Implementing a Subject/Observer pattern with templates

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Introduction

The traditional Subject/Observer pattern as described in the famous Design Patterns book has one annoying disadvantage: the observer does not get a handle to the notifying subject in its update method. This means that:

• The observer needs to store a pointer to the subject to which it is attached.
• If the observer is attached to multiple objects, it has no way of determining which one of them is notifying him.

This article tries to solve these problems by making use of C++ templates.

The original design pattern

The original design pattern makes use of two base classes: Subject and Observer. The Subject base class contains all the logic of storing all the attached observers. The Observer class just contains a pure virtual method (update()), that needs to be filled in by the inheriting observer class. Please read the 'Design Patterns' book for all details. The update() method of the Observer class does not get any parameters, which means that a class that inherits from Observer does not know where the notification came from. It is not difficult to add a 'Subject' parameter to the update method, but since the real subject inherited from the 'Subject' base class, the observing class always needs to perform a down-cast, which could be dangerous.

The solution

Instead of defining two base classes, we will define two template clases. Both template classes will be based on the subject-class that is able to notify other classes (the observers).

template <class T>
class Observer
   {
   public:
      Observer() {}
      virtual ~Observer() {}
      virtual void update(T *subject)= 0;
   };

The first enhancement here is that our pure virtual update method gets a pointer to the subject as argument; not the base Subject class (which is shown hereafter), but the class that was given as parameter to the template definition.

template <class T>
class Subject
   {
   public:
      Subject() {}
      virtual ~Subject() {}
      void attach (Observer<T> &observer)
         {
         m_observers.push_back(&observer);
         }
      void notify ()
         {
         std::vector<Observer<T> *>::iterator it;
         for (it=m_observers.begin();it!=m_observers.end();it++) 
              (*it)->update(static_cast<T *>(this));
         }
   private:
      std::vector<Observer<T> *> m_observers;
   };

Here, we defined the basic Subject class/template. The attach method simply adds the observer (which is of the basic Observer<T> class) to a vector. The notify method simply notifies all observers. Both templates can be used in any situation where the Subject/Observer pattern can be used. The following classes describe how they are used.

class Temperature : public Subject<Temperature>
   {
   public:
      Temperature() {}
      ~Temperature() {}
      void temperatureChanged () {notify();}
      void getTemperature() {std::cout << 
         "   Getting the temperature." << std::endl;}
   };

Our Temperature class is a class that monitors the temperature, and notifies its observers when the temperature changes. As you can see all it has to do is call the notify() method. The getTemperature method simply writes something on the screen, but of course in real-life situations it should return the actual temperature. Taking a look at the implementation of the notify() method. It simply calls the update() method of all attached observers, but with itself as argument. Since 'this' (which is the Subject class) is cast to the type T, the update() method of the observer will get the correct argument type, as shown in the following example:

class PanicSirene : public Observer<Temperature>
   {
   public:
      PanicSirene() {}
      ~PanicSirene() {}
      void update (Temperature *subject)
         {
         std::cout << "Temperature was changed, sound the sirene" 
               << std::endl;
         subject->getTemperature();
         }
   };

As you can see, a pointer to the Temperature instance that trigger the notification is given as argument to the update method. The observing class (PanicSirene in this case) can simply call any method of the notifying subject, in this case simply getTemperature. The following source shows how it is effectively used:

Temperature       temp;
PanicSirene       panic;

temp.attach (panic);

temp.temperatureChanged ();

The following output will be generated:

Temperature was changed, sound the sirene
   Getting the temperature.

Observing multiple subjects of a different type

The templates are still easy to use if you need to attach the observer to multiple objects. Suppose that we have a similar subject-class for measuring the pressure.

class Pressure : public Subject<PRESSURE>
   {
   public:
      Pressure() {}
      ~Pressure() {}
      void pressureChanged () {notify();}
      void getPressure() {std::cout << "   Getting the pressure." 
          << std::endl;}
   };

If we want to show both the temperature and pressure in a window that shows all environment-related information, we simply create our EnvironmentWindow like this:

class EnvironmentWindow : public Observer<Temperature>, 
        public Observer<Pressure>
   {
   public:
      EnvironmentWindow() {}
      ~EnvironmentWindow() {}
      void update (Temperature *subject) {std::cout << 
             "Temperature was changed" << 
             std::endl; subject->getTemperature();}
      void update (Pressure    *subject) {std::cout << 
             "Pressure was changed"    << 
             std::endl; subject->getPressure   ();}
   };

The class simply inherits twice from the Observer template, for both the Temperature and for the Pressure. Notice that we have two update methods here, one for the temperature, one for the pressure. The following example shows how it can be used:

Temperature       temp;
Pressure          press;
EnvironmentWindow win;
PanicSirene       panic;

temp.attach (win  );
temp .attach (panic);
press.attach (win  );

temp.temperatureChanged ();
press.pressureChanged ();

And it shows the following output:

Temperature was changed
   Getting the temperature.
Temperature was changed, sound the sirene
   Getting the temperature.
Pressure was changed
   Getting the pressure.

Observing multiple subjects of the same type

If our PanicSirene class needs to verify both the internal temperature and the external temperature, we don't need to modify anything to our implementation of the PanicSirene class. We simply attach our class instance to both Temperature classes.

Temperature       internalTemp;
Temperature       externalTemp;
PanicSirene       panic;

internalTemp.attach (panic);
externalTemp.attach (panic);