设计模式之五：工厂方法模式（Factory Method）

工厂方法模式：定义了一个创建对象的接口，由子类来决定详细实例化那个对象。工厂方法模式让类的实例化转移到子类中来推断。

Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.

UML图：

主要包含：

1. Product(Page)：定义了工厂类创建的对象的接口
2. ConcreteProduct(SkillPage,EducationPage,ExperiencePage)：实现了Product的详细的类
3. Creator(Document)：声明了一个工厂方法，这种方法返回一个Product类型的对象。
4. ConcreteCreator(Report,Resume)：重写工厂方法来实例化详细的Product

上面的UML是工厂方法模式一般的图例，针对一个详细的有两个ConcreteProductA，ConcreteProductB。以及它们各自工厂类ConcreteCreatorA。ConcreteCreatorB的UML图例如以下所看到的：

C++代码例如以下：

#include <stdlib.h>
#include <stdio.h>
#include <iostream>

class Product
{
    public:
    virtual void show()=0;
};

class ConcreteProductA:public Product
{
    public:
            void show()
            {
                std::cout<<"ConcreteProductA:show"<<std::endl;
            }
};

class ConcreteProductB:public Product
{
    public:
            void show()
            {
                std::cout<<"ConcreteProductB:show"<<std::endl;
            }

};

class Creator
{
    public:
            virtual Product * factoryMethod()=0;

};

class ConcreteCreatorA:public Creator
{
    public:
            Product* factoryMethod()
            {
                return new ConcreteProductA();
            }

};

class ConcreteCreatorB:public Creator
{
    public:
            Product* factoryMethod()
            {
                return new ConcreteProductB();
            }

};

int main()
{
    std::cout<<"工厂方法模式"<<std::endl;
    Creator * creatorA=new ConcreteCreatorA;
    Creator * creatorB=new ConcreteCreatorB;

    Product * pa=creatorA->factoryMethod();
    Product* pb=creatorB->factoryMethod();

    pa->show();
    pb->show();

    delete creatorA;
    delete creatorB;
    delete pa;
    delete pb;
    return 0;
}

測试输出：

事实上还能够一个详细的ConcreteCreator相应多个ConcreteProduct，这里以一个样例为例分析：

1. Product为Page
2. ConcreteProduct包含SkillPage,EducationPage,ExperiencePage
3. Creator为Document（文档）
4. ConcreteCreator为Report(报告文档，报告文档中有SkillPage,EducationPage),Resume（简历文档。简历文档中有SkillPage,EducationPage,ExperiencePage）

这也是一个工厂方法模式的样例

UML图为：

C++代码实现例如以下：

#include <stdlib.h>
#include <stdio.h>
#include <iostream>
#include <list>
#include <algorithm>
using namespace std;

class Page
{
        public:
                virtual void show()=0;
};

class SkillPage:public Page
{
        public:
                void show()
                {
                    std::cout<<"SkillPage::show"<<std::endl;
                }
};

class EducationPage:public Page
{
        public:
                void show()
                {
                    std::cout<<"Education::show"<<std::endl;
                }

};

class ExperiencePage:public Page
{
        public:
                void show()
                {
                    std::cout<<"Experience::show"<<std::endl;
                }
};

class Document
{
    public:
            virtual void factoryMethod()=0;
            list<Page*>& getLists()
            {
                return lists;
            }
            void print()
            {
                list<Page*>::iterator iter;
                for(iter=lists.begin();iter!=lists.end();iter++)
                        (*iter)->show();
            }
            //注意这里要将list中的指针指向的内存删除掉，不然会造成内存泄露
            virtual ~Document(){
                list<Page*>::iterator iter;
                for(iter=lists.begin();iter!=lists.end();iter++)
                {
                    if(*iter)
                            delete *iter;
                }
            }
    private:
            list<Page*> lists;
};

class Report:public Document
{
    public:
            void factoryMethod()
            {
                getLists().push_back(new SkillPage());
                getLists().push_back(new EducationPage());
            }
};

class Resume:public Document
{
    public:
            void factoryMethod()
            {
                getLists().push_back(new SkillPage());
                getLists().push_back(new EducationPage());
                getLists().push_back(new ExperiencePage());
            }
};

int main()
{
    std::cout<<"详细的工厂方法模式測试"<<std::endl;
    Document * report=new Report();
    Document * resume=new Resume();
    report->factoryMethod();
    resume->factoryMethod();
    std::cout<<"report print"<<std::endl;
    report->print();
    std::cout<<"resume print"<<std::endl;
    resume->print();
    return 0;

}

測试输出：