The save and remove Members

Aside from copy control, the Message class has only two public members: save, which puts the Message in the given Folder, and remove, which takes it out:

void Message::save(Folder &f)
{
    folders.insert(&f); // add the given Folder to our list of Folders
    f.addMsg(this);     // add this Message to f's set of Messages
}

void Message::remove(Folder &f)
{
    folders.erase(&f); // take the given Folder out of our list of Folders
    f.remMsg(this);    // remove this Message to f's set of Messages
}

To save (or remove) a Message requires updating the folders member of the Message. When we save a Message, we store a pointer to the given Folder; when we remove a Message, we remove that pointer.

These operations must also update the given Folder. Updating a Folder is a job that the Folder class controls through its addMsg and remMsg members, which will add or remove a pointer to a given Message, respectively.