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Observer

12345678910111213141516171819202122232425262728293031323334 #include <vector> #include <functional> class observer { public: virtual void notify() = 0; }; class observer_concrete : public observer { public: virtual void notify() override { } }; class subject { public: void register_observer(observer& o) { observers.push_back(o); } void notify_observers() { for (observer& o : observers) { o.notify(); } } private: std::vector<std::reference_wrapper<observer>> observers; };

This pattern is licensed under the CC0 Public Domain Dedication.

Requires c++11 or newer.

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.

Contributors

Last Updated

09 December 2017

Source

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