PRB: C2664 Compiler Error in Afxtempl.h Using VC++ 4.2

ID: Q154105

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++, 32-bit Editions, version 4.2

SYMPTOMS

If you are using MFC-templated type safe collection classes, such as CTypedPtrArray, CTypedPtrList, CTypedPtrMap classes or any classes derived from these classes in MFC4.2, and you have a forward declaration of the base classes that are stored as elements in these collection classes, you may get compiler errors similar to the ones listed below depending on which classes are used in the application:


   c:\msdev\mfc\include\afxtempl.h(1561) : error C2664:
     'struct __POSITION *CObList::AddHead(class CObject *)' :
     cannot convert parameter 1 from 'class X *' to 'class CObject *'
     (new behavior; please see help)

- or -


   c:\msdev\mfc\include\afxtempl.h(1587) : error C2664:
   'SetAt' : cannot convert parameter 2 from 'class X *' to
             'class CObject *' (new behavior; please see help)

See the More Information section below for a reproducible case.

CAUSE

The MFC-templated type safe collection classes provided in MFC4.0 and MFC4.1 are not fully type safe. MFC4.2 added more wrapper functions to these classes to make them fully type safe. For example, in MFC4.2 the CTypedPtrList class now contains the following additional wrapper functions that were not in MFC4.0 or MFC4.1:


     // Add a new element before head or after tail of the list
     POSITION AddHead( TYPE newElement );
     POSITION AddTail( TYPE newElement );

     // Add another list of elements before head or after tail
     void AddHead( CTypedPtrList<BASE_CLASS,TYPE> *pNewList );
     void AddTail( CTypedPtrList<BASE_CLASS,TYPE> *pNewList );

     // Modify an element at a given position
     void SetAt( POSITION pos, TYPE newElement );

The implementation of some of these newly-added functions makes calls to their base class' implementation. For example, CTypedPtrList::AddHead(TYPE newElement) calls BASE_CLASS::AddHead(newElement) where newElement is expected to be a pointer to CObject. (BASE_CLASS can be either CObList, CPtrList, or any classes derived from these two classes for a CTypedPtrList.)

The BASE_CLASS must be derived from CObject or you will get the compiler error C2664.

Because there is only a forward declaration of the base class, before instantiating the MFC templated type safe collection class, the compiler has no way to tell if the base class is derived from CObject directly or indirectly, hence the compiler error C2664.

RESOLUTION

Define the base class before instantiating the type safe collection classes.

STATUS

This behavior is by design.

MORE INFORMATION

The following sample code demonstrates the problem:

Sample Code


   #include <afxtempl.h>

   class X;
   class Y
   {
     public:
       CTypedPtrArray<CObArray, X*> m_TextArray;
   };

   class X : public CObject
   {
     ......
   };

You can also reproduce the problem by building step 2 of the Scribble tutorial from Visual C++ 4.1 using Visual C++ 4.2.

For a full list of newly-added functions in classes CTypedPtrList, CTypedPtrArray and CTypePtrMap, please refer to the following Microsoft Knowledge Base article:

Q154482 FIX: Undocumented Functions in Type Safe Collection Classes

Additional query words: 4.00 4.10 4.20

Keywords : kbnokeyword kbMFC kbVC
Version : winnt:4.2
Platform : winnt
Issue type : kbprb