c++ - 迭代列表

Question

我正在尝试将 CD 的歌曲存储在列表中并从 Main 调用它。 Main 应该显示列表中的歌曲。我无法显示它们。如果我要cout << cd.my_disc.size();它显示那里有多少，但我无法显示它们。错误代码在我的代码之后。

光盘类

class CD : public Media
{
public:
    struct Song
    {
        string title;
        double length;
    };

    // List to hold all songs
    list<Song>my_disc;
    list<Song>::iterator iter;


    // Method to add song
    void addSong(string title, double length)
    {
        Song temp = {title, length};
        my_disc.push_back(temp);
    }
    friend ostream& operator<<(ostream& os, const CD& cd);
};

ostream& operator<<(ostream& os, const CD& cd)
{
    os << cd.iter;
    return os;
}

主要

int main()
{
    string title;
    double length;
    char again;
    CD cd;
    do
    {
        cout << "Enter Title: ";
        cin.ignore();
        getline(cin, title);
        cout << "Enter length: ";
        cin >> length;
        cd.addSong(title, length);
        cout << "Again? ";
        cin >> again;
    }while(again == 'Y' || again == 'y');

    for (cd.iter = cd.my_disc.begin(); cd.iter != cd.my_disc.end(); cd.iter++)
        cout << cd.iter << " ";
}

一长串错误

1>------ Build started: Project: CD DVD Collection, Configuration: Debug Win32 ------
1>  Main1.cpp
1>e:\cd dvd collection\cd dvd collection\cd.h(55): error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'const std::_List_iterator<_Mylist>' (or there is no acceptable conversion)
1>          with
1>          [
1>              _Mylist=std::_List_val<CD::Song,std::allocator<CD::Song>>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(679): could be 'std::basic_ostream<_Elem,_Traits> &std::operator <<<char,std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,const char *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(726): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<char,std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,char)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(764): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,const char *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(811): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,char)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(937): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,const signed char *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(944): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,signed char)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(951): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,const unsigned char *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(958): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,unsigned char)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(968): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<char,std::char_traits<char>,std::_List_iterator<_Mylist>>(std::basic_ostream<_Elem,_Traits> &&,_Ty)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>,
1>              _Mylist=std::_List_val<CD::Song,std::allocator<CD::Song>>,
1>              _Ty=std::_List_iterator<std::_List_val<CD::Song,std::allocator<CD::Song>>>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(1085): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<char,std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,const std::error_code &)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          e:\cd dvd collection\cd dvd collection\cd.h(49): or       'std::ostream &operator <<(std::ostream &,const CD &)'
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(186): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(std::basic_ostream<_Elem,_Traits> &(__cdecl *)(std::basic_ostream<_Elem,_Traits> &))'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(192): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(std::basic_ios<_Elem,_Traits> &(__cdecl *)(std::basic_ios<_Elem,_Traits> &))'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(199): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(std::ios_base &(__cdecl *)(std::ios_base &))'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(206): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(std::_Bool)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(226): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(short)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(260): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(unsigned short)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(280): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(int)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(305): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(unsigned int)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(325): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(long)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(345): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(unsigned long)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(366): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(__int64)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(386): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(unsigned __int64)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(407): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(float)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(427): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(double)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(447): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(long double)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(467): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(const void *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(487): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(std::basic_streambuf<_Elem,_Traits> *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          while trying to match the argument list '(std::ostream, const std::_List_iterator<_Mylist>)'
1>          with
1>          [
1>              _Mylist=std::_List_val<CD::Song,std::allocator<CD::Song>>
1>          ]
1>e:\cd dvd collection\cd dvd collection\main1.cpp(24): error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'std::_List_iterator<_Mylist>' (or there is no acceptable conversion)
1>          with
1>          [
1>              _Mylist=std::_List_val<CD::Song,std::allocator<CD::Song>>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(679): could be 'std::basic_ostream<_Elem,_Traits> &std::operator <<<char,std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,const char *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(726): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<char,std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,char)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(764): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,const char *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(811): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,char)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(937): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,const signed char *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(944): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,signed char)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(951): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,const unsigned char *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(958): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,unsigned char)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(968): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<char,std::char_traits<char>,std::_List_iterator<_Mylist>>(std::basic_ostream<_Elem,_Traits> &&,_Ty)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>,
1>              _Mylist=std::_List_val<CD::Song,std::allocator<CD::Song>>,
1>              _Ty=std::_List_iterator<std::_List_val<CD::Song,std::allocator<CD::Song>>>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(1085): or       'std::basic_ostream<_Elem,_Traits> &std::operator <<<char,std::char_traits<char>>(std::basic_ostream<_Elem,_Traits> &,const std::error_code &)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          e:\cd dvd collection\cd dvd collection\cd.h(49): or       'std::ostream &operator <<(std::ostream &,const CD &)'
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(186): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(std::basic_ostream<_Elem,_Traits> &(__cdecl *)(std::basic_ostream<_Elem,_Traits> &))'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(192): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(std::basic_ios<_Elem,_Traits> &(__cdecl *)(std::basic_ios<_Elem,_Traits> &))'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(199): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(std::ios_base &(__cdecl *)(std::ios_base &))'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(206): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(std::_Bool)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(226): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(short)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(260): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(unsigned short)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(280): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(int)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(305): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(unsigned int)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(325): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(long)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(345): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(unsigned long)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(366): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(__int64)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(386): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(unsigned __int64)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(407): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(float)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(427): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(double)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(447): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(long double)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(467): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(const void *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(487): or       'std::basic_ostream<_Elem,_Traits> &std::basic_ostream<_Elem,_Traits>::operator <<(std::basic_streambuf<_Elem,_Traits> *)'
1>          with
1>          [
1>              _Elem=char,
1>              _Traits=std::char_traits<char>
1>          ]
1>          while trying to match the argument list '(std::ostream, std::_List_iterator<_Mylist>)'
1>          with
1>          [
1>              _Mylist=std::_List_val<CD::Song,std::allocator<CD::Song>>
1>          ]
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

