This pattern is licensed under the CC0 Public Domain Dedication.

Intent

Notify generic observer objects when an event occurs.

Description

The observer pattern allows generic observer objects to be registered with a subject object and receive notifications when certain events occur.

The subject class, defined on lines 17–34, contains a std::vector of references to observers line 33. Observers (also known as listeners), in this case, are objects that implement the observer interface (lines 4–8). The register_observer function (lines 20–23) adds observers to this std::vector, which are later to be notified by the notify_observers function (lines 25–30).

We use std::reference_wrapper for the elements of the std::vector (line 33), because the standard containers require the element type to be assignable, which normal reference types are not.