------

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

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

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.