Define operator<< in the proper class namespaces to fix ADL.

[pjungkamp]

Description

The operator<< implementations have been defined in the global namespace instead of the class namespace.

See for example:

libdatachannel/include/rtc/peerconnection.hpp

Lines 123 to 130 in eff6413

	} // namespace rtc
	RTC_CPP_EXPORT std::ostream &operator<<(std::ostream &out, rtc::PeerConnection::State state);
	RTC_CPP_EXPORT std::ostream &operator<<(std::ostream &out, rtc::PeerConnection::IceState state);
	RTC_CPP_EXPORT std::ostream &operator<<(std::ostream &out,
	rtc::PeerConnection::GatheringState state);
	RTC_CPP_EXPORT std::ostream &operator<<(std::ostream &out,
	rtc::PeerConnection::SignalingState state);

This breaks ADL based discovery of operators. I run into this issue when trying to format libdatachannel types using fmt 10.0.0.

An example of broken ADL operator discovery

Here is a very basic reproducer to demonstrate broken ADL based discovery: https://godbolt.org/z/Gx8sb3eb8

#include <fmt/ostream.h>
#include <fmt/format.h>
#include <iostream>

// create a class in a namespace

namespace test {
class Test {};
}

// define the ostream operator outside the namespace

std::ostream &operator<<(std::ostream &s, test::Test) {
    return s << "test";
}

// the fmt::ostream_formatter can't find the operator<< using ADL

template<> class fmt::formatter<test::Test> : public fmt::ostream_formatter {};

// A simple main function to trigger the compilation error

int main() {
  test::Test t;
  std::cout << fmt::format("{}", t);
  return 0;
}

---

This is the crude fix I'm using to enable printing on my side: https://godbolt.org/z/TK3bf786E

#include <fmt/ostream.h>
#include <fmt/format.h>
#include <iostream>

// create a class in a namespace

namespace test {
class Test {};
}

// define the ostream operator outside the namespace

std::ostream &operator<<(std::ostream &s, test::Test) {
    return s << "test";
}

// pull all global defined operator<< definitions into the test namespace

namespace test {
    using ::operator<<;
}

// the fmt::ostream_formatter can now find ::operator<< using ADL

template<> class fmt::formatter<test::Test> : public fmt::ostream_formatter {};

// A simple main function

int main() {
  test::Test t;
  std::cout << fmt::format("{}", t);
  return 0;
}

---

Here is a proper fix for this example: https://godbolt.org/z/bPvj6vqPG

#include <fmt/ostream.h>
#include <fmt/format.h>
#include <iostream>

namespace test {

// create a class in a namespace

class Test {};

// define the ostream operator inside the namespace

std::ostream &operator<<(std::ostream &s, test::Test) {
    return s << "test";
}

} // namespace test

// the fmt::ostream_formatter can now find test::operator<< using ADL

template<> class fmt::formatter<test::Test> : public fmt::ostream_formatter {};

// A simple main function

int main() {
  test::Test t;
  std::cout << fmt::format("{}", t);
  return 0;
}

Expected

This is the expected code to make libdatachannel types formattable using fmt.

// declare a formatter based on the ostream operator provided by the library
template<> fmt::formatter<rtc::PeerConnection> : public fmt::ostream_formatter {}

Workaround

This is actually needed to make the libdatachannel type formattable by fmt.

// pull the operator<< definition into the rtc namespace
namespace rtc { using ::operator<<; }
// declare a formatter based on the ostream operator provided by the library
template<> fmt::formatter<rtc::PeerConnection> : public fmt::ostream_formatter {}

Suggested Fix

Please move all the std::ostream &operator<<(std::ostream &os, T t) definitions into the namespace where the corresponding type is defined.

[melpon]

I encountered the same issue.

void foo() {
  rtc::PeerConnection::GatheringState state =
      rtc::PeerConnection::GatheringState::Complete;
  // error: no match for ‘operator<<’
  PLOG_DEBUG << "onGatheringStateChange: " << state;
}

Thanks to the workaround namespace rtc { using ::operator<<; }, it's now working. Thank you.

[paullouisageneau]

Thank you for reporting, fixed by #1096