第8章工厂方法模式

一 概念

• 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一个类，工厂方法使一个类的实例化延迟到其子类

二 包含的角色

• 抽象工厂
• 具体工厂
• 抽象产品
• 具体产品

三 优势

• 工厂方法模式是对简单工厂模式的稍微的改进。工厂方法模式的用意是定义一个创建产品对象的工厂接口，将实际工作推迟到子类中。
• 与简单工厂模式相比，制造产品的工厂类不再只有一个，而是每种具体产品类都对应一个生产它的具体工厂类。而这些具体工厂类的共同特征再被提取出来形成一个抽象产品类，这些具体产品类都继承自这个抽象产品类。
• 当需要增加一种产品的时候，要做的是：增加一种继承自抽象产品的具体产品类，增加一种继承在抽象工厂的具体工厂类，更改客户端。而不需要在简单工厂模式中那样更改工厂内的switch。

四 简单工厂VS工厂方法

• 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，但是违背了开放-封闭原则

四 C++代码实现

• 计算器的例子

//工厂方法模式
//计算器的例子
#include "pch.h"
#include <iostream>
using namespace std;

//抽象产品类
class Operation
{
public:
	double GetA() const
	{
		return numberA;
	}
	double GetB() const
	{
		return numberB;
	}
	void SetA(const double number)
	{
		numberA = number;
	}
	void SetB(const double number)
	{
		numberB = number;
	}
	virtual double GetResult()
	{
		double result = 0.0;
		return result;
	}

protected:
	double numberA;
	double numberB;
};

//下面是四个具体的产品类
class OperationAdd :public Operation
{
public:
	double GetResult()
	{
		double result = 0;
		result = numberA + numberB;
		return result;
	}
};

class OperationSub :public Operation
{
public:
	double GetResult()
	{
		double result = 0;
		result = numberA - numberB;
		return result;
	}
};

class OperationMul :public Operation
{
public:
	double GetResult()
	{
		double result = 0;
		result = numberA * numberB;
		return result;
	}
};

class OperationDiv :public Operation
{
public:
	double GetResult()
	{
		double result = 0;
		if (numberB != 0)
			result = numberA / numberB;
		return result;
	}
};

//抽象工厂类
class IFactory
{
public:
	virtual Operation* createOperation()
	{
		return new Operation;
	}
};

//下面是四个具体工厂类，分别用于产生四个具体产品
class AddFactory : public IFactory
{
public:
	Operation* createOperation()
	{
		oper = new OperationAdd;
		return oper;
	}
	~AddFactory()
	{
		if (oper != NULL)
		{
			delete oper;
			oper = NULL;
		}
	}
private:
	Operation* oper;
};

class SubFactory : public IFactory
{
public:
	Operation* createOperation()
	{
		oper = new OperationSub;
		return oper;
	}
	~SubFactory()
	{
		if (oper != NULL)
		{
			delete oper;
			oper = NULL;
		}
	}
private:
	Operation* oper;
};

class MulFactory : public IFactory
{
public:
	Operation* createOperation()
	{
		oper = new OperationMul;
		return oper;
	}
	~MulFactory()
	{
		if (oper != NULL)
		{
			delete oper;
			oper = NULL;
		}
	}
private:
	Operation* oper;
};

class DivFactory : public IFactory
{
public:
	Operation* createOperation()
	{
		oper = new OperationDiv;
		return oper;
	}
	~DivFactory()
	{
		if (oper != NULL)
		{
			delete oper;
			oper = NULL;
		}
	}
private:
	Operation* oper;
};

int main()
{
	IFactory *af = NULL;
	af = new SubFactory();

	Operation* oper = af->createOperation();
	oper->SetA(50);
	oper->SetB(19);
	cout << oper->GetResult() << endl;
}

• 雷锋工厂的例子

#include "pch.h"
#include <iostream>
#include <string>
using namespace std;

class LeiFeng
{
public:
	virtual void Sweep()
	{
		cout << "扫地" << endl;
	}
	virtual void Wash()
	{
		cout << "洗衣" << endl;
	}
	virtual void BuyRice()
	{
		cout << "买米" << endl;
	}
};

class Undergraduate : public LeiFeng
{
public:
	void Sweep()
	{
		cout << "学生-扫地" << endl;
	}
	void Wash()
	{
		cout << "学生-洗衣" << endl;
	}
	void BuyRice()
	{
		cout << "学生-买米" << endl;
	}
};

class Volunteer : public LeiFeng
{
public:
	void Sweep()
	{
		cout << "志愿者-扫地" << endl;
	}
	void Wash()
	{
		cout << "志愿者-洗衣" << endl;
	}
	void BuyRice()
	{
		cout << "志愿者-买米" << endl;
	}
};

//雷锋工厂
class IFactory
{
public:
	virtual LeiFeng* CreateLeiFeng()
	{
		oper = new LeiFeng;
		return oper;
	}
	~IFactory()
	{
		if (oper != NULL)
		{
			delete oper;
			oper = NULL;
		}
	}
private:
	LeiFeng* oper;
};

//生成学雷锋的大学生的工厂
class UndergraduateFactory : public IFactory
{
public:
	LeiFeng* CreateLeiFeng()
	{
		return new Undergraduate;
	}
};

//生成社区志愿者的工厂
class VolunteerFactory : public IFactory
{
public:
	LeiFeng* CreateLeiFeng()
	{
		return new Volunteer;
	}
};

int main()
{
	IFactory* factory = new UndergraduateFactory;
	LeiFeng* student = factory->CreateLeiFeng();

	student->BuyRice();
	student->Sweep();
	student->Wash();
	return 0;
}

参考资料：
1 《大话设计模式C++实现-第8章-工厂方法模式》 https://blog.csdn.net/xiqingnian/article/details/40957025