The Message Destructor

When a Message is destroyed, we must remove this Message from the Folders that point to it. This work is shared with the copy-assignment operator, so we’ll define a common function to do it:

void Message::remove_from_Folders()
{
    for (auto f : folders) // for each pointer in folders
        f->remMsg(this);   // remove this Message from that Folder
    folders.clear();       // no Folder points to this Message
}

The implementation of the remove_from_Folders function is similar to that of add_to_Folders, except that it uses remMsg to remove the current Message.

Given the remove_from_Folders function, writing the destructor is trivial:

Message::~Message()
{
    remove_from_Folders();
}

The call to remove_from_Folders ensures that no Folder has a pointer to the Message we are destroying. The compiler automatically invokes the string destructor to free contents and the set destructor to clean up the memory used by those members.