Question

I have a header file that looks like this:

#pragma once

//C++ Output Streams
#include <iostream>

namespace microtask
{
    namespace log
    {
        /**
         * Severity level.
         */
        enum severity
        {
            debug,
            info,
            warning,
            error,
            critical
        };

        /**
         * Output the severity level.
         */
        std::ostream& operator<<(std::ostream& out, const severity& level);
    }
}

and a source file that looks like this:

//Definitions
#include "severity.hpp"

//Namespaces
using namespace std;
using namespace microtask::log;

std::ostream& operator<<(std::ostream& out, const severity& level)
{
    switch(level)
    {
    case debug:
        out << "debug";
        break;
    case info:
        out << "info";
        break;
    case warning:
        out << "warning";
        break;
    case error:
        out << "error";
        break;
    case critical:
        out << "critical";
        break;
    default:
        out << "unknown";
        break;
    }

    return out;
}

that I am trying to compile into a dynamic library. Unfortunately, linking fails with this error message:

undefined reference to `microtask::log::operator<<(std::basic_ostream<char, std::char_traits<char> >&, microtask::log::severity const&)'

What am I doing wrong? I’ve checked other stackoverflow.com questions that seemed similar, but as far as I can tell, I have the format for overloading the operator correct.

Answer 1

In your .cpp file, don’t say using, but instead declare the proper namespace:

namespace microtask
{
    namespace log
    {
        ::std::ostream & operator<<(::std::ostream& out, const severity& level)
        {
            // ...
        }
    }
}

In fact, don’t say using casually at all if you can help it. In my opinion it should be reserved for explicit base member unhiding and ADL requests.