DOCUMENT:Q158541  04-AUG-2001  [visualc]
TITLE   :PRB: C2440 Error When Using CMap and User Defined Key Type
PRODUCT :Microsoft C Compiler
PROD/VER:winnt:4.0,4.1
OPER/SYS:
KEYWORDS:kberrmsg kbtemplate kbMFC kbVC400 kbVC410 kbGrpDSMFCATL kbCollectionClass

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - The Microsoft Foundation Classes (MFC), used with:
    - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1 
-------------------------------------------------------------------------------

SYMPTOMS
========

When using the Cmap-templated collection class with a user-defined class as the
data type for key arguments, the compiler generates the following compiler
error:

   afxtempl.h(97) : error C2440: 'abstract declarator' :

   cannot convert from 'class CClass' to 'unsigned long'

where CClass is the name of the user-defined class.

CAUSE
=====

The default implementation of CMap's HashKey function takes an DWORD (unsigned
long) as a parameter. The compiler does not know how to convert from a
user-defined class type to an unsigned long.

RESOLUTION
==========

Create a static template function to generate a custom HashKey value.

STATUS
======

This behavior is by design.

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

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

This is a sample declaration of a CMap that uses a user-defined class for as
data-type for a Cmap:

      #include <afxtempl.h>

      class CEmployee : public CObject
      {
      public:
      CString m_firstName;
      CString m_lastName;

      public:
      CEmployee(){ };

      // Copy constructor required by Cmap.
      CEmployee(const CEmployee& n)
      {
      m_firstName = n.m_firstName;
      m_lastName = n.m_lastName;
      }

      // Operator= is required by Cmap.
      CEmployee& operator=(const CEmployee& n)
      {
      m_firstName = n.m_firstName;
      m_lastName = n.m_lastName;
      return *this;
      }

      BOOL AFXAPI operator==(const CEmployee& n) const
      {
      return m_firstName == n.m_firstName
      && m_lastName == n.m_lastName;
      }
      };

      template<>
      inline UINT AFXAPI HashKey(CEmployee& key)
      {
      return HashKey((LPCTSTR)key.m_lastName) +
                      HashKey((LPCTSTR)key.m_firstName);
      }

      // Declare the CMap object.
      CMap<CEmployee, CEmployee&, CEmployee, CEmployee&> EmployeeMap;


Additional query words:

======================================================================
Keywords          : kberrmsg kbtemplate kbMFC kbVC400 kbVC410 kbGrpDSMFCATL kbCollectionClass 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.0,4.1
Issue type        : kbprb

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

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