13.4

2019-11-10 17:28:20

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13.33 因为我们需要改变Folder本身，所以必须是Folder&

13.34 @pezy

#include <string>#include <set>class Folder;class Message { friend void swap(Message &, Message &); friend class Folder;public: explicit Message(const std::string &str = ""):contents(str) { } Message(const Message&); Message& Operator=(const Message&); ~Message(); void save(Folder&); void remove(Folder&); void PRint_debug();private: std::string contents; std::set<Folder*> folders; void add_to_Folders(const Message&); void remove_from_Folders(); void addFldr(Folder *f) { folders.insert(f); } void remFldr(Folder *f) { folders.erase(f); }};void swap(Message&, Message&);class Folder { 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; void add_to_Message(const Folder&); void remove_from_Message(); void addMsg(Message *m) { msgs.insert(m); } void remMsg(Message *m) { msgs.erase(m); }};void swap(Folder &, Folder &);#include <iostream>void swap(Message &lhs, Message &rhs) { using std::swap; lhs.remove_from_Folders(); // Use existing member function to avoid duplicate code. rhs.remove_from_Folders(); // Use existing member function to avoid duplicate code. swap(lhs.folders, rhs.folders); swap(lhs.contents, rhs.contents); lhs.add_to_Folders(lhs); // Use existing member function to avoid duplicate code. rhs.add_to_Folders(rhs); // Use existing member function to avoid duplicate code.}// Message Implementationvoid Message::save(Folder &f) { addFldr(&f); // Use existing member function to avoid duplicate code. f.addMsg(this);}void Message::remove(Folder &f) { remFldr(&f); // Use existing member function to avoid duplicate code. f.remMsg(this);}void Message::add_to_Folders(const Message &m) { for (auto f : m.folders) f->addMsg(this);}Message::Message(const Message &m) : contents(m.contents), folders(m.folders) { add_to_Folders(m);}void Message::remove_from_Folders() { for (auto f : folders) f->remMsg(this); // The book added one line here: folders.clear(); but I think it is redundant and more importantly, it will cause a bug: // - In Message::operator=, in the case of self-assignment, it first calls remove_from_Folders() and its folders.clear() // clears the data member of lhs(rhs), and there is no way we can assign it back to lhs. // Refer to: http://stackoverflow.com/questions/29308115/protection-again-self-assignment // - Why is it redundant? As its analogous function Message::add_to_Folders(), Message::remove_from_Folders() should ONLY // take care of the bookkeeping in Folders but not touch the Message's own data members - makes it much clearer and easier // to use. As you can see in the 2 places where we call Message::remove_from_Folders(): in Message::operator=, folders.clear() // introduces a bug as illustrated above; in the destructor ~Message(), the member "folders" will be destroyed anyways, why do // we need to clear it first?}Message::~Message() { remove_from_Folders(); }Message &Message::operator=(const Message &rhs) { remove_from_Folders(); contents = rhs.contents; folders = rhs.folders; add_to_Folders(rhs); return *this;}void Message::print_debug() { std::cout << contents << std::endl; }// Folder Implementationvoid swap(Folder &lhs, Folder &rhs) { using std::swap; lhs.remove_from_Message(); rhs.remove_from_Message(); swap(lhs.msgs, rhs.msgs); lhs.add_to_Message(lhs); rhs.add_to_Message(rhs);}void Folder::add_to_Message(const Folder &f) { for (auto m : f.msgs) m->addFldr(this);}Folder::Folder(const Folder &f) : msgs(f.msgs) { add_to_Message(f); }void Folder::remove_from_Message() { for (auto m : msgs) m->remFldr(this);}Folder::~Folder() { remove_from_Message(); }Folder &Folder::operator=(const Folder &rhs) { remove_from_Message(); msgs = rhs.msgs; add_to_Message(rhs); return *this;}void Folder::print_debug() { for (auto m : msgs) std::cout << m->contents << " "; std::cout << std::endl;}int main() { return 0;}

