策略模式：定义了算法家族，分别封装起来，让它们之间可以相互转换，此模式让算法的变化，不会影响到使用算法的客户。

从概念上来看，所有的这些算法完成的都是相同的工作，只是实现不同，它可以以相同的方式调用所有的算法。

优点：算法可以自由切换，扩展性好。

缺点：策略增多时，上层必须知道具体的策略。

适用于多个类只在算法上不同的地方。

用策略模式实现商场促销：

#include <iostream>

using namespace std;

class CashSuper
{
	public:
		virtual double acceptCash(double money) = 0;
};

class CashNormal : public CashSuper
{
	public:
		double acceptCash(double money)
		{
			return money;
		}
};

class CashRebate : public CashSuper
{
	public:
		CashRebate(double moneyRebate)
		{
			_moneyRebate = moneyRebate;
		}
		double acceptCash(double money)
		{
			return _moneyRebate*money;
		}
	private:
		double _moneyRebate;
};


class CashReturn : public CashSuper
{
	public:
		CashReturn(double moneyReturn,double moneyCondition)
		{
			_moneyReturn = moneyReturn;
			_moneyCondition = moneyCondition;
		}
		double acceptCash(double money)
		{
			double result = 0.0;
			if(money >= _moneyCondition)
				result = money - (money / _moneyCondition)*_moneyReturn;
			else 
				result = money;
			return result;
		}
	private:
		double _moneyReturn;
		double _moneyCondition;
};

class CashContext
{
	public:
		CashContext(CashSuper* cs)
		{
			this->_cs = cs;
		}
		double GetResult(double money)
		{
			return _cs->acceptCash(money);
		}
	private:
		CashSuper* _cs;
};


int main()
{
	int count;
	double price;
	double total = 0.0;
	int type;
	
	cout << "请输入单价：";
	cin >> price;
	cout << "请输入数量：";
	cin >> count;
	cout << "请输入打折类型：";
	cin >> type;
	CashContext* oper = NULL;
	switch(type)
	{
		case 1:
			oper = new CashContext(new CashNormal);
			break;
		case 8:
			oper = new CashContext(new CashRebate(0.8));
			break;
		case 300 - 100:
			oper = new CashContext(new CashReturn(300,100));
			break;
	}
	
	cout << oper->GetResult(price*count) << endl;

	return 0;
}

通过之前学习的简单工厂模式，可以进一步优化商场促销的代码，利用简单工厂和策略模式共同实现：

#include <iostream>

using namespace std;

class CashSuper
{
	public:
		virtual double acceptCash(double money) = 0;
};

class CashNormal : public CashSuper
{
	public:
		double acceptCash(double money)
		{
			return money;
		}
};

class CashRebate : public CashSuper
{
	public:
		CashRebate(double moneyRebate)
		{
			_moneyRebate = moneyRebate;
		}
		double acceptCash(double money)
		{
			return _moneyRebate*money;
		}
	private:
		double _moneyRebate;
};


class CashReturn : public CashSuper
{
	public:
		CashReturn(double moneyReturn,double moneyCondition)
		{
			_moneyReturn = moneyReturn;
			_moneyCondition = moneyCondition;
		}
		double acceptCash(double money)
		{
			double result = 0.0;
			if(money >= _moneyCondition)
				result = money - (money / _moneyCondition)*_moneyReturn;
			else 
				result = money;
			return result;
		}
	private:
		double _moneyReturn;
		double _moneyCondition;
};

class CashContext
{
	public:
		CashSuper* _cs = NULL;
		CashContext(int type)
		{
			switch(type)
			{
				case 1:
					_cs = new  CashNormal;
					break;
				case 8:
					_cs = new  CashRebate(0.8);
					break;
				case 300 - 100:
					_cs = new  CashReturn(300,100);
					break;
			}
		}
		double GetResult(double money)
		{
			return _cs->acceptCash(money);
		}
};


int main()
{
	int count;
	double price;
	double total = 0.0;
	int type;
	
	cout << "请输入单价：";
	cin >> price;
	cout << "请输入数量：";
	cin >> count;
	cout << "请输入打折类型：";
	cin >> type;
	CashContext* oper = new CashContext(type);
	
	cout << oper->GetResult(price*count) << endl;

	return 0;
}