无法重载运算符，因为无法访问类中声明的私有成员

Question

我有一个运算符重载的类，如下所示：

friend ostream operator<< (ostream &stream, const Item &item){
          stream << item.title << " - " << item.format << " (Loaned to " << item.name << ") on " << item.date;
          return stream;
    }

我的程序无法编译并给出以下错误：

error C2248: 'std::basic_ios<_Elem,_Traits>::basic_ios' : cannot access private member declared in class '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 8\vc\include\ios(151) : see declaration of 'std::basic_ios<_Elem,_Traits>::basic_ios'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        This diagnostic occurred in the compiler generated function 'std::basic_ostream<_Elem,_Traits>::basic_ostream(const std::basic_ostream<_Elem,_Traits> &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]

发生了什么事？为什么说私人会员无法访问，我很困惑。

Answer 1

您必须返回参考。

friend ostream& operator<< (ostream &stream, const Item &item)
//            ^

stream的copy-constructor被删除/私有。

Answer 2

问题在于：

friend ostream operator<< (ostream &stream, const Item &item){

您需要通过引用返回流，ostream对象不可复制。