Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of Boost. Click here to view this page for the latest version.


// logger_service.hpp
// ~~~~~~~~~~~~~~~~~~
// Copyright (c) 2003-2023 Christopher M. Kohlhoff (chris at kohlhoff dot com)
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at


#include <boost/asio.hpp>
#include <functional>
#include <fstream>
#include <sstream>
#include <string>
#include <thread>

namespace services {

/// Service implementation for the logger.
class logger_service
  : public boost::asio::execution_context::service
  /// The type used to identify this service in the execution context.
  typedef logger_service key_type;

  /// The backend implementation of a logger.
  struct logger_impl
    explicit logger_impl(const std::string& ident) : identifier(ident) {}
    std::string identifier;

  /// The type for an implementation of the logger.
  typedef logger_impl* impl_type;

  /// Constructor creates a thread to run a private io_context.
  logger_service(boost::asio::execution_context& context)
    : boost::asio::execution_context::service(context),
      work_thread_([this]{; })

  logger_service(const logger_service&) = delete;
  logger_service& operator=(const logger_service&) = delete;;

  /// Destructor shuts down the private io_context.
    /// Indicate that we have finished with the private io_context. Its
    /// io_context::run() function will exit once all other work has completed.
    if (work_thread_.joinable())

  /// Destroy all user-defined handler objects owned by the service.
  void shutdown()

  /// Return a null logger implementation.
  impl_type null() const
    return 0;

  /// Create a new logger implementation.
  void create(impl_type& impl, const std::string& identifier)
    impl = new logger_impl(identifier);

  /// Destroy a logger implementation.
  void destroy(impl_type& impl)
    delete impl;
    impl = null();

  /// Set the output file for the logger. The current implementation sets the
  /// output file for all logger instances, and so the impl parameter is not
  /// actually needed. It is retained here to illustrate how service functions
  /// are typically defined.
  void use_file(impl_type& /*impl*/, const std::string& file)
    // Pass the work of opening the file to the background thread.
        std::bind(&logger_service::use_file_impl, this, file));

  /// Log a message.
  void log(impl_type& impl, const std::string& message)
    // Format the text to be logged.
    std::ostringstream os;
    os << impl->identifier << ": " << message;

    // Pass the work of writing to the file to the background thread.
        std::bind(&logger_service::log_impl, this, os.str()));

  /// Helper function used to open the output file from within the private
  /// io_context's thread.
  void use_file_impl(const std::string& file)

  /// Helper function used to log a message from within the private io_context's
  /// thread.
  void log_impl(const std::string& text)
    ofstream_ << text << std::endl;

  /// Private io_context used for performing logging operations.
  boost::asio::io_context work_io_context_;

  /// Work for the private io_context to perform. If we do not give the
  /// io_context some work to do then the io_context::run() function will exit
  /// immediately.
      boost::asio::io_context::executor_type> work_;

  /// Thread used for running the work io_context's run loop.
  std::thread work_thread_;

  /// The file to which log messages will be written.
  std::ofstream ofstream_;

} // namespace services