A swap Function for Message

The library defines versions of swap for both string and set (§ 9.2.5, p. 339). As a result, our Message class will benefit from defining its own version of swap. By defining a Message-specific version of swap, we can avoid extraneous copies of the contents and folders members.

However, our swap function must also manage the Folder pointers that point to the swapped Messages. After a call such as swap(m1, m2), the Folders that had pointed to m1 must now point to m2, and vice versa.

We’ll manage the Folder pointers by making two passes through each of the folders members. The first pass will remove the Messages from their respective Folders. We’ll next call swap to swap the data members. We’ll make the second pass through folders this time adding pointers to the swapped Messages:

void swap(Message &lhs, Message &rhs)
{
    using std::swap; // not strictly needed in this case, but good habit
    // remove pointers to each Message from their (original) respective Folders
    for (auto f: lhs.folders)
        f->remMsg(&lhs);
    for (auto f: rhs.folders)
        f->remMsg(&rhs);
    // swap the contents and Folder pointer sets
    swap(lhs.folders, rhs.folders);     // uses swap(set&, set&)
    swap(lhs.contents, rhs.contents);   // swap(string&, string&)
    // add pointers to each Message to their (new) respective Folders
    for (auto f: lhs.folders)
        f->addMsg(&lhs);
    for (auto f: rhs.folders)
        f->addMsg(&rhs);
}