设计模式之工厂方法模式＆＆抽象工厂模式

简单工厂模式的优点在于工厂类中包含了必要的逻辑判断，根据客户端的选择动态的实例化相关类，对于客客户端来说是去除了对产品的依赖。但是就是因为这样，每次添加一个新的选择，就要修改工厂类中的switch逻辑分支，这显然违背了开放-封闭原则；

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

工厂方法实现时，客户端需要决定实例化哪一个工厂来实现运算类，选择判断的问题依然存在，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进行，如果想要加功能。则是由更改工厂类转变为了更改客户端。

下面引入两个示例来介绍这个模式方法：

1 用工厂方法模式实现计算器功能：

#include <iostream>
using namespace std;
//抽象产品类
class operation
{protected:double _num_A;double _num_B;
public:double get_A(){return _num_A;}double get_B(){return _num_B;}void set_A(double num){_num_A=num;}void set_B(double num){_num_B=num;}//基类中将操作实现方法设置为虚函数virtual double getResult()=0;virtual ~operation()//当子类含有动态分配的内存时，需要把基类的析构函数声明为虚函数。//                                     此处可不用声明为虚函数。因为子类中没有动态分配的内存。此处写出知识为了举例{cout<<"base destruction"<<endl;}
};//具体产品类
//实现子类继承父类的具体实现
//实现加法功能
class operationAdd:public operation   //加法类继承自operation
{double getResult(){return _num_A+_num_B;}//构造之后进行析构 防止内存泄漏~operationAdd(){cout<<"ADD destruction"<<endl;}
};//实现减法功能
class operationSub:public operation
{double getResult(){return _num_A-_num_B;}
}; //实现乘法功能
class operationMul:public operation
{double getResult(){return _num_A*_num_B;}
};//实现除法功能
class operationdiv:public operation
{double getResult(){return _num_A/_num_B;}
};//工厂方法类
class factory//先构建一个工厂接口
{public:virtual operation* createOperation()=0;  //纯虚函数
};class addFactory:public factory
{public:operation* createOperation(){return new operationAdd();}
};class subFactory:public factory
{public:operation* createOperation(){return new operationSub();}
};class mulFactory:public factory
{public:operation* createOperation(){return new operationMul();}
};class divFactory:public factory
{public:operation* createOperation(){return new operationdiv();}
};//主界面函数体
int main()
{factory* operfactory=new addFactory();operation* add=operfactory->createOperation();add->set_A(1);add->set_B(1);cout<<add->getResult();return 0;
}
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2 工厂方法模式实现雷锋功能：

#include <iostream>
#include <string>
using namespace std;class leifeng
{public:void saodi(){cout<<"扫地 ";}void maifan(){cout<<"买饭 ";}
};class daxuesheng:public leifeng
{};class shehuizhiyuanzhe:public leifeng
{};//雷锋工厂实现
class leifengfactory
{public:virtual leifeng* CreateLeiFeng()=0;
};
//大学生工厂实现
class daxueshengFactory:public leifengfactory
{public:leifeng* CreateLeiFeng(){cout<<"大学生雷锋";return new daxuesheng;}
};class shehuizhiyuanzheFactory:public leifengfactory
{public:leifeng* CreateLeiFeng(){return new shehuizhiyuanzhe;}
};int main(){leifengfactory* lff=new daxueshengFactory;//父类指针指向子类对象 返回一个子类new出来的对象leifeng* xiaoli=lff->CreateLeiFeng();//创建雷锋类对象接收雷锋工厂对象调用子类函数的结果xiaoli->saodi();//实现雷锋对象的具体功能return 0;
}
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抽象工厂模式

提供一个创建一系列相关或相互依赖对象的接口，而不需指定他们具体的类；

抽象工厂模式的优缺点

好处便是易于交换产品系列，有与具体工厂类，在一个应用中只需要初始化的时候出现一次，这就使得改变一个应用的具体工厂变得非常容易，他只需要改变具体工厂即可使用不同的产品配置。
它让具体的创建实例过程与客户端分离，客户端是通过他们的抽象接口曹忠实例，产品的具体类名也被具体工厂的实现分离，不会出现在客户代码中。

