设计模式之策略模式学习

        策略模式：它定义了算法家族，分别封装起来，让他么之间可以互相替换。此模式让算法的变化，不会影响使用这些算法的客户。

        在引出策略模式时，讨论了超市收银系统。可以将各种收费项目抽象成几种操作，比如正常收费、打折、返现、返积分。。。对打折来说，之所以可以这样进行抽象因为各种折扣之间存在着相互的联系，故将其抽象为一种。返现、返积分也类似。

       算法本身是一种策略，这些算法可能随时被替换，这就是变化点，而封装变化点是面向对象一种很重要的思维方式。

策略模式是定义了一系列算法的方法，从概念上来看所有这些算法完成相同的工作，只是实现不同。它可以以相同的方式调用所有的算法，减少了各种算法类与使用它的算法类之间的耦合。   

看代码： 

//收费对象虚基类。

<span style="font-size:18px;">class SuperCashier { public: double price; int num; public: SuperCashier(double price,int num) { this->price=price; this->num=num; } virtual double getTotalMoney()=0; } ;</span>

//正常收费

<span style="font-size:18px;">class Normal:public SuperCashier { public: double discount; public: Normal(double price,int num) :SuperCashier(price,num) { } virtual double getTotalMoney() { return price*num; } } ;</span><span style="font-size:18px;"> </span>
<span style="font-size:18px;">//打折 class Discount:public SuperCashier { public: double discount; public: Discount(double price,int num,double discount) :SuperCashier(price,num) { this->discount=discount; } virtual double getTotalMoney() { return price*num*discount; } } ;</span>

 

<pre class="cpp" name="code"><span style="font-size:18px;">//返点 class ReturnPoint :public SuperCashier { public: double conditionMoney; double point; public : ReturnPoint(double price,double num,double cm,double rp) :SuperCashier(price,num),conditionMoney(cm),point(rp) { } virtual double getTotalMoney() { double total=price*num; if(total>=conditionMoney) { double n=total/conditionMoney; return point*n; } return 0; } };</span>

 

<span style="font-size:18px;">//返现金 class ReturnMoney:public SuperCashier { public: double conditionMoney; double returnNum; public: ReturnMoney(double price,double num,double cm,double returnNum) :SuperCashier(price,num),conditionMoney(cm) { this->returnNum=returnNum; } virtual double getTotalMoney() { double total=num*price; if(total>=conditionMoney) { double n=total/conditionMoney; return total-n*returnNum; } } } ;</span>

<span style="font-size:18px;">/************************************************************************/ /* 策略模式。 */ /*********************************************************************** class CashContext { public: SuperCashier*sc; public: CashContext(SuperCashier*sc) { this->sc=sc; } double getResult() { return sc->getTotalMoney(); } }; </span><span style="font-size:18px;"> </span><span style="font-size:18px;">//在客户端进行判断。</span><span style="font-size:18px;">int main(int argc,char**argv) { double price; double num; CashContext *cc; while(1) { cout<<"1..正常收费。2..打折。3..返点 4..返利"<<endl; int ch; cin>>ch;getchar(); switch(ch) { case 1: { cin>>price>>num; cc=new CashContext(new Normal(price,num)); } break; case 2: { double discount; cin>>price>>num>>discount; cc=new CashContext(new Discount(price,num,discount)); } break; case 3: { double conditionMoney; double returnPoint; cin>>price>>num>>conditionMoney>>returnPoint; cc=new CashContext(new ReturnPoint(price,num,conditionMoney,returnPoint)); } break; case 4: { double conditionMoney; double returnMoney; cin>>price>>num>>conditionMoney>>returnMoney; cc=new CashContext(new ReturnMoney(price,num,conditionMoney,returnMoney)); } break; default: break; } cout<<cc->getResult()<<endl; delete cc; } return 0; }*/</span>

 

<span style="font-size:18px;">/************************************************************************/ /* 策略模式与简单工厂模式结合，即将在main中的switch循环移到CashContext类中 */ /************************************************************************/ class CashContext { public: int type; double price; double num; double discount; double conditionMoney; double ret; SuperCashier *sc; public: CashContext(int t,double price,int num,double discount) :type(t) { this->price=price; this->num=num; this->discount=discount; sc=NULL; } CashContext(int t,double price,int num) :type(t) { this->price=price; this->num=num; sc=NULL; } CashContext(int t,double price,int num,double conditionMoney,double ret) :type(t) { this->price=price; this->num=num; this->conditionMoney=conditionMoney; this->ret=ret; sc=NULL; } double getResult() { switch(type) { case 1://正常收费 { sc=new Normal(price,num); } break; case 2://打折 { sc=new Discount(price,num,discount); } break; case 3://返点 { sc=new ReturnPoint(price,num,conditionMoney,ret); } break; case 4://返现 { sc=new ReturnPoint(price,num,conditionMoney,ret); } break; default: break; } return sc->getTotalMoney(); } }; int main(int argc,char**argv) { CashContext *cc; double price,num,discount,conditionMoney,ret; cin>>price>>num; cc=new CashContext(1,price,num); cout<<cc->getResult()<<endl; delete cc; cin>>price>>num>>discount; cc=new CashContext(2,price,num,discount); cout<<cc->getResult()<<endl; delete cc; cin>>price>>num>>conditionMoney>>ret; cc=new CashContext(3,price,num,conditionMoney,ret); cout<<cc->getResult()<<endl; delete cc; cin>>price>>num>>conditionMoney>>ret; cc=new CashContext(4,price,num,conditionMoney,ret); cout<<cc->getResult()<<endl; delete cc; return 0; }</span>

       此处觉得有不妥，一旦超市推出另外一些活动，CashContext类中用于switch判断的代码就要修改，程序需要重新编译。这在实际应用中是不可能的。第二章的最后提了一下使用映射可以解决这个问题。但不知映射为何物，拭目以待吧    

       注：此处实现的收费对象类，比书上的多实现了返点类。关于此类不知应该将其归为哪一部分，暂将其放入收费对象类中。但觉得不应该与收费对象类放在一起，应该有另外一个类处理返点。毕竟积分信息的管理对于大型超市来说也是很重要的一部分。