error C2664 can't convert const char [40] to char *

I am having trouble resolving the issue in the following code:

typedef struct _symbol_key
{
      // constructor
      _symbol_key( const char *pszSym )
      {
            if ( pszSym )
                  strncpy( szSym, pszSym, sizeof(szSym));
            else
                  szSym[0] = 0;
      }

      // overload the less than operator
      bool operator < ( const _symbol_key &key ) const
      {
            int iRet = strcmp( szSym, key.szSym );
            return (iRet < 0);
      }

      operator size_t() const
      {
            return hash_compare( szSym );    // error on this line...
      }

      unsigned long  hash_compare(char *str) const
      {
            unsigned long hash = 5381;
            int c;

            while (c = *str++)
                  hash = ((hash << 5) + hash) + c; /* hash * 33 + c */

            return hash;
      }

      char szSym[40];
} SYMBOL_KEY;



error C2664: 'mcapi::_symbol_key::hash_compare' : cannot convert parameter 1 from 'const char [40]' to 'char *'Start Free Trial