引入一个抽象工厂模式的示例：

#include <iostream>
#include <string>using namespace std;
//抽象工厂实现数据库访问。有两个数据库 sqlUser accessUser，每个数据库有两个表项sqldepartment  accessdepartment
class IUser
{public:virtual void setuser()=0;//设置用户virtual void getuser()=0;//获取用户
};class sqlUser:public IUser
{public:virtual void setuser(){cout<<"在SQL Server中给User增加一条记录"<<endl;}virtual void getuser(){cout<<"在SQL Server中根据ID得到User一条记录"<<endl;}
};class AccessUser:public IUser
{public:virtual void setuser(){cout<<"在Acess Server中给User增加一条记录"<<endl;}virtual void getuser(){cout<<"在Acess Server中根据ID得到User一条记录"<<endl;}
};class IDepartment
{public:virtual void getDepartment()=0;virtual void setDepartment()=0;
};class SqlDepartment:public IDepartment
{public:void getDepartment(){cout<<"在Sql Server中根据ID得到Department一条记录"<<endl;}void setDepartment(){cout<<"在Sql Server中给Department增加一条记录"<<endl;}
};class AccessDepartment:public IDepartment
{public:void getDepartment(){cout<<"在Acess Server中根据ID得到Department一条记录"<<endl;}void setDepartment(){cout<<"在Acess Server中给Department增加一条记录"<<endl;}
};class IFactory
{public:virtual IUser *createUser()=0;virtual IDepartment *createDepartment()=0;
};class SqlFactory:public IFactory
{public:IUser *createUser() {return new sqlUser();}IDepartment *createDepartment() {return new SqlDepartment();}
};class AccessFactory:public IFactory
{public:IUser *createUser(){return new AccessUser();}IDepartment *createDepartment() {return new AccessDepartment();}
};int main()
{IFactory* sql=new SqlFactory;IUser * user=sql->createUser();IDepartment* department=sql->createDepartment();user->setuser();user->getuser();department->setDepartment();department->getDepartment();return 0;}
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用简单工厂模式重写上面例子：

#include <iostream>
#include <string>using namespace std;
//抽象工厂实现//用户父类IUer  它有俩子类 即对应两个数据库
class IUser
{public:virtual void setuser()=0;virtual void getuser()=0;
};//用户子类sqlUser
class sqlUser:public IUser
{public:virtual void setuser(){cout<<"在SQL Server中给User增加一条记录"<<endl;}virtual void getuser(){cout<<"在SQL Server中根据ID得到User一条记录"<<endl;}
};
//子类用户AccessUser
class AccessUser:public IUser
{public:virtual void setuser(){cout<<"在Acess Server中给User增加一条记录"<<endl;}virtual void getuser(){cout<<"在Acess Server中根据ID得到User一条记录"<<endl;}
};//表项父类
class IDepartment
{public:virtual void getDepartment()=0;virtual void setDepartment()=0;
};//表项子类SqlDepartment
class SqlDepartment:public IDepartment
{public:void getDepartment(){cout<<"在Sql Server中根据ID得到Department一条记录"<<endl;}void setDepartment(){cout<<"在Sql Server中给Department增加一条记录"<<endl;}
};//表项子类 AccessDepartment
class AccessDepartment:public IDepartment
{public:void getDepartment(){cout<<"在Acess Server中根据ID得到Department一条记录"<<endl;}void setDepartment(){cout<<"在Acess Server中给Department增加一条记录"<<endl;}
};//获取数据
class Accessdata
{public:static string db;static IUser* createUser(){if(db=="sql")return new sqlUser;else if(db=="access")return new AccessUser;}static IDepartment* createDepartment(){if(db=="sql")return new SqlDepartment;else if(db=="access")return new AccessDepartment;}
};string Accessdata::db="sql"; //作用域int main()
{Accessdata DB;IUser * user=DB.createUser(); //获取数据库IDepartment* department=DB.createDepartment();//获取表项user->setuser();user->getuser();department->setDepartment();department->getDepartment();return 0;}
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