Q102327: INFO: Const Versus Non-Const CObList Objects

Article: Q102327
Product(s): Microsoft C Compiler
Version(s): 7.0
Operating System(s): 
Keyword(s): kbMFC kbVC100 kbVC150 kbVC200 kbVC400 kbGrpDSMFCATL kbCollectionClass
Last Modified: 07-MAY-2001

7.00   | 1.00 1.50 1.51 1.52 | 1.00 2.00 2.10 4.00
MS-DOS | WINDOWS             | WINDOWS NT
kbprg


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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft C/C++, version 7.0 
   - Microsoft Visual C++ for Windows 
   - Microsoft Visual C++ 32-bit Edition 
-------------------------------------------------------------------------------

SUMMARY
=======

When you write an application with the CObList class provided by the Microsoft
Foundation Class (MFC) libraries, you may note that many of the member functions
have both const and non-const forms. For example, the CObList class has two
GetNext() member functions, as follows:

  CObject * & GetNext(POSITION & rPosition);
  CObject *   GetNext(POSITION & rPosition) const;

The first form of the GetNext() member function is for non-const CObList objects
and implements the left-side of the assignment operator. The second form of the
GetNext() member function is used when the CObList object is const (generally,
this means that you are using a pointer to a const CObList to prevent any
modification to the contents of the CObList).

Note that the first member function returns a reference to a CObject pointer. The
item returned is the actual CObject pointer in the list, not its value. Through
this reference, you can modify the CObject pointer and use the return value of
the member function to modify the list. The second member function does not
provide access to modify the CObList.

MORE INFORMATION
================

The code sample below demonstrates using the GetNext() member function that
returns a reference to a CObject pointer. Note the comment in the code. If you
remove the comment from the indicated line, an error results.

Sample Code
-----------

  // Sample program to create a CObList, fill it, display the "set"
  // member of each CObject pointed to by the CObList, reassign the
  // elements of the CObList, and display the "set" member of each
  // CObject pointed to by the CObList a second time.

  /*

* Compiler options needed: Visual C++ 4.0 - /MT[d] or /MD[d] * Others - None

   */ 

  #define _DOS
  #include <iostream.h>
  #include <afx.h>
  #include <afxcoll.h>

class CMyObject: public CObject { public:

  CMyObject(int i):set(i) { }
  int set;

} MyObject(2); // Create a CObject to place in the CObList.

                  // assign set = 2

  void main(void)

{

  CObList * pMyList = new CObList;
  const CObList * pMyListAlso = pMyList;

     // load list
     for (int nCount = 0; nCount < 10; nCount++)
        pMyList->AddHead(new CMyObject(1));   // Fill up list with
                     // CMyObject objects that have set equal to 1

  for (POSITION p = pMyList->GetHeadPosition(); p != NULL; )

  cout << ((CMyObject*)pMyList->GetNext(p))->set;

  for (p = pMyList->GetHeadPosition(); p != NULL; )

  (pMyList->GetNext(p)) = &MyObject;

  // If you remove the comments from the following lines, the compiler
  // generates the following error:
  // 
  //    error C2106: '=' : left operand must be lvalue
  // 
  // The error occurs because pMyListAlso is a pointer to a const
  // CObList object and the GetNext() member function that returns a
  // CObject pointer is called.
  // 
  // for (p = pMyList->GetHeadPosition(); p != NULL; )
  //    (pMyListAlso->GetNext(p)) = &MyObject;

  for (p = pMyList->GetHeadPosition(); p != NULL; )

  cout << ((CMyObject*)pMyList->GetNext(p))->set;

}

Additional query words: 7.00 1.00 1.50 2.00 2.10 2.50 2.51 2.52 3.00 kbinf 4.00

======================================================================
Keywords          : kbMFC kbVC100 kbVC150 kbVC200 kbVC400 kbGrpDSMFCATL kbCollectionClass 
Technology        : kbAudDeveloper kbMFC
Version           : :7.0
Issue type        : kbinfo

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