Custom events in C++?

Is it possible to create custom events in C++? For example, say I have the variable X, and the variable Y. Whenever X changes, I would like to execute a function that sets Y equal to 3X. Is there a way to create such a trigger/event? (triggers are common in some databases)

From stackoverflow Adam

• Think you should read a little about Design Patterns, specifically the Observer Pattern.

  Qt from Trolltech have implemented a nice solutions they call Signals and Slots.

  From epatel

• Use the Observer pattern

  code project example

  wiki page

  From slf

• As far as I am aware you can't do it with default variables, however if you wrote a class that took a callback function you could let other classes register that they want to be notified of any changes.

  From Denice

• This is basically an instance of the Observer pattern (as others have mentioned and linked). However, you can use template magic to render it a little more syntactically palettable. Consider something like...

  template <typename T>
  class Observable
  {
    T underlying;
  
  public:
    Observable<T>& operator=(const T &rhs) {
     underlying = rhs;
     fireObservers();
  
     return *this;
    }
    operator T() { return underlying; }
  
    void addObserver(ObsType obs) { ... }
    void fireObservers() { /* Pass every event handler a const & to this instance /* }
  };
  

  Then you can write...

  Observable<int> x;
  x.registerObserver(...);
  
  x = 5;
  int y = x;
  

  What method you use to write your observer callback functions are entirely up to you; I suggest http://www.boost.org's function or functional modules (you can also use simple functors). I also caution you to be careful about this type of operator overloading. Whilst it can make certain coding styles clearer, reckless use an render something like

  seemsLikeAnIntToMe = 10;

  a very expensive operation, that might well explode, and cause debugging nightmares for years to come.

  Scott Saad : Great explanation! I've used boost in past projects and have always been happy with the results.
  From Adam Wright

• Boost signals is another commonly used library you might come across to do Observer Pattern (aka Publish-Subscribe). Buyer beware here, I've heard its performance is terrible.

  From Doug T.