我认为这与重载运算符有关<<但我不知道出了什么问题

Answer 1

很可能你想输出迭代器指向的对象，而不是迭代器本身，所以 cout 代码会变成这样:

    for (cd.iter = cd.my_disc.begin(); cd.iter != cd.my_disc.end(); cd.iter++)
        cout << *(cd.iter) << " "; //notice the *, to dereference the iterator
//it should work without the parentheses, but i like with those because it's clearer for the reader what is dereferenced

运算符中的相同内容<< 而且您不能将 CD::Song 放在输出流上，因此您将需要第二个运算符<<:

ostream& operator<<(ostream& os, const CD::Song& song)
{ 
    os << song.title << " " << song.length;
    return os;
}

并且 cin.ignore 也应该在您的代码中以不同的方式放置。

但是我会写一些不同的代码，如果你定义一个输出 CD 的运算符 <<，我会把歌曲循环写入运算符 << 这样代码 cout << cd;将工作:

ostream& operator<<(ostream& os, const CD::Song& song)
{
    os << song.title << " " << song.length;
    return os;
}

ostream& operator<<(ostream& os, const CD& cd)
{
    for (list<CD::Song>::const_iterator i = cd.my_disc.cbegin(); i != cd.my_disc.cend(); i++)
        os << *i << "\n";
    return os;
}

然后 main 看起来像这样:

int main()
{
    string title;
    double length;
    char again;
    CD cd;
    do
    {
        cout << "Enter Title: ";
        //cin.ignore(); //ignore here is not necessary
        getline(cin, title);
        cout << "Enter length: ";
        cin >> length;
        cd.addSong(title, length);
        cout << "Again? ";
        cin >> again;
        cin.ignore();//here might be necessary because a getline might be called after reading with cin
    }while(again == 'Y' || again == 'y');

    cout << cd << "\n"; //this will output all the Songs on the CD
}

如果您进行了这些修改，则不需要将迭代器成员放入 CD 类中，您可以进行一些代码清理。