The Message Class

Given this design, we can write our Message class as follows:

class Message {
    friend class Folder;
public:
    // folders is implicitly initialized to the empty set
    explicit Message(const std::string &str = ""):
        contents(str) { }
    // copy control to manage pointers to this Message
    Message(const Message&);            // copy constructor
    Message& operator=(const Message&); // copy assignment
    ~Message();                         // destructor
    // add/remove this Message from the specified Folder's set of messages
    void save(Folder&);
    void remove(Folder&);
private:
    std::string contents;      // actual message text
    std::set<Folder*> folders; // Folders that have this Message
    // utility functions used by copy constructor, assignment, and destructor
    // add this Message to the Folders that point to the parameter
    void add_to_Folders(const Message&);
    // remove this Message from every Folder in folders
    void remove_from_Folders();
};

The class defines two data members: contents, to store the message text, and folders, to store pointers to the Folders in which this Message appears. The constructor that takes a string copies the given string into contents and (implicitly) initializes folders to the empty set. Because this constructor has a default argument, it is also the Message default constructor (§ 7.5.1, p. 290).