more better solution

ch13/README.md

@@ -220,4 +220,7 @@ some existing `Folders` will out of sync with the `Message` after assignment.
 >We did not use copy and swap to define the Message assignment operator. Why do you suppose this is so? 
 
 @Mooophy
-The copy and swap is an elegant way when working with dynamicly allocated memory. In the Message class ,noing is allocated dynamically. Thus using this idiom makes no sense and will make it more complicated to implement due to the pointers that point back.
+The copy and swap is an elegant way when working with dynamicly allocated memory. In the Message class ,noing is allocated dynamically. Thus using this idiom makes no sense and will make it more complicated to implement due to the pointers that point back.
+
+@pezy
+In this case, `swap` function is special. It will be clear two `Message`'s folders , then swap members, and added themselves to each folders. But, `Message` assignment operator just clear itself, and copy the members, and added itself to each folders. The `rhs` don't need to clear and add to folders. So, if using copy and swap to define, it will be very inefficiency.

ch13/ex13_34_36_37.cpp

@@ -1,4 +1,5 @@
-#include "ex12_34_36_37.h"
+#include "ex13_34_36_37.h"
+#include <iostream>
 
 void swap(Message &lhs, Message &rhs)
 {
@@ -62,8 +63,31 @@ Message& Message::operator=(const Message &rhs)
     return *this;
 }
 
+void Message::print_debug()
+{
+    std::cout << contents << std::endl;
+}
+
 // Folder Implementation
 
+void swap(Folder &lhs, Folder &rhs)
+{
+	using std::swap;
+	for (auto m : lhs.msgs)
+		m->remFldr(&lhs);
+
+	for (auto m : rhs.msgs)
+		m->remFldr(&rhs);
+
+	swap(lhs.msgs, rhs.msgs);
+
+	for (auto m : lhs.msgs)
+		m->addFldr(&lhs);
+
+	for (auto m : rhs.msgs)
+		m->addFldr(&rhs);
+}
+
 void Folder::add_to_Message(const Folder &f)
 {
     for (auto m : f.msgs)
@@ -93,3 +117,10 @@ Folder& Folder::operator =(const Folder &rhs)
     add_to_Message(rhs);
     return *this;
 }
+
+void Folder::print_debug()
+{
+    for (auto m : msgs)
+        std::cout << m->contents << " ";
+    std::cout << std::endl;
+}

ch13/ex13_34_36_37.h

@@ -22,6 +22,7 @@ class Folder;
 
 class Message {
     friend void swap(Message &, Message &);
+    friend void swap(Folder &, Folder &);
     friend class Folder;
 public:
     explicit Message(const std::string &str = ""):contents(str) {}
@@ -31,6 +32,8 @@ class Message {
     void save(Folder&);
     void remove(Folder&);
 
+    void print_debug();
+
 private:
     std::string contents;
     std::set<Folder*> folders;
@@ -46,13 +49,16 @@ void swap(Message&, Message&);
 
 class Folder {
     friend void swap(Message&, Message&);
+    friend void swap(Folder &, Folder &);
     friend class Message;
 public:
     Folder() = default;
     Folder(const Folder &);
     Folder& operator=(const Folder &);
     ~Folder();
 
+    void print_debug();
+
 private:
     std::set<Message*> msgs;
 
@@ -63,5 +69,7 @@ class Folder {
     void remMsg(Message *m) { msgs.erase(m); }
 };
 
+void swap(Folder &, Folder &);
+
 #endif // MESSAGE