https://www.bilibili.com/video/BV1kW411P7KS?p=4

//strategy1.cpp

//變化：增加對法國的支援，違背開閉原則：對擴充套件開放，對更改封閉
enum TaxBase {
    CN_Tax,
    US_Tax,
    DE_Tax,
    FR_Tax //更改
};

class SalseOrder{
    TaxBase tax;
public:
    double CalculateTax(){
        //...
        
        if (tax == CN_Tax){ //一個個的演算法
            
 
//CN**********
        }
        else if (tax == US_Tax){
            //US***********
        }
        else if (tax == DE_Tax){
            //DE***********
        }
        else if (tax == FR_Tax){    //更改
            //FR***********
        }
        //....
    }
};

//strategy2.cp

//工程上，每個類單獨放一個檔案
class TaxStrategy{
 
public:
    virtual double Caculate(const Context &context) = 0;
    virtual ~TaxStrategy(){}
};

//把一個個的演算法，變成了一個個子類
class CNTax : public TaxStrategy{
public:
    virtual double Caculate(const Context &context){
        //*************
    }
};

class USTax : public TaxStrategy{
public:
    virtual
 
 double Caculate(const Context &context){
        //*************
    }
};

class DETax : public TaxStrategy{
public:
    virtual double Caculate(const Context &context){
        //*************
    }
};

//增加對法國的支援，增加一個子類，SalesOrder沒有任何改變，複用性強
//開放封閉原則
class FRTax : public TaxStrategy{
public:
    virtual double Caculate(const Context &context){
        //....
    }
};

class SalesOrder{
private:
    TaxStrategy *strategy; //指標為變化帶來了可能
    
public:
    SalesOrder(StrategyFactory * strategyFactory){
        this->strategy = strategyFactory->NewStrategy();
    }
    ~SalesOrder(){}
    
    double CalculateTax(){
        //...
        Context context();
        
        double val = strategy->Calculate(context); //多型呼叫
        
        //...
    }
};