13.4

最新推荐文章于 2025-06-13 10:42:53 发布

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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 Implementation

void 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 Implementation

void 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 Implementation

void 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 Implementation

void 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 Implementation

void 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 Implementation

void 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.