Re: CMap and struct

From:

=?Utf-8?B?QXJtYW4gU2FoYWt5YW4=?= <armancho_x@rambler.ru(donotspam)>

Newsgroups:

microsoft.public.vc.mfc

Date:

Wed, 2 May 2007 22:42:36 -0700

Message-ID:

<C1D339B9-58B6-411B-87F8-C95DCFBA452E@microsoft.com>

Now you have two problems;
* == operator is not const [it is required because inside CMap are compared
constant objects]. So make it const.
* The HashKey function inside CMap cannot produce any hash value from
SDataType objects; SDataType objects should be able to turn into integer
value somehow [to calculate the hash code]. So define a DWORD operator
returning some unique value (usually returning the object's address will do).

After all, the following code is working;

struct SDataType {
  unsigned long nNamespace;
  unsigned long nDataType;
  SDataType& operator = (const SDataType &s2)
  {
     nNamespace = s2.nNamespace;
     nDataType = s2.nDataType;
     return *this;
  }
  bool operator == (const SDataType &s2) const
  {
    return ((nNamespace == s2.nNamespace) && (nDataType == s2.nDataType));
  }

  operator DWORD_PTR () { return (DWORD_PTR ) this; }
};

typedef CMap<SDataType, SDataType &, UINT , UINT> CDataMap;

CDataMap myMap;

And another thing, you should check for equallity at the first line of
assignment operator to avoid harmful consequences of a=a statements;
if (this == &s2) return *this;

--
======
Arman

"Vahid" wrote:

Thanks for your reply Joseph.

I have already tried a few combinations such as the one you suggested.

Yours will create the following errors:

error C2440: 'type cast' : cannot convert from 'SDataType' to
'DWORD_PTR' (refers to the "return" line in

template<class ARG_KEY>
AFX_INLINE UINT AFXAPI HashKey(ARG_KEY key)
{
    // default identity hash - works for most primitive values
    return (DWORD)(((DWORD_PTR)key)>>4);
}
)

plus the following error:

error C2678: binary '==' : no operator found which takes a left-hand
operand of type 'const SDataType' (or there is no acceptable
conversion)
        D:\Program Files\Microsoft Visual Studio .NET 2003\Vc7\atlmfc
\include\afxtempl.h(1544) : see reference to function template
instantiation 'BOOL CompareElements<KEY,SDataType>(const TYPE *,const
ARG_TYPE *)' being compiled
        with
        [
            KEY=SDataType,
            TYPE=SDataType,
            ARG_TYPE=SDataType
        ]

refering to

template<class TYPE, class ARG_TYPE>
BOOL AFXAPI CompareElements(const TYPE* pElement1, const ARG_TYPE*
pElement2)
{
    ASSERT(AfxIsValidAddress(pElement1, sizeof(TYPE), FALSE));
    ASSERT(AfxIsValidAddress(pElement2, sizeof(ARG_TYPE), FALSE));

    return *pElement1 == *pElement2; // this line
}