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<title>boost/asio/strand.hpp - 1.67.0</title> <link rel="icon" href="/favicon.ico" type="image/ico" />

<title>boost/asio/io_context_strand.hpp - 1.67.0</title> <link rel="icon" href="/favicon.ico" type="image/ico" />

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<h3>boost/asio/strand.hpp</h3>

<h3>boost/asio/io_context_strand.hpp</h3>

<pre>

// strand.hpp

// io_context_strand.hpp

// ~~~~~~~~~~

// ~~~~~~~~~~~~~~~~~~~~~

// Distributed under the Boost Software License, Version 1.0. (See accompanying

// file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)

#ifndef BOOST_ASIO_STRAND_HPP

#ifndef BOOST_ASIO_IO_CONTEXT_STRAND_HPP

#define BOOST_ASIO_STRAND_HPP

#define BOOST_ASIO_IO_CONTEXT_STRAND_HPP

#if defined(_MSC_VER) && (_MSC_VER >= 1200)

# pragma once

#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)

#include <<a href="../../boost/asio/detail/config.hpp">boost/asio/detail/config.hpp</a>>

#include <<a href="../../boost/asio/detail/strand_executor_service.hpp">boost/asio/detail/strand_executor_service.hpp</a>>

#include <<a href="../../boost/asio/detail/type_traits.hpp">boost/asio/detail/type_traits.hpp</a>>

#if !defined(BOOST_ASIO_NO_EXTENSIONS)

#include <<a href="../../boost/asio/async_result.hpp">boost/asio/async_result.hpp</a>>

#include <<a href="../../boost/asio/detail/handler_type_requirements.hpp">boost/asio/detail/handler_type_requirements.hpp</a>>

#include <<a href="../../boost/asio/detail/strand_service.hpp">boost/asio/detail/strand_service.hpp</a>>

#include <<a href="../../boost/asio/detail/wrapped_handler.hpp">boost/asio/detail/wrapped_handler.hpp</a>>

#include <<a href="../../boost/asio/io_context.hpp">boost/asio/io_context.hpp</a>>

#include <<a href="../../boost/asio/detail/push_options.hpp">boost/asio/detail/push_options.hpp</a>>

namespace boost {

namespace asio {

/// Provides serialised function invocation for any executor type.

/// Provides serialised handler execution.

template <typename Executor>

/**

class strand

* The io_context::strand class provides the ability to post and dispatch

* handlers with the guarantee that none of those handlers will execute

* concurrently.

* @par Order of handler invocation

* Given:

* @li a strand object @c s

* @li an object @c a meeting completion handler requirements

* @li an object @c a1 which is an arbitrary copy of @c a made by the

* implementation

* @li an object @c b meeting completion handler requirements

* @li an object @c b1 which is an arbitrary copy of @c b made by the

* implementation

* if any of the following conditions are true:

* @li @c s.post(a) happens-before @c s.post(b)

* @li @c s.post(a) happens-before @c s.dispatch(b), where the latter is

* performed outside the strand

* @li @c s.dispatch(a) happens-before @c s.post(b), where the former is

* performed outside the strand

* @li @c s.dispatch(a) happens-before @c s.dispatch(b), where both are

* performed outside the strand

* then @c asio_handler_invoke(a1, &a1) happens-before

* @c asio_handler_invoke(b1, &b1).

* Note that in the following case:

* @code async_op_1(..., s.wrap(a));

* async_op_2(..., s.wrap(b)); @endcode

* the completion of the first async operation will perform @c s.dispatch(a),

* and the second will perform @c s.dispatch(b), but the order in which those

* are performed is unspecified. That is, you cannot state whether one

* happens-before the other. Therefore none of the above conditions are met and

* no ordering guarantee is made.

* @note The implementation makes no guarantee that handlers posted or

* dispatched through different @c strand objects will be invoked concurrently.

* @par Thread Safety

* @e Distinct @e objects: Safe.@n

* @e Shared @e objects: Safe.

* @par Concepts:

* Dispatcher.

class io_context::strand

{

public:

/// The type of the underlying executor.

/// Constructor.

typedef Executor inner_executor_type;

/// Default constructor.

