Copy Control for the Message Class

When we copy a Message, the copy should appear in the same Folders as the original Message. As a result, we must traverse the set of Folder pointers adding a pointer to the new Message to each Folder that points to the original Message. Both the copy constructor and the copy-assignment operator will need to do this work, so we’ll define a function to do this common processing:

// add this Message to Folders that point to m
void Message::add_to_Folders(const Message &m)
{
    for (auto f : m.folders) // for each Folder that holds m
        f->addMsg(this); // add a pointer to this Message to that Folder
}

Here we call addMsg on each Folder in m.folders. The addMsg function will add a pointer to this Message to that Folder.

The Message copy constructor copies the data members of the given object:

Message::Message(const Message &m):
    contents(m.contents), folders(m.folders)
{
    add_to_Folders(m); // add this Message to the Folders that point to m
}

and calls add_to_Folders to add a pointer to the newly created Message to each Folder that contains the original Message.