Re: 'CMap' of 'CString' to 'CString'

"Michael K. O'Neill" <>
Sat, 14 Apr 2007 11:01:28 -0800
"Martin" <> wrote in message


I'm trying to create a dictionary with both key and value of type
'CString'. I declare the object of 'CMap' so it is as closer to
std::map, as possible:

CMap<CString, const CString&, CString, const CString&> m_appLangs;

and semantically it's like this:

std::map<std::string, std::string> m_appLangs;

But it seems 'CMap' is not realized as an RB-tree, I guess it must be
realized through hash table. The problem is when I make the above
declaration, my source file refuse to compile. Here is the simplified
error message:

cannot convert from 'const CString' to 'DWORD_PTR'
see reference to function template instantiation 'UINT
HashKey<ARG_KEY>(ARG_KEY)' being compiled with
[ARG_KEY=const CString &]

Here is the code of 'HashKey' function from "afxtempl.h":

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

And what does it mean? Can't I create a map of string to string just
because there is no conversion from 'CString' to 'DWORD_PTR'? I think/
hope I'm wrong. Would you please show me my mistake?

Thanks in advance

When you use a CMap where CString is the key, you need to provide your own
hash function. The default one will not work/compile for a CString, as you
have discovered.

Here's one that has worked for me. I define it as global function:

// implementation of hash function

template< > UINT AFXAPI HashKey( CString& key )
 LPCTSTR pp = (LPCTSTR)(key);
 UINT uiRet = 0;
 while (*pp)
  uiRet = (uiRet<<5) + uiRet + *pp++;

 return uiRet;

In the .h file for the class where you define the CMap (I'm assuming it's a
member variable), you will need a forward declaration of the function at the

// forward declaration of hash function for the CMap

template< > UINT AFXAPI HashKey( CString& key );


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