SourceMaking

#include <iostream>  #include <string>  using namespace std;

class Person;

 

class Command

{

    // 1. Create a class that encapsulates an object

    Person *object; //    and a member function

    void(Person:: *method)(); // a pointer to a member function (the attri-

  public:

    //    bute's name is "method")

    Command(Person *obj = 0, void(Person:: *meth)() = 0)

    {

        object = obj; // the argument's name is "meth"

        method = meth;

    }

    void execute()

    {

        (object-> *method)(); // invoke the method on the object

    }

};

 

class Person

{

    string name;

    Command cmd; // cmd is a "black box", it is a method invoca-

  public:

    //    tion promoted to "full object status"

    Person(string n, Command c): cmd(c)

    {

        name = n;

    }

    void talk()

    {

        // "this" is the sender, cmd has the receiver

        cout << name << " is talking" << endl;

        cmd.execute(); // ask the "black box" to callback the receiver

    }

    void passOn()

    {

        cout << name << " is passing on" << endl;

        cmd.execute(); // 4. When the sender is ready to callback to

    } //    the receiver, it calls execute()

    void gossip()

    {

        cout << name << " is gossiping" << endl;

        cmd.execute();

    }

    void listen()

    {

        cout << name << " is listening" << endl;

    }

};

 

int main()

{

  // Fred will "execute" Barney which will result in a call to passOn()

  // Barney will "execute" Betty which will result in a call to gossip()

  // Betty will "execute" Wilma which will result in a call to listen()

  Person wilma("Wilma", Command());

  // 2. Instantiate an object for each "callback"

  // 3. Pass each object to its future "sender"

  Person betty("Betty", Command(&wilma, &Person::listen));

  Person barney("Barney", Command(&betty, &Person::gossip));

  Person fred("Fred", Command(&barney, &Person::passOn));

  fred.talk();

}