C++ Primer 练习13.36和13.37

---------------------------------Message.h---------------------------------
#ifndef MESSAGE_H
#define MESSAGE_H

#include <string>
#include <set>
#include <iostream>

using namespace std;

class Folder;
class Message{
friend class Folder;
friend void swap(Message &lmsg, Message &rmsg);
public:
explicit Message(const string &str = "") :contents(str) {}
Message(const Message &str);
Message &operator=(const Message &str);
~Message();
void save(Folder &);//本类用以添加Folder对象，同时在Folder中添加本类（调用addMsg）
void remove(Folder &);//本类用以移除Folder对象，同时在Folder中移除本类（调用remMsg）

private:
string contents;
set<Folder*> folders;
void add_to_Folders( Message&);//向Folder类添加Message，有目的的声明为private
void remove_from_Folders();//移除Folder类的Message

/*声明为private。一般非本类对象调用，只能被Folder调用（其目的是在本类中添加Folder）。
否则无法向Folder中Message。若想要被本类对象调用，则必须将本类成员函数add_to_Folder声明为public，
之后调用此函数和add_to_folder函数向本类对象中添加Folder和向Folder中添加Message，
为了避免影响故而声明为private*/
void addFolder(Folder *f);
void remFolder(Folder *f);//目的：移除本类对象的Folder。用法同上
};
#endif

void swap(Message &lmsg, Message &rmsg);

---------------------------------Message.cpp---------------------------------
#include "Message.h"
#include "Folder.h"

void Message::addFolder(Folder *f){
folders.insert(f);
}

void Message::remFolder(Folder *f){
folders.erase(f);
}

void Message::save(Folder &f){
folders.insert(&f);
f.addMsg(this);
}

void Message::remove(Folder &f){
folders.erase(&f);
f.remMsg(this);
}

void Message::add_to_Folders( Message &msg){
for (auto be : msg.folders)
be->addMsg(&msg);
}

void Message::remove_from_Folders(){
for (auto be : this->folders)
be->remMsg(this);
}
Message::Message(const Message &str){
contents = str.contents;
folders = str.folders;
add_to_Folders(*this);
}

Message& Message::operator=(const Message &msg){
remove_from_Folders();
folders = msg.folders;
contents = msg.contents;
add_to_Folders(*this);
return *this;
}

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

void swap(Message &lmsg, Message &rmsg){
lmsg.remove_from_Folders();
rmsg.remove_from_Folders();
swap(lmsg.contents, rmsg.contents);
swap(lmsg.folders, rmsg.folders);
lmsg.add_to_Folders(lmsg);
rmsg.add_to_Folders(rmsg);
}

---------------------------------Folder.h---------------------------------
#ifndef FOLDER_H
#define FOLDER_H

#include <iostream>
#include <string>
#include <set>

using namespace std;

class Message;
class Folder{
friend class Message;
public:
Folder() = default;
Folder(const Folder&f);
Folder& operator=(const Folder &f);
~Folder();
void save(Message &msg);
void remove(Message &msg);
private:
set<Message*> messages;
void add_to_msg(Folder &f);
void remove_from_msg();
void addMsg(Message *msg);
void remMsg(Message *msg);
};
#endif

---------------------------------Folder.cpp---------------------------------
#include "Folder.h"
#include "Message.h"

void Folder::save(Message &msg){
messages.insert(&msg);
msg.addFolder(this);
}

void Folder::remove(Message &msg){
messages.erase(&msg);
msg.remFolder(this);
}

void Folder::add_to_msg(Folder &f){
for (auto be : f.messages)
be->addFolder(&f);
}

void Folder::remove_from_msg(){
for (auto be : messages)
be->remFolder(this);
}

void Folder::addMsg( Message *msg){
messages.insert(msg);
}

void Folder::remMsg(Message *msg){
messages.erase(msg);
}

Folder::Folder(const Folder &f){
messages = f.messages;
add_to_msg(*this);
}

Folder& Folder::operator=(const Folder &f){
remove_from_msg();
messages = f.messages;
add_to_msg(*this);
return *this;
}

Folder::~Folder(){
remove_from_msg();
}

---------------------------------main.cpp---------------------------------
#include "Folder.h"
#include "Message.h"

int main(){
Message m1("xingxiuyan"),m2("lindaiyu"),m3("xuebaochai");
Folder f1, f2, f3, f4, f5;
m1.save(f1);
f1.save(m2);
return 0;
}