/**

* This constructor is only valid if the underlying executor type is default

* constructible.

strand()

: executor_(),

impl_(use_service<detail::strand_executor_service>(

executor_.context()).create_implementation())

{

}

/// Construct a strand for the specified executor.

explicit strand(const Executor& e)

: executor_(e),

impl_(use_service<detail::strand_executor_service>(

executor_.context()).create_implementation())

{

}

/// Copy constructor.

strand(const strand& other) BOOST_ASIO_NOEXCEPT

: executor_(other.executor_),

impl_(other.impl_)

{

}

/// Converting constructor.

/**

* This constructor is only valid if the @c OtherExecutor type is convertible

* Constructs the strand.

* to @c Executor.

* @param io_context The io_context object that the strand will use to

* dispatch handlers that are ready to be run.

template <class OtherExecutor>

explicit strand(boost::asio::io_context& io_context)

strand(

: service_(boost::asio::use_service<

const strand<OtherExecutor>& other) BOOST_ASIO_NOEXCEPT

boost::asio::detail::strand_service>(io_context))

: executor_(other.executor_),

impl_(other.impl_)

{

}

/// Assignment operator.

strand& operator=(const strand& other) BOOST_ASIO_NOEXCEPT

{

executor_ = other.executor_;

service_.construct(impl_);

impl_ = other.impl_;

return *this;

}

/// Converting assignment operator.

/// Destructor.

/**

* This assignment operator is only valid if the @c OtherExecutor type is

* Destroys a strand.

* convertible to @c Executor.

* Handlers posted through the strand that have not yet been invoked will

* still be dispatched in a way that meets the guarantee of non-concurrency.

template <class OtherExecutor>

~strand()

strand& operator=(

const strand<OtherExecutor>& other) BOOST_ASIO_NOEXCEPT

{

executor_ = other.executor_;

impl_ = other.impl_;

return *this;

}

#if defined(BOOST_ASIO_HAS_MOVE) || defined(GENERATING_DOCUMENTATION)

/// Move constructor.

strand(strand&& other) BOOST_ASIO_NOEXCEPT

: executor_(BOOST_ASIO_MOVE_CAST(Executor)(other.executor_)),

impl_(BOOST_ASIO_MOVE_CAST(implementation_type)(other.impl_))

{

}

/// Converting move constructor.

#if !defined(BOOST_ASIO_NO_DEPRECATED)

/// (Deprecated: Use context().) Get the io_context associated with the

/// strand.

/**

* This constructor is only valid if the @c OtherExecutor type is convertible

* This function may be used to obtain the io_context object that the strand

* to @c Executor.

* uses to dispatch handlers for asynchronous operations.

* @return A reference to the io_context object that the strand will use to

* dispatch handlers. Ownership is not transferred to the caller.

template <class OtherExecutor>

boost::asio::io_context& get_io_context()

strand(strand<OtherExecutor>&& other) BOOST_ASIO_NOEXCEPT

: executor_(BOOST_ASIO_MOVE_CAST(OtherExecutor)(other)),

impl_(BOOST_ASIO_MOVE_CAST(implementation_type)(other.impl_))

{

}

/// Move assignment operator.

strand& operator=(strand&& other) BOOST_ASIO_NOEXCEPT

{

executor_ = BOOST_ASIO_MOVE_CAST(Executor)(other);

return service_.get_io_context();

impl_ = BOOST_ASIO_MOVE_CAST(implementation_type)(other.impl_);

return *this;

}

/// Converting move assignment operator.

/// (Deprecated: Use context().) Get the io_context associated with the

/// strand.

/**

* This assignment operator is only valid if the @c OtherExecutor type is

* This function may be used to obtain the io_context object that the strand

* convertible to @c Executor.

* uses to dispatch handlers for asynchronous operations.

* @return A reference to the io_context object that the strand will use to

* dispatch handlers. Ownership is not transferred to the caller.

template <class OtherExecutor>

boost::asio::io_context& get_io_service()

strand& operator=(

const strand<OtherExecutor>&& other) BOOST_ASIO_NOEXCEPT

{

executor_ = BOOST_ASIO_MOVE_CAST(OtherExecutor)(other);

impl_ = BOOST_ASIO_MOVE_CAST(implementation_type)(other.impl_);

return *this;

}

#endif // defined(BOOST_ASIO_HAS_MOVE) || defined(GENERATING_DOCUMENTATION)

/// Destructor.

