Class A wanna class B do what he want to (command). Simple way, invoke B's method, that's a good idea, but don't follow the OO principle: decouple

How to do it?  

        Firstly, B is a stable class; he can in charge of all his responsibility. Maybe there are too many classes like B that A wants to invoke. So how to do it, A composition or aggregation all classes that he needs? Of course not, too much dependence with A. So let’s think about this with OO principle: Encapsulate What Varies. 

       There are many unpredictable commands that are the variety in this case. We need to encapsulate the command and plugin to A. So at that time, A just knows he is invoking a command, and doesn’t know who and how to execute that command.

       Secondly, how to plugin to A. I don’t wanna talk about DI or Java mechanism. I am handling C++, so just add a method named SetCommand(Command* pCommand) to A, that's fine.

       Lastly, I showed a UML figure 1-1 that described all the means.

                                                                Figure 1-1 
        Everybody’ll shout: that is command pattern.  Yes.

        As the title said: we used but we didn't recognize. So let’s start with its official definition:

        The Command Pattern encapsulate a request as an object, thereby letting you parameterize other objects with different request, queue or log requests, and support undoable operations.