Re: Use of CString key in CMap

"David Ching" <>
Sat, 05 Apr 2008 02:32:16 GMT
"Giovanni Dicanio" <> wrote in message

Hi David,

I'm sorry if I wrote my ideas badly.

I would like to give you a C++ sample code:


 // CMap
 CMap< CString, const CString &, CString, const CString & > map1;
 map1[ _T("Seattle") ] = _T("Washington");
 map1[ _T("Napoli") ] = _T("Campania");

 // std::map
 std::map< CString, CString > map2;
 map2[ _T("Seattle") ] = _T("Washington");
 map2[ _T("Napoli") ] = _T("Campania");


This code does not compile under Visual C++ 9 (VS2008).

I get the following error:

error C2440: 'type cast' :
cannot convert from 'const CString' to 'DWORD_PTR'
j:\programmi\microsoft visual studio 9.0\vc\atlmfc\include\afxtempl.h 163

The error points in this template function in afxtempl.h:


template<class ARG_KEY>
// default identity hash - works for most primitive values
return (DWORD)(((DWORD_PTR)key)>>4);


If I comment out the CMap version (where the problem is), the STL std::map
version compiles fine.

What should I do to use CMap *generic template* with CString?
Why doesn't it work simply, just like std::map?

I think you've exposed a limitation of the MFC maps. Apparently the key
type must be a type that can be converted to a DWORD_PTR (which is a 32-bit
DWORD on 32-bit Windows and a 64-bit value on 64-). So unless the type has
a conversion operator producing the DWORD_PTR, it can't be a key. CString
can't convert to a DWORD_PTR.

I did workaround this problem by declaring

   CMap<LPCTSTR, LPCTSTR, CString, CString&> map1;

It's interesting that trying to follow the pattern of making the 2nd
parameter a reference, i.e..

   CMap<LPCTSTR, LPCTSTR&, CString, CString&> map1;

resulted in a compiler error. I'm not sure what we give up by not making
the 2nd parameter a reference.

MFC does have a built in class CMapStringToString, but if you look at the
source code, it doesn't rely on templates at all, and both the key and value
are of type LPCTSTR and not CString!

This excellent CodeProject article offers a lot more detail:


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