二、工厂方法(Factory Method) 模式

工厂方法模式是类的创建模式，又叫虚拟构造子（Virtual Constructor）模式或者多态性工厂（Polymorphic Factory）模式。

该模式中，核心的工厂类不再是负责所有的产品创建，而是将具体创建的工作交给具体子类去做。

即提供一个抽象的接口工厂类。针对不同的产品从抽样的接口工厂类实现对应的产品工厂类。工厂类和产品类一一对应。

工厂方法模式添加新的产品，只需要对应的向系统中增加一个对应产品的工厂类即可。因此其实完全支持“开闭”原则的。

一个工厂方法的示例如下：

fruit的代码如下：

// fruit.h
#ifndef FRUIT_H_
#define FRUIT_H_
class Fruit {
  public:
    virtual ~Fruit();
    virtual void Name() = 0;
  protected:
    Fruit();
  private:
};

class Apple : public Fruit {
  public:
    virtual void Name();
};

class Pear : public Fruit {
  public:
    virtual void Name();
};

class Orange : public Fruit {
  public:
    virtual void Name();
};
#endif  // FRUIT_H_

//fruit.cc
#include <iostream>
#include "fruit.h"
using namespace std;

Fruit::~Fruit() {
}

Fruit::Fruit() {
}

void Apple::Name() {
  cout << "This is Apple !" << endl;
}

void Pear::Name() {
  cout << "This is Pear !" << endl;
}

void Orange::Name() {
  cout << "This is Orange !" << endl;
}

fruit_factory代码如下:

// fruit_factory.h
#ifndef FRUIT_FACTORY_H_
#define FRUIT_FACTORY_H_
class Fruit;
class FruitFactory {
  public:
    virtual Fruit* GetFruit() = 0;
  private:
};

class AppleFactory : public FruitFactory {
  public:
    virtual Fruit* GetFruit();
};

class PearFactory : public FruitFactory {
  public:
    virtual Fruit* GetFruit();
  private:
};

class OrangeFactory : public FruitFactory {
  public:
    virtual Fruit* GetFruit();
  private:
};
#endif  // FRUIT_FACTORY_H_

// fruit_factory.cc
#include "fruit_factory.h"

#include <iostream>
#include <stdio.h>
#include "fruit.h"

Fruit* AppleFactory::GetFruit() {
  return new Apple();
}

Fruit* PearFactory::GetFruit() {
  return new Pear();
}

Fruit* OrangeFactory::GetFruit() {
  return new Orange();
}

client 的代码如下:

#include "fruit.h"
#include "fruit_factory.h"

using namespace std;

int main(int argc, char** argv) {
  {
    FruitFactory* factory = new AppleFactory();
    Fruit* fruit = factory->GetFruit();
    fruit->Name();
    delete fruit;
    delete factory;
  }
  {
    FruitFactory* factory = new PearFactory();
    Fruit* fruit = factory->GetFruit();
    fruit->Name();
    delete fruit;
    delete factory;
  }
  {
    FruitFactory* factory = new OrangeFactory();
    Fruit* fruit = factory->GetFruit();
    fruit->Name();
    delete fruit;
    delete factory;
  }
  return 0;
}

makefile文件

# UNIX makefile
CXX = g++
LD  = g++
CXXFLAGS = -g
AR = ar
TARGETLIB = libfactory.a
object = fruit_factory.o fruit.o
main: main.o $(TARGETLIB)
    $(CXX) -o main main.o $(TARGETLIB)
$(TARGETLIB): $(object)
    $(AR) cq $@ $^
main.o: fruit.h fruit_factory.h
fruit_factory.o: fruit.h fruit_factory.h
fruit.o: fruit.h
.PHONY: clean
clean:
    rm -rf *.o *.a main

运行结果如下:

This is Apple !
This is Pear !
This is Orange !

对于工厂方法.当需要增加一个水果时,则只需要从Fruit派生出一个具体的水果类.然后从FruitFactory派生出对应水果的工厂类即可.不需要修改原有的代码.

因此,工厂方法满足“开闭”原则