Re: Use of CString key in CMap

"Giovanni Dicanio" <>
Fri, 4 Apr 2008 18:14:55 +0200
"David Ching" <> ha scritto nel messaggio

I'm not sure what you mean.

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?

This is true, however, using templates you could just as well say:

 CMap<CString, CString &, CMyClass, CMyClass &> m_map;

and this has the additional advantage that CMyClass does not need to
derive from CObject.

You are right about using CMap generic template.
But my problem is that it seems that it does not compile with CString key...

Or am I missing something?

// CMap
// (You must specify 4 types...)
CMap<CString, CString &, csomecobject, csomecobject &> m_map

This is true, but siince the 2nd and 4th parameters are simply reference
types of the 1st and 3rd parameters, it's not as if it causes much brain
power to figure these out. OTOH, making sense out of STL causes
significant brain power....

You are right that 2nd and 4th parameters are simply reference types, so no
big brain power is required, but I don't like that: I mean: why add this
complexity of 2nd and 4th parameter types? It seems to me useless

For STL, I believe that you can master that library better than me!
Moreover, you don't need to master *all* STL (I don't master all of it, of
course), you can just use some useful containers like vector or map...

However, I'm not in a position to "teach" anything to anyone :)
Mine are just simple advices, just IMHO.

I think that every programmer should use what he/she is familiar with and


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