~strand()

{

}

/// Obtain the underlying executor.

inner_executor_type get_inner_executor() const BOOST_ASIO_NOEXCEPT

{

return executor_;

return service_.get_io_context();

}

#endif // !defined(BOOST_ASIO_NO_DEPRECATED)

/// Obtain the underlying execution context.

execution_context& context() const BOOST_ASIO_NOEXCEPT

boost::asio::io_context& context() const BOOST_ASIO_NOEXCEPT

{

return executor_.context();

return service_.get_io_context();

}

/// Inform the strand that it has some outstanding work to do.

/**

* The strand delegates this call to its underlying executor.

* The strand delegates this call to its underlying io_context.

void on_work_started() const BOOST_ASIO_NOEXCEPT

{

executor_.on_work_started();

context().get_executor().on_work_started();

}

/// Inform the strand that some work is no longer outstanding.

/**

* The strand delegates this call to its underlying executor.

* The strand delegates this call to its underlying io_context.

void on_work_finished() const BOOST_ASIO_NOEXCEPT

{

executor_.on_work_finished();

context().get_executor().on_work_finished();

}

/// Request the strand to invoke the given function object.

/**

* This function is used to ask the strand to execute the given function

* object on its underlying executor. The function object will be executed

* object on its underlying io_context. The function object will be executed

* inside this function if the strand is not otherwise busy and if the

* underlying executor's @c dispatch() function is also able to execute the

* underlying io_context's executor's @c dispatch() function is also able to

* function before returning.

* execute the function before returning.

* @param f The function object to be called. The executor will make

* a copy of the handler object as required. The function signature of the

* function object must be: @code void function(); @endcode

* @param a An allocator that may be used by the executor to allocate the

* internal storage needed for function invocation.

template <typename Function, typename Allocator>

void dispatch(BOOST_ASIO_MOVE_ARG(Function) f, const Allocator& a) const

{

detail::strand_executor_service::dispatch(impl_,

typename decay<Function>::type tmp(BOOST_ASIO_MOVE_CAST(Function)(f));

executor_, BOOST_ASIO_MOVE_CAST(Function)(f), a);

service_.dispatch(impl_, tmp);

(void)a;

}

#if !defined(BOOST_ASIO_NO_DEPRECATED)

/// (Deprecated: Use boost::asio::dispatch().) Request the strand to invoke

/// the given handler.

/**

* This function is used to ask the strand to execute the given handler.

* The strand object guarantees that handlers posted or dispatched through

* the strand will not be executed concurrently. The handler may be executed

* inside this function if the guarantee can be met. If this function is

* called from within a handler that was posted or dispatched through the same

* strand, then the new handler will be executed immediately.

* The strand's guarantee is in addition to the guarantee provided by the

* underlying io_context. The io_context guarantees that the handler will only

* be called in a thread in which the io_context's run member function is

* currently being invoked.

* @param handler The handler to be called. The strand will make a copy of the

* handler object as required. The function signature of the handler must be:

* @code void handler(); @endcode

template <typename LegacyCompletionHandler>

BOOST_ASIO_INITFN_RESULT_TYPE(LegacyCompletionHandler, void ())

dispatch(BOOST_ASIO_MOVE_ARG(LegacyCompletionHandler) handler)

{

// If you get an error on the following line it means that your handler does

// not meet the documented type requirements for a LegacyCompletionHandler.

BOOST_ASIO_LEGACY_COMPLETION_HANDLER_CHECK(

LegacyCompletionHandler, handler) type_check;

async_completion<LegacyCompletionHandler, void ()> init(handler);

service_.dispatch(impl_, init.completion_handler);

return init.result.get();

}

#endif // !defined(BOOST_ASIO_NO_DEPRECATED)

/// Request the strand to invoke the given function object.

/**

* This function is used to ask the executor to execute the given function

* object. The function object will never be executed inside this function.

* Instead, it will be scheduled by the underlying executor's defer function.

* Instead, it will be scheduled to run by the underlying io_context.

