简易计算器

老师布置了个作业：可拓展性或灵活性，指软件可快速改进以满足业务需求的变化，以一个简单系统为例理解在设计上要如何实现这个性能。可以一个简单的计算器为例，使用C++实现。

然后经过各种查，用已懂的知识写了个，下面贴上计算器的代码。

期间遇到错误提示："A"的初始化操作由"case"标签跳过，这个错误是因为怕用了别的case而导致这个变量不创建，解决方法为把case(0)，case(1)...后代码用{}括起来。

#include<iostream>
#include<string>
using namespace std;

//运算类
class COperation{   
private:
	double dNumA; //数字A
	double dNumB; //数字B
public:
	void SetNumA(double n){ //设置数字A
		dNumA = n;
	}
	void SetNumB(double n){ //设置数字B
		dNumB = n;
	}
	double GetNumA(){ //取得数字A
		return dNumA;
	}
	double GetNumB(){ //取得数字B
		return dNumB;
	}
	virtual double GetResult()  // 得到运算结果
	{
		double result = 0;
		return result;
	}
};

//加法类
class COperationAdd :public  COperation  
{
public:
	double  GetResult()override
	{
		double result = 0;
		result = GetNumA() + GetNumB();
		return result;
	}
};

//减法类
class COperationSub :public COperation 
{
public:
	double GetResult()override
	{
		double result = 0;
		result = GetNumA() - GetNumB();
		return result;
	}
};

//乘法类
class COperationMul :public COperation  
{
public:
	double GetResult()override
	{
		double result = 0;
		result = GetNumA() * GetNumB();
		return result;
	}
};

//除法类
class COperationDiv :public COperation 
{
public:
	double GetResult()override
	{
		double result = 0;
		if (GetNumB() == 0){
			std::cout << "除数不能为0。";
			return NULL;
		}
		result = GetNumA() / GetNumB();
		return result;
	}
};

int main()
{
	int n;
	double x, y;

	while(true)
	{
		cout << " == == == == == == == == == == == == == == == == == == ==" << endl;
		cout << "                0. 加法运算" << endl;
		cout << "                1. 减法运算" << endl;
		cout << "                2. 乘法运算" << endl;
		cout << "                3. 除法运算" << endl;
		cout << "                4. 退出    " << endl;
		cout << " == == == == == == == == == == == == == == == == == == ==" << endl;
		cout << "运算类型选择：";
		cin >> n;
		switch(n){
		case(0) : {
					  COperationAdd A;
					  cout << "请输入数据x和y：";
					  cin >> x >> y;
					  A.SetNumA(x);
					  A.SetNumB(y);
					  cout << x << " + " << y << " = " << A.GetResult() << endl; break;
		}
		case(1) : {
					  COperationSub A;
					  cout << "请输入数据x和y：";
					  cin >> x >> y;
					  A.SetNumA(x);
					  A.SetNumB(y);
					  cout << x << " - " << y << " = " << A.GetResult() << endl; break;
		}
		case(2) : {
					  COperationMul A;
					  cout << "请输入数据x和y：";
					  cin >> x >> y;
					  A.SetNumA(x);
					  A.SetNumB(y);
					  cout << x << " * " << y << " = " << A.GetResult() << endl; break;
		}
		case(3) : {
					  COperationDiv A;
					  cout << "请输入数据x和y：";
					  cin >> x >> y;
					  A.SetNumA(x);
					  A.SetNumB(y);
					  cout << x << " / " << y << " = " << A.GetResult() << endl; break;
		}
		case(4):exit(0);
		default: break;
		}
	}
	return 0;
}