Controlling BOOST_ASIO_ENABLE_HANDLER_TRACKING Output at Runtime

I'm using the BOOST_ASIO_ENABLE_HANDLER_TRACKING flag in Boost.Asio to enable handler tracking logs, which provides useful debugging output to the standard error stream.

I have a couple of questions regarding the flexibility of this debugging feature:

1. Is it possible to enable or disable the handler tracking output at runtime, without rebuilding the application?

2. Is there a way to redirect the output to a different destination (e.g., a log file or custom stream) instead of standard error?

I’ve gone through the documentation but couldn't find any support for these capabilities.

It is not directly possible to toggle handler tracking at runtime because BOOST_ASIO_ENABLE_HANDLER_TRACKING is a preprocessor macro used to compile different code paths depending on its value. The desired behavior can be achieved by implementing custom runtime dispatch. Injection can be done via the Custom Tracking mechanism. The same mechanism can also be used to redirect output to a different destination, as well as to perform any other customization supported by the provided interface.

Assuming existing project with Boost::Asio installed, here is minimal example demonstrating injection of custom logging function via Custom Tracking mechanism:

"main.cpp":

#define BOOST_ASIO_CUSTOM_HANDLER_TRACKING "custom_handler_tracking_example.hpp"

#include <boost/asio.hpp>
#include <iostream>
#include <chrono>

int main() {
    boost::asio::io_context   io;
    boost::asio::steady_timer timer{io, std::chrono::seconds{1}};
    timer.async_wait([](const boost::system::error_code& ec) {
        if (!ec) { std::cout << "Done!\n"; }
    });
    io.run();
}

"custom_handler_tracking_example.hpp":

#ifndef CUSTOM_HANDLER_TRACKING_EXAMPLE_HPP
#define CUSTOM_HANDLER_TRACKING_EXAMPLE_HPP

#include <fstream>
#include <string_view>

inline void log(const std::string_view& prefix, const std::string_view& message) {
    static std::ofstream logFile{"custom_handler_tracking.log"};
    if (logFile.is_open()) { logFile << prefix << ": " << message << "\n"; }
}

#define BOOST_ASIO_INHERIT_TRACKED_HANDLER
#define BOOST_ASIO_ALSO_INHERIT_TRACKED_HANDLER

#define BOOST_ASIO_HANDLER_TRACKING_INIT  ((void)0)
#define BOOST_ASIO_HANDLER_LOCATION(args) static int dummyLocation{0};

#define BOOST_ASIO_HANDLER_CREATION(args)         log("CREATE", __func__)
#define BOOST_ASIO_HANDLER_COMPLETION(args)       log("COMPLETE", __func__)
#define BOOST_ASIO_HANDLER_INVOCATION_BEGIN(args) log("BEGIN", __func__)
#define BOOST_ASIO_HANDLER_INVOCATION_END         log("END", __func__)

#define BOOST_ASIO_HANDLER_OPERATION(args)              ((void)0)
#define BOOST_ASIO_HANDLER_REACTOR_REGISTRATION(args)   ((void)0)
#define BOOST_ASIO_HANDLER_REACTOR_DEREGISTRATION(args) ((void)0)

#define BOOST_ASIO_HANDLER_REACTOR_READ_EVENT      0x01
#define BOOST_ASIO_HANDLER_REACTOR_WRITE_EVENT     0x02
#define BOOST_ASIO_HANDLER_REACTOR_ERROR_EVENT     0x04
#define BOOST_ASIO_HANDLER_REACTOR_EVENTS(args)    ((void)0)
#define BOOST_ASIO_HANDLER_REACTOR_OPERATION(args) ((void)0)

#endif  // CUSTOM_HANDLER_TRACKING_EXAMPLE_HPP

Example output:

CREATE: async_wait
COMPLETE: do_complete
BEGIN: do_complete
END: do_complete

Note that this code is for demonstration purposes only and ignores good coding practices in favor of brevity.