text: //
  // detail/impl/strand_service.hpp
  // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  //
  // Copyright (c) 2003-2018 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 ASIO_DETAIL_IMPL_STRAND_SERVICE_HPP
  #define ASIO_DETAIL_IMPL_STRAND_SERVICE_HPP
  
  #if defined(_MSC_VER) && (_MSC_VER >= 1200)
  # pragma once
  #endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
  
  #include "asio/detail/call_stack.hpp"
  #include "asio/detail/completion_handler.hpp"
  #include "asio/detail/fenced_block.hpp"
  #include "asio/detail/handler_alloc_helpers.hpp"
  #include "asio/detail/handler_invoke_helpers.hpp"
  #include "asio/detail/memory.hpp"
  
  #include "asio/detail/push_options.hpp"
  
  namespace asio {
  namespace detail {
  
  inline strand_service::strand_impl::strand_impl()
    : operation(&strand_service::do_complete),
      locked_(false)
  {
  }
  
  struct strand_service::on_dispatch_exit
  {
    io_context_impl* io_context_;
    strand_impl* impl_;
  
    ~on_dispatch_exit()
    {
      impl_->mutex_.lock();
      impl_->ready_queue_.push(impl_->waiting_queue_);
      bool more_handlers = impl_->locked_ = !impl_->ready_queue_.empty();
      impl_->mutex_.unlock();
  
      if (more_handlers)
        io_context_->post_immediate_completion(impl_, false);
    }
  };
  
  template <typename Handler>
  void strand_service::dispatch(strand_service::implementation_type& impl,
      Handler& handler)
  {
    // If we are already in the strand then the handler can run immediately.
    if (call_stack<strand_impl>::contains(impl))
    {
      fenced_block b(fenced_block::full);
      asio_handler_invoke_helpers::invoke(handler, handler);
      return;
    }
  
    // Allocate and construct an operation to wrap the handler.
    typedef completion_handler<Handler> op;
    typename op::ptr p = { asio::detail::addressof(handler),
      op::ptr::allocate(handler), 0 };
    p.p = new (p.v) op(handler);
  
    ASIO_HANDLER_CREATION((this->context(),
          *p.p, "strand", impl, 0, "dispatch"));
  
    bool dispatch_immediately = do_dispatch(impl, p.p);
    operation* o = p.p;
    p.v = p.p = 0;
  
    if (dispatch_immediately)
    {
      // Indicate that this strand is executing on the current thread.
      call_stack<strand_impl>::context ctx(impl);
  
      // Ensure the next handler, if any, is scheduled on block exit.
      on_dispatch_exit on_exit = { &io_context_, impl };
      (void)on_exit;
  
      completion_handler<Handler>::do_complete(
          &io_context_, o, asio::error_code(), 0);
    }
  }
  
  // Request the io_context to invoke the given handler and return immediately.
  template <typename Handler>
  void strand_service::post(strand_service::implementation_type& impl,
      Handler& handler)
  {
    bool is_continuation =
      asio_handler_cont_helpers::is_continuation(handler);
  
    // Allocate and construct an operation to wrap the handler.
    typedef completion_handler<Handler> op;
    typename op::ptr p = { asio::detail::addressof(handler),
      op::ptr::allocate(handler), 0 };
    p.p = new (p.v) op(handler);
  
    ASIO_HANDLER_CREATION((this->context(),
          *p.p, "strand", impl, 0, "post"));
  
    do_post(impl, p.p, is_continuation);
    p.v = p.p = 0;
  }
  
  } // namespace detail
  } // namespace asio
  
  #include "asio/detail/pop_options.hpp"
  
  #endif // ASIO_DETAIL_IMPL_STRAND_SERVICE_HPP
  