Overloading operator<<: cannot bind lvalue to ‘std::basic_ostream<char>&&’

Question

I have a class that uses a nested class, and want to use the nested class operator<< to define operator<< in the upper class. Here is how my code looks like:

#include <memory> #include <iostream>  template<typename T> struct classA {   struct classB   {     template<typename U>     friend inline std::ostream& operator<< (std::ostream &out,                                             const typename classA<U>::classB &b);   };    classB root;    template<typename U>   friend std::ostream& operator<< (std::ostream &out,                                    const classA<U> &tree); };  template<typename T> inline std::ostream& operator<< (std::ostream &out,                                  const classA<T> &tree) {   out << tree.root;   return out; }  template<typename T> inline std::ostream& operator<< (std::ostream &out,                                  const typename classA<T>::classB &b) {   return out; }  int main() {   classA<int> a;   std::cout << a; } 

• When compiling without support for C++11, the definition of operator<< for the inner class seems not to be found by the compiler:

  so.hpp:24:7: error: no match for ‘operator<<’ in ‘out << tree.classA<int>::root’ so.hpp:24:7: note: candidates are: ... 

• With GCC 4.6 and 4.7 when compiling with std=c++0x:

  so.hpp:21:3: error: cannot bind ‘std::ostream {aka std::basic_ostream<char>}’ lvalue to ‘std::basic_ostream<char>&&’ In file included from /usr/include/c++/4.7/iostream:40:0,                  from so.hpp:2: /usr/include/c++/4.7/ostream:600:5: error:   initializing argument 1 of ‘std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = classA<int>::classB]’ 

Can someone tell me why this code is not legal, and what's the best way to do what I want?

Answer 1

You have a problem with a "non-deducible context" in this operator

template<typename T> inline std::ostream& operator<< (std::ostream &out,                                  const typename classA<T>::classB &b) {   return out; } 

The compiler cannot figure out what values of T will result in a classB that matches the parameter you want to pass. So this template is not considered!

In C++11 mode, the compiler then goes on to find a close match from the standard library

operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) 

where it can match _Tp to just about any type, including classA<T>::classB, but notes that the first parameter doesn't match.