C++ Primer（第5版） 练习 13.34
练习 13.34 编写本节所描述的Message。
环境：Linux Ubuntu（云服务器）
工具：vim
代码块
#include<iostream>
using namespace std;class Messsage{friend class Folder;friend void swap(Message &lhs, Message &rhs);public:explicit Message(const string &str = ""): contents(str) {}Message(const Message &);Message &operator= (const Message &);~Message();void save(Folder &);void remove(Folder &);private:string contents;set<Folder*> folders;void add_to_Folders(const Message &);void remove_from_Folders();
};Message::Message(const Message &m): contents(m.contents), folders(m.folders){add_to_Folders(m);
}Message &Message::operator= (const Message &rhs){remove_from_Folders();contents = rhs.contents;folders = rhs.folders;add_to_Folders(rhs);return *this;
}Message::~Message(){remove_from_Folders();
}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(const Message &m){for(auto f : m.folders){f->addMsg(this);}
}void Message::remove_from_Folders(){for(auto f : folders){f->remMsg(this);}
}void swap(Messsage &lhs, Messsage &rhs){using std::swap;for(auto f : lhs.folders){f->remMsg(&lhs);}for(auto f : rhs.folders){f->remMsg(&rhs);}swap(lhs.folders, rhs.folders);swap(lhs.contents, rhs.contents);for(auto f : lhs.folders){f->addMsg(&lhs);}for(auto f : rhs.folders){f->addMsg(&rhs);}
}
