Q158541: PRB: C2440 Error When Using CMap and User Defined Key Type

Article: Q158541
Product(s): Microsoft C Compiler
Version(s): winnt:4.0,4.1
Operating System(s): 
Keyword(s): kberrmsg kbtemplate kbMFC kbVC400 kbVC410 kbGrpDSMFCATL kbCollectionClass
Last Modified: 04-AUG-2001

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