三种工厂模式详解

策略模式

在GOF的《设计模式:可复用面向对象软件的基础》一书中对策略模式是这样说的：

定义一系列的算法，把它们一个个封装起来，并且使它们可相互替换。该模式使得算法可独立于使用它的客户而变化。

策略模式为了适应不同的需求，只把变化点封装了，这个变化点就是实现不同需求的算法，例如加班工资，不同的加班情况，有不同的计算加班工资的方法。

我们不能在程序中将计算工资的算法进行硬编码，而能够自由变化。

这就是策略模式。

实验1 策略模式+简单工厂

代码

#include <iostream>
using namespace std;
// Define the strategy type
typedef enum StrategyType
{
    StrategyA,
    StrategyB,
    StrategyC
}STRATEGYTYPE;
// The abstract strategy
class Strategy
{
public:
    virtual void AlgorithmInterface(int x, int y) = 0;
    virtual ~Strategy() = 0; 
};
Strategy::~Strategy()
{}
class ConcreteStrategyA : public Strategy
{
public:
    void AlgorithmInterface(int x, int y)
    {
        cout << "I am from ConcreteStrategyA: result=X+Y, = " << x+y << endl;
    }
    ~ConcreteStrategyA(){
        cout << "~ConcreteStrategyA()." << endl;
    }
};
class ConcreteStrategyB : public Strategy
{
public:
    void AlgorithmInterface(int x, int y)
    {
        cout << "I am from ConcreteStrategyB: result=X*Y, = " << x*y << endl;
    }
    ~ConcreteStrategyB(){
        cout << "~ConcreteStrategyB()." << endl;
    }
};
class ConcreteStrategyC : public Strategy
{
public:
    void AlgorithmInterface(int x, int y)
    {
        cout << "I am from ConcreteStrategyC: result=X-Y, = " << x-y << endl;
    }
    ~ConcreteStrategyC(){
        cout << "~ConcreteStrategyC()." << endl;
    }
};
// 用户类 ： 使用算法的用户
class Context
{
public:
    // 该构造函数内使用简单工厂模式
    Context(STRATEGYTYPE strategyType)
    {
        switch (strategyType)
        {
        case StrategyA:
            pStrategy = new ConcreteStrategyA;
            break;
        case StrategyB:
            pStrategy = new ConcreteStrategyB;
            break;
        case StrategyC:
            pStrategy = new ConcreteStrategyC;
            break;
        default:
            break;
        }
    }
    ~Context()
    {
        if (pStrategy) 
            delete pStrategy;
    }
    void ContextInterface(int x, int y) // 定义一个接口来让Stategy访问用户的数据
    {
        if (pStrategy)
            pStrategy->AlgorithmInterface(x, y);
    }
private:
    Strategy *pStrategy; // 维护一个对Stategy对象的引用或指针
    int a;               // 私有数据,这是用户数据,该用户是独立于算法的用户,用户不需要了解算法内部的数据结构
    int b;
};
int main()
{
    Context *pContext = new Context(StrategyA);
    pContext->ContextInterface(100, 6);
    if (pContext) 
        delete pContext;
}

