Strategy 模式

enum TaxBase {
	CN_Tax,
	US_Tax,
	DE_Tax,
};
class SaleOrder {
	TaxBase tax;
public:
	if(tax == CN_Tax) {
	//....
	}
	else if (tax == US_Tax) {
		//....
	}
	else if (tax == DE_Tax)
	{
		//....
	}
	//.......
};
//如果加个法国税款，则需改写枚举类型，改写类内方法，打破了类的复用性  绝对不变的可以用if else,如果场景或者业务变换

class TaxStrategy {
public:
	virtual double Calculate(const Context& context）=0;
	virtual ~TaxStrategy() {}

};
class CNTax :public TaxStrategy {
public:
	virtual double Calculate(Context& context)
	{
		//......
	}
};
class USTax :public TaxStrategy {
public:
	virtual double Calculate(Context& context) {
	//...
	}
};
class DETax :public TaxStrategy {
public:
	virtual double Calculate(Context& context) {
		//....
	}
};

class SaleOrder {
	TaxStrategy *strategy;
public:
	SaleOrder(StrategyFactory* strategy) {               //不知道是哪国税款，工厂模式未学
		this->strategy = strategyFactory->NewStrategy();
	}
	~SaleOrder() {
		delete this->strategy;
	}
public: 
	double CalculateTax() {
		Context context();

		double val = strategy->Calculate(context);
		//....
	}
};

//假如加个法国税款 只需扩展一个类
class FRTax :public TaxStrategy {
public:
	virtual double Calculate(Context& context) {
		//....
	}
};

 红圈圈起来是指稳定不变的 左边中的Contex相当于saleorder.