* @param f The function object to be called. The executor will make

* a copy of the handler object as required. The function signature of the

* function object must be: @code void function(); @endcode

* @param a An allocator that may be used by the executor to allocate the

* internal storage needed for function invocation.

template <typename Function, typename Allocator>

void post(BOOST_ASIO_MOVE_ARG(Function) f, const Allocator& a) const

{

detail::strand_executor_service::post(impl_,

typename decay<Function>::type tmp(BOOST_ASIO_MOVE_CAST(Function)(f));

executor_, BOOST_ASIO_MOVE_CAST(Function)(f), a);

service_.post(impl_, tmp);

(void)a;

}

#if !defined(BOOST_ASIO_NO_DEPRECATED)

/// (Deprecated: Use boost::asio::post().) Request the strand to invoke the

/// given handler and return immediately.

/**

* This function is used to ask the strand to execute the given handler, but

* without allowing the strand to call the handler from inside this function.

* The strand object guarantees that handlers posted or dispatched through

* the strand will not be executed concurrently. The strand's guarantee is in

* addition to the guarantee provided by the underlying io_context. The

* io_context guarantees that the handler will only be called in a thread in

* which the io_context's run member function is currently being invoked.

* @param handler The handler to be called. The strand will make a copy of the

* handler object as required. The function signature of the handler must be:

* @code void handler(); @endcode

template <typename LegacyCompletionHandler>

BOOST_ASIO_INITFN_RESULT_TYPE(LegacyCompletionHandler, void ())

post(BOOST_ASIO_MOVE_ARG(LegacyCompletionHandler) handler)

{

// If you get an error on the following line it means that your handler does

// not meet the documented type requirements for a LegacyCompletionHandler.

BOOST_ASIO_LEGACY_COMPLETION_HANDLER_CHECK(

LegacyCompletionHandler, handler) type_check;

async_completion<LegacyCompletionHandler, void ()> init(handler);

service_.post(impl_, init.completion_handler);

return init.result.get();

}

#endif // !defined(BOOST_ASIO_NO_DEPRECATED)

/// Request the strand to invoke the given function object.

/**

* This function is used to ask the executor to execute the given function

* object. The function object will never be executed inside this function.

* Instead, it will be scheduled by the underlying executor's defer function.

* Instead, it will be scheduled to run by the underlying io_context.

* @param f The function object to be called. The executor will make

* a copy of the handler object as required. The function signature of the

* function object must be: @code void function(); @endcode

* @param a An allocator that may be used by the executor to allocate the

* internal storage needed for function invocation.

template <typename Function, typename Allocator>

void defer(BOOST_ASIO_MOVE_ARG(Function) f, const Allocator& a) const

{

detail::strand_executor_service::defer(impl_,

typename decay<Function>::type tmp(BOOST_ASIO_MOVE_CAST(Function)(f));

executor_, BOOST_ASIO_MOVE_CAST(Function)(f), a);

service_.post(impl_, tmp);

(void)a;

}

#if !defined(BOOST_ASIO_NO_DEPRECATED)

/// (Deprecated: Use boost::asio::bind_executor().) Create a new handler that

/// automatically dispatches the wrapped handler on the strand.

/**

* This function is used to create a new handler function object that, when

* invoked, will automatically pass the wrapped handler to the strand's

* dispatch function.

* @param handler The handler to be wrapped. The strand will make a copy of

* the handler object as required. The function signature of the handler must

* be: @code void handler(A1 a1, ... An an); @endcode

* @return A function object that, when invoked, passes the wrapped handler to

* the strand's dispatch function. Given a function object with the signature:

* @code R f(A1 a1, ... An an); @endcode

* If this function object is passed to the wrap function like so:

* @code strand.wrap(f); @endcode

* then the return value is a function object with the signature

* @code void g(A1 a1, ... An an); @endcode

* that, when invoked, executes code equivalent to:

* @code strand.dispatch(boost::bind(f, a1, ... an)); @endcode

template <typename Handler>

#if defined(GENERATING_DOCUMENTATION)

unspecified

#else

detail::wrapped_handler<strand, Handler, detail::is_continuation_if_running>

#endif

wrap(Handler handler)

{

return detail::wrapped_handler<io_context::strand, Handler,

detail::is_continuation_if_running>(*this, handler);

}

#endif // !defined(BOOST_ASIO_NO_DEPRECATED)

/// Determine whether the strand is running in the current thread.