上述的例子是策略模式，我们还能看到一点简单工厂模式的影子。

百度到的绝大多数都是这种例子了。这个代码完美吗？看下GoF的要领：

如果我们需要新增新的算法，我们需要修改Context类的构造函数，在里面新增新的算法，这显然不符合GoF采用扩展、子类化的精要。

所以引出了我下面的策略模式+抽象工厂的解决方案。

实验2 策略模式+抽象工厂

代码

#include <iostream>
using namespace std;
// The abstract strategy
class Strategy
{
public:
    virtual void AlgorithmInterface(int x, int y) = 0;
    virtual ~Strategy() = 0; 
};
Strategy::~Strategy()
{}
class ConcreteStrategyA : public Strategy
{
public:
    void AlgorithmInterface(int x, int y)
    {
        cout << "I am from ConcreteStrategyA: result=X+Y, = " << x+y << endl;
    }
    ~ConcreteStrategyA(){
        cout << "~ConcreteStrategyA().析够" << endl;
    }
};
class ConcreteStrategyB : public Strategy
{
public:
    void AlgorithmInterface(int x, int y)
    {
        cout << "I am from ConcreteStrategyB: result=X*Y, = " << x*y << endl;
    }
    ~ConcreteStrategyB(){
        cout << "~ConcreteStrategyB().析够" << endl;
    }
};
class ConcreteStrategyC : public Strategy
{
public:
    void AlgorithmInterface(int x, int y)
    {
        cout << "I am from ConcreteStrategyC: result=X-Y, = " << x-y << endl;
    }
    ~ConcreteStrategyC(){
        cout << "~ConcreteStrategyC().析够" << endl;
    }
};
// 用户类 ： 使用算法的用户
class Context
{
public:
    Context(Strategy* paraStrategy)
    {
        pStrategy = paraStrategy;
    }
    ~Context()
    {
        // 这里不需要析够pStrategy， pStrategy对应对象的析够会在delete pAbstractStrategyFactory时进行。
        // 即，销毁具体某个策略工厂对象时会负责析够具体的策略对象。所以这里就不需要了。
    }
    void ContextInterface(int x, int y) // 定义一个接口来让Stategy访问用户的数据
    {
        if (pStrategy)
            pStrategy->AlgorithmInterface(x, y);
    }
private:
    Strategy *pStrategy; // 维护一个对Stategy对象的引用或指针
    int a;               // 私有数据,这是用户数据,该用户是独立于算法的用户,用户不需要了解算法内部的数据结构
    int b;
};
// 点评该类： 一条产品线下的产品，通常存在共性，
// 也就是说，维护一个对产品的抽象类通常是需要的，而不是必须的。   
class AbstractStrategyFactory   // 抽象工厂这里可以实现多个纯虚方法
{
public:
    virtual Strategy* createStrategy(string Strategy)=0;
    virtual ~AbstractStrategyFactory(){}
};
  
class StrategyA_Factory:public AbstractStrategyFactory
{
Strategy *pStrategy;
public:
    StrategyA_Factory():pStrategy(NULL)
    {}
    Strategy* createStrategy(string Strategy)
    {
        if(Strategy == "StrategyA"){
            pStrategy = new ConcreteStrategyA();
            return pStrategy;
        }
        return NULL;
    }
    ~StrategyA_Factory()
    {
        cout << "~StrategyA_Factory(). 析够" << endl;
        if(pStrategy)
            delete pStrategy;
    }
};
class StrategyB_Factory:public AbstractStrategyFactory
{
Strategy *pStrategy;
public:
    StrategyB_Factory():pStrategy(NULL)
    {}
    Strategy* createStrategy(string Strategy)
    {
        if(Strategy == "StrategyB"){
            pStrategy = new ConcreteStrategyB();
            return pStrategy;
        }
        return NULL;
    }
    ~StrategyB_Factory()
    {
        if(pStrategy)
            delete pStrategy;
    }
};
class StrategyC_Factory:public AbstractStrategyFactory
{
Strategy *pStrategy;
public:
    StrategyC_Factory():pStrategy(NULL)
    {}
    Strategy* createStrategy(string Strategy)
    {
        if(Strategy == "StrategyC"){
            pStrategy = new ConcreteStrategyC();
            return pStrategy;
        }
        return NULL;
    }
    ~StrategyC_Factory()
    {
         if(pStrategy)
            delete pStrategy;       
    }
};
int main(int argc, char *argv[])
{
    AbstractStrategyFactory* pAbstractStrategyFactory;  // 创建抽象工厂指针
 
    pAbstractStrategyFactory = new StrategyA_Factory();       // 创建策略A的工厂
    Strategy* pStrategy = pAbstractStrategyFactory->createStrategy("StrategyA");  //使用策略A工厂来生产出策略A
    if(pStrategy == NULL)
    {
        cout << "pStrategy is NULL" << endl;
    }
    else
    {
    // 供用户来使用策略A : 用户使用策略A 和 策略A的实现，是松耦合的。
    Context *pContext = new Context(pStrategy);
    pContext->ContextInterface(100, 6);
    delete pAbstractStrategyFactory;
    if (pContext) 
        delete pContext;
    }
    return 0;
  
}