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

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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。