工厂方法模式---重写计算器（C++）

       简单工厂模式和工厂模式的区别：

       简单工厂模式的最大优点在于工厂类中包含了必要的逻辑判断，根据客户端的选择条件动态的实例化相关的类，对于客户端来说，除去了与具体产品的依赖。就像计算器，让客户端不用管该用哪个类的实例，只需把'+'给工厂，工厂自动就给出了相应的实例，客户端只要做运算就可以了。但是问题也在这里，如果要增加新的运算功能，就一定需要修改这个运算工厂类，违背了开放-封闭原则。

       工厂方法模式，定义一个用于创造对象的接口，让子类决定实例化哪一个类。工厂方法使一个类的实例化延迟到其子类。

 

    工厂方法模式实现时，客户端需要决定实例化哪一个工厂来实现运算类，选择判断的问题还是存在的，也就是说，工厂模式把简单工厂的内部逻辑判断转移到了客户端代码来进行，增加功能时，需要修改客户端。

#include <iostream>#include <stdexcept>#include <memory>using namespace std;class Operation{protected:double numberA;double numberB;public:void GetNumberA(double value){numberA = value;}void GetNumberB(double value){numberB = value;}virtual double GetResult() = 0;};class OperationAdd : public Operation{public:virtual double GetResult(){double result = numberA + numberB;return result;}};class OperationSub : public Operation{public:virtual double GetResult(){double result = numberA - numberB;return result;}};class OperationMul : public Operation{public:virtual double GetResult(){double result = numberA * numberB;return result;}};class OperationDiv : public Operation{public:virtual double GetResult(){if( (-0.0000001< numberB) && (numberB < 0.0000001) )throw  exception("除数不能为0");double result = numberA / numberB;return result;}};/*运算符工厂类*/class OperationFactory{public:virtual Operation* CreateOperation() = 0;  //子类实现这个方法，返回具体的运算法对象};/*加法运算符工厂类，继承OperationFactory抽象类，实现CreateOperation方法，返回加法运算符对象*/class AddOperationFactory : public OperationFactory{public:virtual Operation* CreateOperation(){return new OperationAdd();}};class SubOperationFactory : public OperationFactory{public:virtual Operation* CreateOperation(){return new OperationSub();}};class MulOperationFactory : public OperationFactory{public:virtual Operation* CreateOperation(){return new OperationMul();}};class DivOperationFactory : public OperationFactory{public:virtual Operation* CreateOperation(){return new OperationDiv();}};int main(){double numberA;double numberB;char operate;cout<<"请输入数字A: ";cin>>numberA;cout<<"请输入数字B: ";cin>>numberB;AddOperationFactory addfa;SubOperationFactory subfa;MulOperationFactory mulfa;DivOperationFactory divfa;OperationFactory* factory = &divfa; /* 只需替换具体的操作符对象即可 */Operation* oper = factory->CreateOperation();oper->GetNumberA(numberA);oper->GetNumberB(numberB);try{cout<<oper->GetResult()<<endl;}catch(exception& e){cout<<e.what()<<endl;}delete oper;return 0;}