/**

* @return @c true if the current thread is executing a function that was

* @return @c true if the current thread is executing a handler that was

* submitted to the strand using post(), dispatch() or defer(). Otherwise

* submitted to the strand using post(), dispatch() or wrap(). Otherwise

* returns @c false.

bool running_in_this_thread() const BOOST_ASIO_NOEXCEPT

{

return detail::strand_executor_service::running_in_this_thread(impl_);

return service_.running_in_this_thread(impl_);

}

/// Compare two strands for equality.

/**

* Two strands are equal if they refer to the same ordered, non-concurrent

* state.

friend bool operator==(const strand& a, const strand& b) BOOST_ASIO_NOEXCEPT

{

return a.impl_ == b.impl_;

}

/// Compare two strands for inequality.

/**

* Two strands are equal if they refer to the same ordered, non-concurrent

* state.

friend bool operator!=(const strand& a, const strand& b) BOOST_ASIO_NOEXCEPT

{

return a.impl_ != b.impl_;

}

private:

Executor executor_;

boost::asio::detail::strand_service& service_;

typedef detail::strand_executor_service::implementation_type

mutable boost::asio::detail::strand_service::implementation_type impl_;

implementation_type;

implementation_type impl_;

};

} // namespace asio

} // namespace boost

#include <<a href="../../boost/asio/detail/pop_options.hpp">boost/asio/detail/pop_options.hpp</a>>

// If both io_context.hpp and strand.hpp have been included, automatically

// include the header file needed for the io_context::strand class.

#if !defined(BOOST_ASIO_NO_EXTENSIONS)

# if defined(BOOST_ASIO_IO_CONTEXT_HPP)

# include <<a href="../../boost/asio/io_context_strand.hpp">boost/asio/io_context_strand.hpp</a>>

# endif // defined(BOOST_ASIO_IO_CONTEXT_HPP)

#endif // !defined(BOOST_ASIO_NO_EXTENSIONS)

#endif // BOOST_ASIO_STRAND_HPP

#endif // BOOST_ASIO_IO_CONTEXT_STRAND_HPP

</pre>

</div>

</div>

<p>Revised $Date$</p>

</div>

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              <h3>boost/asio/strand.hpp</h3>
<pre>
//
// strand.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 <a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)
//

#ifndef BOOST_ASIO_STRAND_HPP
#define BOOST_ASIO_STRAND_HPP

#if defined(_MSC_VER) &amp;&amp; (_MSC_VER &gt;= 1200)
# pragma once
#endif // defined(_MSC_VER) &amp;&amp; (_MSC_VER &gt;= 1200)

#include &lt;<a href="../../boost/asio/detail/config.hpp">boost/asio/detail/config.hpp</a>&gt;
#include &lt;<a href="../../boost/asio/detail/strand_executor_service.hpp">boost/asio/detail/strand_executor_service.hpp</a>&gt;
#include &lt;<a href="../../boost/asio/detail/type_traits.hpp">boost/asio/detail/type_traits.hpp</a>&gt;

#include &lt;<a href="../../boost/asio/detail/push_options.hpp">boost/asio/detail/push_options.hpp</a>&gt;

namespace boost {
namespace asio {

/// Provides serialised function invocation for any executor type.
template &lt;typename Executor&gt;
class strand
{
public:
  /// The type of the underlying executor.
  typedef Executor inner_executor_type;

/// Default constructor.
  /**
   * This constructor is only valid if the underlying executor type is default
   * constructible.
   */
  strand()
    : executor_(),
      impl_(use_service&lt;detail::strand_executor_service&gt;(
            executor_.context()).create_implementation())
  {
  }

/// Construct a strand for the specified executor.
  explicit strand(const Executor&amp; e)
    : executor_(e),
      impl_(use_service&lt;detail::strand_executor_service&gt;(
            executor_.context()).create_implementation())
  {
  }

/// Copy constructor.
  strand(const strand&amp; other) BOOST_ASIO_NOEXCEPT
    : executor_(other.executor_),
      impl_(other.impl_)
  {
  }

/// Converting constructor.
  /**
   * This constructor is only valid if the @c OtherExecutor type is convertible
   * to @c Executor.
   */
  template &lt;class OtherExecutor&gt;
  strand(
      const strand&lt;OtherExecutor&gt;&amp; other) BOOST_ASIO_NOEXCEPT
    : executor_(other.executor_),
      impl_(other.impl_)
  {
  }

