CDXLScalar类作为Base class for representing scalar DXL operators，该类只是定义一些接口，其中实现了GetDXLOperatorType函数，其返回EdxloptypeScalar，代表scalar DXL operators。

class CDXLScalar : public CDXLOperator{
private:	
	CDXLScalar(CDXLScalar &); // private copy ctor
public:	
	explicit CDXLScalar(CMemoryPool *mp); // ctor/dtor
	virtual ~CDXLScalar(){};

	Edxloptype GetDXLOperatorType() const;

	// does the operator return a boolean result
	virtual BOOL HasBoolResult(CMDAccessor *md_accessor) const = 0;
};

//---------------------------------------------------------------------------
//	@function:
//		CDXLScalar::CDXLScalar
//	@doc:
//		Constructor
//---------------------------------------------------------------------------
CDXLScalar::CDXLScalar(CMemoryPool *mp) : CDXLOperator(mp){}

//---------------------------------------------------------------------------
//      @function:
//              CDXLScalar::GetDXLOperatorType
//      @doc:
//              Operator Type
//---------------------------------------------------------------------------
Edxloptype CDXLScalar::GetDXLOperatorType() const {
	return EdxloptypeScalar;
}