13.35 新的Message可能不会正确的匹配带到Folder中

13.36 @pezy

#include <string>#include <set>class Folder;class Message { friend void swap(Message &, Message &); friend class Folder;public: explicit Message(const std::string &str = ""):contents(str) { } Message(const Message&); Message& operator=(const Message&); ~Message(); void save(Folder&); void remove(Folder&); void print_debug();private: std::string contents; std::set<Folder*> folders; void add_to_Folders(const Message&); void remove_from_Folders(); void addFldr(Folder *f) { folders.insert(f); } void remFldr(Folder *f) { folders.erase(f); }};void swap(Message&, Message&);class Folder { 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; void add_to_Message(const Folder&); void remove_from_Message(); void addMsg(Message *m) { msgs.insert(m); } void remMsg(Message *m) { msgs.erase(m); }};void swap(Folder &, Folder &);#include <iostream>void swap(Message &lhs, Message &rhs) { using std::swap; lhs.remove_from_Folders(); // Use existing member function to avoid duplicate code. rhs.remove_from_Folders(); // Use existing member function to avoid duplicate code. swap(lhs.folders, rhs.folders); swap(lhs.contents, rhs.contents); lhs.add_to_Folders(lhs); // Use existing member function to avoid duplicate code. rhs.add_to_Folders(rhs); // Use existing member function to avoid duplicate code.}// Message Implementationvoid Message::save(Folder &f) { addFldr(&f); // Use existing member function to avoid duplicate code. f.addMsg(this);}void Message::remove(Folder &f) { remFldr(&f); // Use existing member function to avoid duplicate code. f.remMsg(this);}void Message::add_to_Folders(const Message &m) { for (auto f : m.folders) f->addMsg(this);}Message::Message(const Message &m) : contents(m.contents), folders(m.folders) { add_to_Folders(m);}void Message::remove_from_Folders() { for (auto f : folders) f->remMsg(this); // The book added one line here: folders.clear(); but I think it is redundant and more importantly, it will cause a bug: // - In Message::operator=, in the case of self-assignment, it first calls remove_from_Folders() and its folders.clear() // clears the data member of lhs(rhs), and there is no way we can assign it back to lhs. // Refer to: http://stackoverflow.com/questions/29308115/protection-again-self-assignment // - Why is it redundant? As its analogous function Message::add_to_Folders(), Message::remove_from_Folders() should ONLY // take care of the bookkeeping in Folders but not touch the Message's own data members - makes it much clearer and easier // to use. As you can see in the 2 places where we call Message::remove_from_Folders(): in Message::operator=, folders.clear() // introduces a bug as illustrated above; in the destructor ~Message(), the member "folders" will be destroyed anyways, why do // we need to clear it first?}Message::~Message() { remove_from_Folders(); }Message &Message::operator=(const Message &rhs) { remove_from_Folders(); contents = rhs.contents; folders = rhs.folders; add_to_Folders(rhs); return *this;}void Message::print_debug() { std::cout << contents << std::endl; }// Folder Implementationvoid swap(Folder &lhs, Folder &rhs) { using std::swap; lhs.remove_from_Message(); rhs.remove_from_Message(); swap(lhs.msgs, rhs.msgs); lhs.add_to_Message(lhs); rhs.add_to_Message(rhs);}void Folder::add_to_Message(const Folder &f) { for (auto m : f.msgs) m->addFldr(this);}Folder::Folder(const Folder &f) : msgs(f.msgs) { add_to_Message(f); }void Folder::remove_from_Message() { for (auto m : msgs) m->remFldr(this);}Folder::~Folder() { remove_from_Message(); }Folder &Folder::operator=(const Folder &rhs) { remove_from_Message(); msgs = rhs.msgs; add_to_Message(rhs); return *this;}void Folder::print_debug() { for (auto m : msgs) std::cout << m->contents << " "; std::cout << std::endl;}int main() { return 0;}

