Where is the benefit in using the Strategy Pattern?

Question

I\'ve looked at this explanation on Wikipedia, specifically the C++ sample, and fail to recognize the difference between just defining 3 classes, creating instances and call

Answer 1

There is a difference between strategy and decision/choice. Most of the time a we would be handling decisions/choices in our code, and realise them using if()/switch() constructs. Strategy pattern is useful when there is a need to decouple the logic/algorithm from usage.

As an example, Think of a polling mechanism, where different users would check for resources/updates. Now we may want some of the priveliged users to be notified with a quicker turnaround time or with more details. Essentailly the logic being used changes based on user roles. Strategy makes sense from a design/architecture view point, at lower levels of granularity it should always be questioned.

Answer 2

This design pattern allows to encapsulate algorithms in classes.

The class that uses the strategy, the client class, is decoupled from the algorithm implementation. You can change the algorithms implementation, or add new algorithm without having to modify the client. This can also be done dynamically: the client can choose the algorithm he will use.

For an example, imagine an application that needs to save an image to a file ; the image can be saved in different formats (PNG, JPG ...). The encoding algorithms will all be implemented in different classes sharing the same interface. The client class will choose one depending on the user preference.