定义：

观察者模式定义了对象之间的一对多依赖，这样一来，当一个对象改变状态时，它的所有依赖者都会收到通知并自动更新。

场景：

我们有一个气象站，我们通过WeatherData获取气象站的最新数据，并更新布告板上的数据。我们定义了两个布告板，一个是温度计，显示最新的温度，另一个布告板就是显示当前的所有温度、湿度和气压的最新数据。

类图：

Observer，Subject，DisplayElement代码如下：

1 #ifndef SUBJECT_H  2 #define SUBJECT_H  3 #include 
      
         4 #include 
       
          5 #include 
        
           6 #include 
         
            7 using namespace std;  8  9 class Observer; 10 11 // 主题 12 class Subject 13 {
    14 public: 15     virtual void registerObserver(Observer* o) = 0; 16     virtual void removeObserver(Observer* o) = 0; 17     virtual void notifyObserver() = 0; 18     float getTemperature(); 19     float getHumidity(); 20     float getPressure(); 21 protected: 22     float m_temperature; 23     float m_humidity; 24     float m_pressure; 25 }; 26 float Subject::getTemperature() 27 {
    28     return m_temperature; 29 } 30 31 float Subject::getHumidity() 32 {
    33     return m_humidity; 34 } 35 36 float Subject::getPressure() 37 {
    38     return m_pressure; 39 } 40 // 观察者 41 class Observer 42 {
    43 public: 44     Observer(Subject* pWeaterData); 45     virtual ~Observer() {}; 46     virtual void update() = 0; 47     Subject* getWeaterData(); 48 protected: 49     Subject* m_weaterData; 50 }; 51 Observer::Observer(Subject* pWeaterData) 52 {
    53     m_weaterData = pWeaterData; 54 } 55 56 Subject* Observer::getWeaterData() 57 {
    58     return m_weaterData; 59 } 60 61 class DisplayElement 62 {
    63 public: 64     virtual void display() = 0; 65 }; 66 #endif
         
        
       
      

WeatherData，CurrentConditionsDisplay，Thermometer代码如下：

1 #ifndef WEATHER_H   2 #define WEATHER_H   3 #include "subject.h"   4 #include 
      
          5 class WeatherData:public Subject   6 {
      7 public:   8     WeatherData();   9     ~WeatherData();  10     void registerObserver(Observer* o);  11     void removeObserver(Observer* o);  12     void notifyObserver();  13     void measurementsChanged();  14     void setMeasurements(float temperature, float humidity, float pressure);  15 private:  16     list
       
         m_lObserver;  17 };  18 WeatherData::WeatherData()  19 {
     20     m_temperature = 0;  21     m_humidity = 0;  22     m_pressure = 0;  23 }  24  25 WeatherData::~WeatherData()  26 {
     27     list
        
         ::iterator iter, temp;  28     for (iter=m_lObserver.begin(); iter!=m_lObserver.end(); ++iter)  29     {
     30         delete (*iter);  31     }  32     m_lObserver.clear();  33 }  34  35 void WeatherData::registerObserver(Observer* o)  36 {
     37     m_lObserver.push_back(o);  38 }  39  40 void WeatherData::removeObserver(Observer* o)  41 {
     42     list
         
          ::iterator iter;  43  44     iter = std::find(m_lObserver.begin(), m_lObserver.end(), o);  45  46     if (m_lObserver.end() != iter)  47     {
     48         m_lObserver.erase(iter);  49     }  50 }  51  52 void WeatherData::notifyObserver()  53 {
     54     list
          
           ::iterator iter;  55     for (iter=m_lObserver.begin(); iter!=m_lObserver.end(); ++iter)  56     {
     57         (*iter)->update();  58     }  59 }  60  61 void WeatherData::measurementsChanged()  62 {
     63     notifyObserver();  64 }  65  66 void WeatherData::setMeasurements(float temperature, float humidity, float pressure)  67 {
     68     m_temperature = temperature;  69     m_humidity = humidity;  70     m_pressure = pressure;  71     measurementsChanged();  72 }  73  74 // 当前条件显示  75 class CurrentConditionsDisplay:public Observer,public DisplayElement  76 {
     77 public:  78     CurrentConditionsDisplay(Subject* pWeaterData);  79     ~CurrentConditionsDisplay();  80     void update();  81     void display();  82 private:  83     float m_template; // 温度  84     float m_humidity; // 湿度  85     float m_pressure; // 气压  86 };  87 CurrentConditionsDisplay::CurrentConditionsDisplay(Subject* pWeaterData):Observer(pWeaterData)  88 {
     89     pWeaterData->registerObserver(this);  90 }  91 CurrentConditionsDisplay::~CurrentConditionsDisplay()  92 {
     93     m_weaterData = NULL;  94 }  95 void CurrentConditionsDisplay::update()  96 {
     97     m_template = m_weaterData->getTemperature();  98     m_humidity = m_weaterData->getHumidity();  99     m_pressure = m_weaterData->getPressure(); 100     display(); 101 } 102 void CurrentConditionsDisplay::display() 103 {
    104     printf("Current conditions %f0.2F degrees and %0.2f%s humidity and pressure %0.2f\n",m_template,m_humidity,"%",m_pressure); 105 } 106 107 // 温度计 108 class Thermometer:public Observer,public DisplayElement 109 {
    110 public: 111     Thermometer(Subject* pWeaterData); 112     ~Thermometer(); 113     void update(); 114     void display(); 115 private: 116     float m_template; // 温度 117 }; 118 Thermometer::Thermometer(Subject* pWeaterData):Observer(pWeaterData) 119 {
    120     pWeaterData->registerObserver(this); 121 } 122 123 Thermometer::~Thermometer() 124 {
    125     m_weaterData = NULL; 126 } 127 128 void Thermometer::update() 129 {
    130     m_template = m_weaterData->getTemperature(); 131     display(); 132 } 133 134 void Thermometer::display() 135 {
    136     printf("Current conditions %f0.2F degrees\n",m_template); 137 } 138 #endif
          
         
        
       
      

主程序：

1 #include 
     
        2 #include "subject.h"  3 #include "weather.h"  4 int main()  5 {
    6     WeatherData* pWeatherData = new WeatherData();  7     CurrentConditionsDisplay* pCurrentConditionsDisplay = new CurrentConditionsDisplay(pWeatherData);  8     Thermometer* pThermometer = new Thermometer(pWeatherData);  9     pWeatherData->setMeasurements(123,456,789); 10 11     // 当移除观察者后，需要手动释放内存 12     pCurrentConditionsDisplay->getWeaterData()->removeObserver(pCurrentConditionsDisplay); 13     pWeatherData->setMeasurements(789,456,123); 14 15     // 当再次注册后，内存交由主题管理 16     pCurrentConditionsDisplay->getWeaterData()->registerObserver(pCurrentConditionsDisplay); 17     pWeatherData->setMeasurements(135,257,479); 18 19     delete pWeatherData; 20     return 0; 21 }
     

运行结果：