Understanding the Observer pattern

In the Observer pattern, an object of interest will maintain a list of observers who are interested in state updates of the main object. The observers will maintain a link to their object of interest. We will refer to the main object of interest as the Subject. The list of interested objects is known collectively as the Observers. The Subject will inform any Observer of relevant state changes. The Observers, once notified of any state changes of the Subject, will take any appropriate next action themselves (usually through a virtual function invoked on each Observer by the Subject).

Already, we can imagine how an Observer pattern may be implemented using associations. In fact, the Observer represents a one-to-many association. The Subject, for example, may use an STL list (or vector) to collect a set of Observers. Each Observer will contain an association to the Subject. We can imagine an important operation on the Subject, corresponding to a state...