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。