c++ - 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?

question from:https://stackoverflow.com/questions/10651161/overloading-operator-cannot-bind-lvalue-to-stdbasic-ostreamchar

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.