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Question

I was reading about copy-control and came across the following sample in the book C++ Primer (chapter 13.4).

My question is about the remove_from_Folders(); inside copy assignment operator:
If we firstly do remove_from_Folders(); , in the case of self assignment, doesn't its folders.clear(); clear the data member of rhs and cause a failure?

 Message& Message::operator=(const Message &rhs) { // handle self-assignment by removing pointers before inserting them remove_from_Folders(); // update 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; } // remove this Message from the corresponding Folders 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(); } 

The class is defined as:

 class Message { friend class Folder; public: // folders is implicitly 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; // Folders 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(); }; 

Answer 1

https://github.com/Mooophy/Cpp-Primer/tree/master/ch13

the sampe code listed above is more appropriate for handling operator=(self)

Message& Message::operator=(const Message &rhs) {
    if (this != &rhs) { // self-assigment check is necessary
                        // while remove_from_Folders() do folders.clear()
        remove_from_Folders();    // update 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;
}