设计模式C++实现五：工厂方法模式

工厂方法模式（Factory Method)：定义一个用于封建对象的接口，让子类觉得史丽华哪一个类，工厂方法使一个类的实例化延迟到其子类。

简单工厂模式优点 ：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实例化相关的类，对于客户端来说，去除了与具体产品的依赖。但是因为我们需要根据客户端的输入来修实例化类，如果我们要增加类的话，我们需要修改工厂类方法，把该类加入其中，这意味着我们不但对扩展开发了，对修改也开发了，违背了开放-封闭原则。这就可以用工厂方法来解决这个问题。

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，工厂方法把简单工厂的内部逻辑判断移到了客户端代码来进行，你想要增加功能，本来是改工厂类的，而现在是修改客户端。

<pre name="code" class="cpp">#ifndef FACTORY_MEANS_H#define FACTORY_MEANS_H#include<iostream>using namespace std;class Operation{protected:double opA, opB;public:bool SetValue(double& n, double& m);virtual double GetResult()const = 0;};class OperationAdd :public Operation{double GetResult()const;};class  OperationSub :public Operation{double GetResult()const;};class OperationMul :public Operation{double GetResult()const;};class OperationDiv :public Operation{double GetResult()const;};class IFactory{public:virtual Operation *CreatOperation()=0;}; class AddFactory : public IFactory{public:Operation *CreatOperation(){return new OperationAdd();}};class SubFactory : public IFactory{public:Operation *CreatOperation(){return new OperationSub();}};class MulFactory : public IFactory{public:Operation *CreatOperation(){return new OperationMul();}};class DivFactory : public IFactory{public:Operation *CreatOperation(){return new OperationDiv();}};bool Operation::SetValue(double &n, double &m){opA = n;opB = m;return true;}double OperationAdd::GetResult()const{double result;result = opA + opB;return result;}double OperationSub::GetResult()const{double result;result = opA - opB;return result;}double OperationMul::GetResult()const{double result;result = opA * opB;return result;}double OperationDiv::GetResult()const{if (opB == 0){ cout << "opB in OperationDiv can't be zero.\n"; return 0.00000001; }double result;result = opA / opB;return result;}#endif

#include"FactoryMeans.h"int main(){IFactory *operA = new AddFactory;Operation *opera = operA->CreatOperation();IFactory *operS = new SubFactory;Operation *opers = operS->CreatOperation();IFactory *operM = new MulFactory;Operation *operm = operM->CreatOperation();IFactory *operD = new DivFactory;Operation *operd = operD->CreatOperation();double a, b;while (cin >> a&&cin >> b){opera->SetValue(a,b );cout << "the result of " << a << " add " << b << " equal " << opera->GetResult() << endl;opers->SetValue(a, b);cout << "the result of " << a << " sub " << b << " equal " << opers->GetResult() << endl;operm->SetValue(a, b);cout << "the result of " << a << " mul " << b << " equal " << operm->GetResult() << endl;operd->SetValue(a, b);cout << "the result of " << a << " div " << b << " equal " << operd->GetResult() << endl;}return 0;}