具有同一对象的 C++ 复制分配

问：

在著名的C++入门书中，我找到了以下代码片段

Message& Message::operator=(const Message &rhs)
{
    // handle self-assignment by removing pointer before inserting them
    remove_from_Folders(); // updating existing Folders
    contents = rhs.contents; // copy message contents from rhs
    folders = rhs.folders; // copy Folder pointers from rhs
    add_to_Folders(rhs); // add this Message to those Folders
    return *this;
}

Message 类：

class Message {
    friend class Folder;
public: 
    // folders is implicity 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; // Folder's 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();
}

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
}

// 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
}

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
}

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

现在我的问题是：我们看到整个文件夹集被清除了。我们还看到 copy-assignment 运算符中的参数是一个引用。那么，为什么召唤它，然后做它，在这里起作用呢？既然是引用，那么当我们使用同一个对象（即进行自我（复制）赋值）时，现在不会是空集吗？这不就是引用的工作方式吗？或者编译器在使用赋值运算符进行操作时是否满足了特殊规则？remove_from_Foldersrhsremove_from_Folders()folders = rhs.folders;rhsrhs.folders