Observer pattern

[mem]

Class subject
{
private:
List<Observer*> *_observers;
....
};

Why use a pointer, _observers, above?

Is it really necessary to have Subject and Observer base classes in the
pattern? Instead, just pass a pointer of a class into another, and the
latter just informs the former by calling its member function, whenever
needed?

How to assign _observers to a List object?

Thanks!

 

[John Harrison]

Class subject
{
private:
List<Observer*> *_observers;
...
};

Why use a pointer, _observers, above?

Why indeed? I can't think of a good reason.

Is it really necessary to have Subject and Observer base classes in the
pattern? Instead, just pass a pointer of a class into another, and the
latter just informs the former by calling its member function, whenever
needed?

How would you deal with pointers to different classes? How you you write a
list that can take an arbitary pointer and call use that pointer to call
specific member functions.

How to assign _observers to a List object?

That question makes no sense to me. Do you mean 'how to add an observer to
the _observers list?'. The answer to that would be use the appropriate
method of the List class.

john

 

[adiian]

Observers are really necessary. Just imagine you need to add another observer. You just derive from base observer class and that's it, no need to change existing classes. You can live without subjects but what to do when you need to add a new one?

see http://www.oodesign.com/oo_design_patterns/behavioral_patterns/observer_pattern.html