13.37 @pezy

#include <string>#include <set>class Folder;class Message { friend void swap(Message &, Message &); friend class Folder;public: explicit Message(const std::string &str = ""):contents(str) { } Message(const Message&); Message& operator=(const Message&); ~Message(); void save(Folder&); void remove(Folder&); void print_debug();private: std::string contents; std::set<Folder*> folders; void add_to_Folders(const Message&); void remove_from_Folders(); void addFldr(Folder *f) { folders.insert(f); } void remFldr(Folder *f) { folders.erase(f); }};void swap(Message&, Message&);class Folder { 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; void add_to_Message(const Folder&); void remove_from_Message(); void addMsg(Message *m) { msgs.insert(m); } void remMsg(Message *m) { msgs.erase(m); }};void swap(Folder &, Folder &);#include <iostream>void swap(Message &lhs, Message &rhs) { using std::swap; lhs.remove_from_Folders(); // Use existing member function to avoid duplicate code. rhs.remove_from_Folders(); // Use existing member function to avoid duplicate code. swap(lhs.folders, rhs.folders); swap(lhs.contents, rhs.contents); lhs.add_to_Folders(lhs); // Use existing member function to avoid duplicate code. rhs.add_to_Folders(rhs); // Use existing member function to avoid duplicate code.}// Message Implementationvoid Message::save(Folder &f) { addFldr(&f); // Use existing member function to avoid duplicate code. f.addMsg(this);}void Message::remove(Folder &f) { remFldr(&f); // Use existing member function to avoid duplicate code. f.remMsg(this);}void Message::add_to_Folders(const Message &m) { for (auto f : m.folders) f->addMsg(this);}Message::Message(const Message &m) : contents(m.contents), folders(m.folders) { add_to_Folders(m);}void Message::remove_from_Folders() { for (auto f : folders) f->remMsg(this); // The book added one line here: folders.clear(); but I think it is redundant and more importantly, it will cause a bug: // - In Message::operator=, in the case of self-assignment, it first calls remove_from_Folders() and its folders.clear() // clears the data member of lhs(rhs), and there is no way we can assign it back to lhs. // Refer to: http://stackoverflow.com/questions/29308115/protection-again-self-assignment // - Why is it redundant? As its analogous function Message::add_to_Folders(), Message::remove_from_Folders() should ONLY // take care of the bookkeeping in Folders but not touch the Message's own data members - makes it much clearer and easier // to use. As you can see in the 2 places where we call Message::remove_from_Folders(): in Message::operator=, folders.clear() // introduces a bug as illustrated above; in the destructor ~Message(), the member "folders" will be destroyed anyways, why do // we need to clear it first?}Message::~Message() { remove_from_Folders(); }Message &Message::operator=(const Message &rhs) { remove_from_Folders(); contents = rhs.contents; folders = rhs.folders; add_to_Folders(rhs); return *this;}void Message::print_debug() { std::cout << contents << std::endl; }// Folder Implementationvoid swap(Folder &lhs, Folder &rhs) { using std::swap; lhs.remove_from_Message(); rhs.remove_from_Message(); swap(lhs.msgs, rhs.msgs); lhs.add_to_Message(lhs); rhs.add_to_Message(rhs);}void Folder::add_to_Message(const Folder &f) { for (auto m : f.msgs) m->addFldr(this);}Folder::Folder(const Folder &f) : msgs(f.msgs) { add_to_Message(f); }void Folder::remove_from_Message() { for (auto m : msgs) m->remFldr(this);}Folder::~Folder() { remove_from_Message(); }Folder &Folder::operator=(const Folder &rhs) { remove_from_Message(); msgs = rhs.msgs; add_to_Message(rhs); return *this;}void Folder::print_debug() { for (auto m : msgs) std::cout << m->contents << " "; std::cout << std::endl;}int main() { return 0;}

13.38 @Mooophy The copy and swap is an elegant way when working with dynamicly allocated memory. In the Message class , nothing 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.

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