设计模式-计算器

小王正在编写一个简单的计算器程序，要求输入两个整数和运算符号(加、减、乘、除)，输出计算结果。小王用面向过程方法编写了下面的代码。请采用面向对象方法通过恰当的设计模式对小王的代码进行重构。
int main()
{
  int numa,numb; char oper; double result;
  cin>>numa>>numb;  cin>>oper;
  switch(oper) {
    case '+': result=numa+numb; break;
    case '-': result=numa-numb; break;
    case '*': result=numa*numb; break;
    case '/': result=numa/(double)numb; break;
  }
  cout<<result<<endl;
  return 0;
}

#include<string>
#include<iostream>
using namespace std;
//抽象产品角色
class COperation{
public:
  int m_nFirst;
  int m_nSecond;
  virtual double GetResult(){
    double dResult = 0;
    return dResult;
  }
};
//具体产品角色
class AddOperation :public COperation{
public:
  virtual double GetResult(){
    return m_nFirst + m_nSecond;
  }
};
class SubOperation :public COperation{
public:
  virtual double GetResult(){
    return m_nFirst - m_nSecond;
  }
};
class MultiOperation :public COperation{
public:
  virtual double GetResult(){
    return m_nFirst*m_nSecond;
  }
};
class DiviOperation :public COperation{
public:
  virtual double GetResult(){
    return m_nFirst / m_nSecond;
  }
};
//工厂角色
class CalculatorFactory{
public:
  static COperation* Create(char COperator);
};
COperation* CalculatorFactory::Create(char Coperator){
  COperation *oper;
  switch (Coperator){
  case '+':
    oper = new AddOperation();
    break;
  case '-':
    oper = new SubOperation();
    break;
  case '*':
    oper = new MultiOperation();
    break;
  case '/':
    oper = new DiviOperation();
    break;
  default:
    oper = new AddOperation();
  }
  return oper;
}
//客户
int main(){
  int a, b;
  cin >> a >> b;
  COperation *op = CalculatorFactory::Create('-');
  op->m_nFirst = a;
  op->m_nSecond = b;
  cout << op->GetResult() << endl;
  return 0;
}