g++ issues incomprehensible warning

Question

system.h:

#include <iostream>

namespace ss
{
  class system
  {
  private:
    // ...
  public:
    // ...
    friend std::ostream& operator<< (std::ostream& out, const system& sys);

   };
}

system.cpp:

#include "system.h"

std::ostream& ss::operator<< (std::ostream& out, const ss::system& sys)
  {
    // print a representation of the ss::system
    // ...
    return out;
  }

Compiling the above code with g++ 8.30 yields the following output:

[db@dbPC test]$ LANG=en g++ -Wall -Wextra system.cpp
system.cpp:2:15: warning: 'std::ostream& ss::operator<<(std::ostream&, const ss::system&)' has not been declared within 'ss'
 std::ostream& ss::operator<< (std::ostream& out, const ss::system& sys)
               ^~
In file included from system.cpp:1:
system.h:11:26: note: only here as a 'friend'
     friend std::ostream& operator<< (std::ostream& out, const system& sys);
                          ^~~~~~~~
system.cpp: In function 'std::ostream& ss::operator<<(std::ostream&, const ss::system&)':
system.cpp:2:68: warning: unused parameter 'sys' [-Wunused-parameter]
 std::ostream& ss::operator<< (std::ostream& out, const ss::system& sys)
                                                  ~~~~~~~~~~~~~~~~~~^~~

The compiler tells me, that the operator<< function was not declared within the namespace ss. However it is declared within that namespace.

I also tried to compile this with clang++. clang complains only about the unused parameter, but not about the 'not within namespace' issue I do not understand.

What is the cause of the g++ warning? Is this a false warning?

Versions:

g++ (GCC) 8.3.0
clang version: 8.00 (tags/RELEASE_800/final)

Answer 1

You simply missed declaring the operator << in namespace.

Try the following:

namespace ss
{
  std::ostream& operator << (std::ostream& out, const system& sys);
  class system
  {
  private:
    // ...
  public:
    // ...
    friend std::ostream& operator<< (std::ostream& out, const system& sys);

   };
}

// in cpp

namespace ss
{
  std::ostream& operator << (std::ostream& out, const system& sys)
  {
    // the body
  }
}