设计模式之C++实现（一）---简单工厂模式（Simple Factory）

简单工厂模式又称静态工厂方法模式（Static Factory Method），它不是23种模式之一，但是却是我们常用的设计模式之一。代码先行：

1: // abstract base class

2: class Operation

3: {

4: public:

5:     int m_nFirst;

6:     int m_nSecond;

7: 

8:     virtual double GetResult() = 0;

9: 

10:     virtual ~Operation()

11:     {

12:

13:     }

14: };

15: 

16: // AddOperation class, Operation's child

17: class AddOperation: public Operation

18: {

19:     double GetResult()

20:     {

21:         return m_nFirst + m_nSecond;

22:     }

23: };

24: 

25: // SubOperation class, Operation's child

26: class SubOperation: public Operation

27: {

28:     double GetResult()

29:     {

30:         return m_nFirst - m_nSecond;

31:     }

32: };

33: 

34: // Factory class

35: class SimpleFactory

36: {

37: public:

38:     static Operation* CreateOperation(const char a);

39: };

40: 

41: Operation* SimpleFactory::CreateOperation(const char a)

42: {

43:     switch(a)

44:     {

45:         case '+':

46:             return new AddOperation;

47: 

48:         case '-':

49:             return new SubOperation;

50:

51:     }

52: }

简单解析：在上面的代码中有四个类:Operation、AddOperation、SubOperation和SimpleFactory。AddOperation和SubOperation是Operation的子类，继承了Operation的两个数据成员和GetResult()方法，一个是加法操作，一个是减法操作。SimpleFactory是简单工厂模式的核心，它负责根据用户传递的信息来创建具体的对象，进而在多态的支持下实现程序预想的功能。

客户端用户需要知道基类Operation和工厂类来创建相应的对象并调用对应的方法，耦合性不算好。

客户代码示例：

1: #include <iostream>

2: #include "simpleFactory.h"

3: 

4: using namespace std;

5: 

6: int main(int argc, char* argv[])

7: {

8:     // client code

9:     Operation* op = NULL;

10:

11:     op = SimpleFactory::CreateOperation('+');

12:op->m_nFirst  = 4;

13:     op->m_nSecond = 5;

14:     int sum = op->GetResult();

15:     delete op;    // don't forget to delete

16:     op = NULL;

17:     cout<<"sum is:"<<sum<<endl;

18: 

19:     op = SimpleFactory::CreateOperation('-');

20:     op->m_nFirst  = 4;

21:     op->m_nSecond = 5;

22:     int sub = op->GetResult();

23:     delete op;

24:     op = NULL;

25:     cout<<"sub is:"<<sub<<endl;

26:

27:     return 0;

28: }

上面示例代码的输出如下：

1: sum is:9

2: sub is:-1

模式的本质：新添加类时，不会影响以前的系统代码。核心思想是用一个工厂来根据输入的条件产生不同的类，然后根据不同类的virtual函数得到不同的结果。