Passing objects to data structures

[Max Payne]

I have a problem with design that I need some help. I have classes as shown below, my problem is on COperator:oOperation,
explained in the comment. please help me through this. thank you.

CElement : public CObject
{
}

CNumber : public CElement
{
	friend CNumber operator+(const CNumber& x, const CNumber& y);
	{
		CNumber result;
		result.m_nNumber = x.m_nNumber + y.m_nNumber;
		return result;
	}

public:
	CNumber& operator=(const CNumber& x);
	{
		if(this == &x)
			return;
		this->m_nNumber = x.m_nNumber;
		return *this;
	}

private:
	int m_nNumber;
}

COperator : public CElement
{
public:
	enum OP_TYPE {ET_ADD=0,ET_SUB,ET_MULTIPLY,ET_DIVIDE};

	CElement* doOperation(CElement* op1,CElement* op2)
	{
		//here is the problem, when the result type is of CElement*, 
                //then returning this object would be not correct, 
		//becoz im expection a pointer to CNumber, 
                //or soon i'm adding another class called CFraction, so how do i do this?
		//should I get the type of op1 and op2 then create a new object of that Type?
                //if i do that, then how about if i'm doing the operation on CFraction class??
		
		CElement* result;
		switch(m_operator)
		{
		case ET_ADD:
			result = op1 + op2;
                case ET_SUB:
			result = op1 - op2;
                case ET_MULTIPLY:
			result = op1 * op2;
                case ET_DIVIDE:
			result = op1 / op2;
		};
	}

private:
	OP_TYPE m_operator;
}

CMathExpression : public CArray<CElement*,CElement*>
{
	//this class would construct the math expression 
	//example,
	this->AddTail(new CNumber(10));
	this->AddTail(new COperator(ET_ADD));
	this->AddTail(new CNumber(10));
}

CAlgorithm
{

public:
  
  int evaluatePostfix()
  {
     CList<CElement*,CElement*> objTemp;

     //convert "m_objExpr" to postfix and store in "objTemp"

     //now evaluate the postfix expr and get the result
     Stack s;// stack is a derived CList implimentation
 
     POSITION pos = objTemp.GetHeadPosition();
     while(pos)
     {
          switch(objTemp.GetType())//getType function is virtual common to all derived class from CElement
          {
                case ET_ELEMET:
                case ET_NUMBER:
                case ET_FRACTION:
                         s.push(objTemp.GetAt(i))
					     break;
                case ET_OPERATOR:
                         CElement* operand2 = s.getTop();
                         s.pop();
                         CElement* operand1 = s.getTop();
                         s.pop();
                         CElement* result = ((COperaotr*)objTemp.GetAt(i))->doOperation(operand1,operand2);
                         s.push(result);
                         break;
          };
     }

	 //get and display the result which is at the top of the Stack
	 s.getTop()->toString(); //toString function is virtual common to all derived class from CElement
  }

private:
  CMathExpression m_objExpr; //this is an array of elements in "infix"
}

somewhereinafarfarawayclass::calculate()
{
   CAlgorithm algo;
   //... here all work done creating the expression 
   int nResult  = algo. evaluatePostfix();
}