CMap and struct

I have a struct in my program defined as below:

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

And I have defined a CMap variable as below:

CMap<SDataType, SDataType&, unsigned int, unsigned int> map;

However, I get this compilation error:

error C2679: binary '=' : no operator found which takes a right-hand
operand of type 'const SDataType' (or there is no acceptable
conversion)

which refers to this portion of "template<class KEY, class ARG_KEY,
class VALUE, class ARG_VALUE>
void CMap<KEY, ARG_KEY, VALUE, ARG_VALUE>::GetNextAssoc(POSITION&
rNextPosition, KEY& rKey, VALUE& rValue) const" function in
afxtempl.h:

rKey = pAssocRet->key;

Does anyone know how this problem can be resolved? I'd appreciate your
help.

.

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