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doc/html/boost_asio/example/cpp03/services/logger_service.hpp

//
// logger_service.hpp
// ~~~~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2021 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 http://www.boost.org/LICENSE_1_0.txt)
//

#ifndef SERVICES_LOGGER_SERVICE_HPP
#define SERVICES_LOGGER_SERVICE_HPP

#include <boost/asio.hpp>
#include <boost/thread/thread.hpp>
#include <boost/bind/bind.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/noncopyable.hpp>
#include <boost/scoped_ptr.hpp>
#include <fstream>
#include <sstream>
#include <string>

namespace services {

/// Service implementation for the logger.
class logger_service
  : public boost::asio::execution_context::service
{
public:
  /// 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_io_context_(),
      work_(boost::asio::require(work_io_context_.get_executor(),
            boost::asio::execution::outstanding_work.tracked)),
      work_thread_(new boost::thread(
            boost::bind(&boost::asio::io_context::run, &work_io_context_)))
  {
  }

  /// Destructor shuts down the private io_context.
  ~logger_service()
  {
    /// Indicate that we have finished with the private io_context. Its
    /// io_context::run() function will exit once all other work has completed.
    work_ = boost::asio::any_io_executor();
    if (work_thread_)
      work_thread_->join();
  }

  /// 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.
    boost::asio::post(work_io_context_, boost::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.
    boost::asio::post(work_io_context_, boost::bind(
          &logger_service::log_impl, this, os.str()));
  }

private:
  /// Helper function used to open the output file from within the private
  /// io_context's thread.
  void use_file_impl(const std::string& file)
  {
    ofstream_.close();
    ofstream_.clear();
    ofstream_.open(file.c_str());
  }

  /// 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_;

  /// A work-tracking executor giving 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::any_io_executor work_;

  /// Thread used for running the work io_context's run loop.
  boost::scoped_ptr<boost::thread> work_thread_;

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

} // namespace services

#endif // SERVICES_LOGGER_SERVICE_HPP