简单工厂模式----控制台计算器的设计(C++实现)

        一个计算器的设计却也体现出了面向对象设计的封装、继承与多态三大特性。

       让计算数据和过程与显示结果分开体现了封装的特性；为了程序的可扩展性，比如以后要加入开根号的运算而不去重写整个类，可以将每个操作符单独作为一个类，他们都继承共同的父类------抽象类：Operation类，这里体现了继承和多态的特性。

       如何让计算器知道该用哪个操作符的类呢，这里用到了“简单工厂模式”，也就是，到底要实例化哪个操作符类，应该考虑用一个单独的类来做这个创造实例的过程，这就是工厂。

#include 
#include 

using namespace std;

class Operation
{
public:
	virtual double GetResult() = 0;

	void SetNumberA( double a )
	{
		numberA = a;
	}
	double GetNumberA()
	{
		return numberA;
	}

	void SetNumberB( double b )
	{
		numberB = b;
	}
	double GetNumberB()
	{
		return numberB;
	}

protected:
	double numberA;
	double numberB;
};

class OperationAdd : public Operation
{
public:
	virtual double GetResult()
	{
		return numberA + numberB;
	}
};

class OperationSub : public Operation
{
public:
	virtual double GetResult()
	{
		return numberA - numberB;
	}
};

class OperationMul : public Operation
{
public:
	virtual double GetResult()
	{
		return numberA * numberB;
	}
};

class OperationDiv : public Operation
{
public:
	virtual double GetResult()
	{
		if( -0.00000001 < numberB && numberB < 0.00000001 )
			throw exception("除数不能为0");

		return numberA / numberB;
	}
};

class OperationFactory
{
public:
	OperationFactory()
	{
		operation = NULL;
	}

	Operation* CreateOperate( char o )
	{
		switch( o )
		{
		case '+':
			operation = new OperationAdd();
			break;
		case '-':
			operation = new OperationSub();
			break;
		case '*':
			operation = new OperationMul();
			break;
		case '/':
			operation = new OperationDiv();
			break;
		default:
			throw exception("操作符错误");
		}

		return operation;
	}

	~OperationFactory()
	{
		delete operation;
	}

private:
	Operation* operation;
};

int main()
{
	double numberA;
	double numberB;
	char operate;

	cout<<"输入数字A: ";
	cin>>numberA;

	cout<<"运算符(+ - * /): ";
	cin>>operate;

	cout<<"输入数字B: ";
	cin>>numberB;

	OperationFactory factory;
	Operation* operation;

	try
	{
		operation = factory.CreateOperate( operate );
	}
	catch(exception& e)
	{
		cout<SetNumberA( numberA );
	operation->SetNumberB( numberB );
	
	int result;
	try
	{
		result = operation->GetResult();
	}
	catch( exception& e)
	{
		cout<

        delete operation;

	return 0;
}