/// Assignment operator.
  strand&amp; operator=(const strand&amp; other) BOOST_ASIO_NOEXCEPT
  {
    executor_ = other.executor_;
    impl_ = other.impl_;
    return *this;
  }

/// Converting assignment operator.
  /**
   * This assignment operator is only valid if the @c OtherExecutor type is
   * convertible to @c Executor.
   */
  template &lt;class OtherExecutor&gt;
  strand&amp; operator=(
      const strand&lt;OtherExecutor&gt;&amp; other) BOOST_ASIO_NOEXCEPT
  {
    executor_ = other.executor_;
    impl_ = other.impl_;
    return *this;
  }

#if defined(BOOST_ASIO_HAS_MOVE) || defined(GENERATING_DOCUMENTATION)
  /// Move constructor.
  strand(strand&amp;&amp; other) BOOST_ASIO_NOEXCEPT
    : executor_(BOOST_ASIO_MOVE_CAST(Executor)(other.executor_)),
      impl_(BOOST_ASIO_MOVE_CAST(implementation_type)(other.impl_))
  {
  }

/// Converting move constructor.
  /**
   * This constructor is only valid if the @c OtherExecutor type is convertible
   * to @c Executor.
   */
  template &lt;class OtherExecutor&gt;
  strand(strand&lt;OtherExecutor&gt;&amp;&amp; other) BOOST_ASIO_NOEXCEPT
    : executor_(BOOST_ASIO_MOVE_CAST(OtherExecutor)(other)),
      impl_(BOOST_ASIO_MOVE_CAST(implementation_type)(other.impl_))
  {
  }

/// Move assignment operator.
  strand&amp; operator=(strand&amp;&amp; other) BOOST_ASIO_NOEXCEPT
  {
    executor_ = BOOST_ASIO_MOVE_CAST(Executor)(other);
    impl_ = BOOST_ASIO_MOVE_CAST(implementation_type)(other.impl_);
    return *this;
  }

/// Converting move assignment operator.
  /**
   * This assignment operator is only valid if the @c OtherExecutor type is
   * convertible to @c Executor.
   */
  template &lt;class OtherExecutor&gt;
  strand&amp; operator=(
      const strand&lt;OtherExecutor&gt;&amp;&amp; other) BOOST_ASIO_NOEXCEPT
  {
    executor_ = BOOST_ASIO_MOVE_CAST(OtherExecutor)(other);
    impl_ = BOOST_ASIO_MOVE_CAST(implementation_type)(other.impl_);
    return *this;
  }
#endif // defined(BOOST_ASIO_HAS_MOVE) || defined(GENERATING_DOCUMENTATION)

/// Destructor.
  ~strand()
  {
  }

/// Obtain the underlying executor.
  inner_executor_type get_inner_executor() const BOOST_ASIO_NOEXCEPT
  {
    return executor_;
  }

/// Obtain the underlying execution context.
  execution_context&amp; context() const BOOST_ASIO_NOEXCEPT
  {
    return executor_.context();
  }

/// Inform the strand that it has some outstanding work to do.
  /**
   * The strand delegates this call to its underlying executor.
   */
  void on_work_started() const BOOST_ASIO_NOEXCEPT
  {
    executor_.on_work_started();
  }

/// Inform the strand that some work is no longer outstanding.
  /**
   * The strand delegates this call to its underlying executor.
   */
  void on_work_finished() const BOOST_ASIO_NOEXCEPT
  {
    executor_.on_work_finished();
  }

/// Request the strand to invoke the given function object.
  /**
   * This function is used to ask the strand to execute the given function
   * object on its underlying executor. The function object will be executed
   * inside this function if the strand is not otherwise busy and if the
   * underlying executor's @c dispatch() function is also able to execute the
   * function before returning.
   *
   * @param f The function object to be called. The executor will make
   * a copy of the handler object as required. The function signature of the
   * function object must be: @code void function(); @endcode
   *
   * @param a An allocator that may be used by the executor to allocate the
   * internal storage needed for function invocation.
   */
  template &lt;typename Function, typename Allocator&gt;
  void dispatch(BOOST_ASIO_MOVE_ARG(Function) f, const Allocator&amp; a) const
  {
    detail::strand_executor_service::dispatch(impl_,
        executor_, BOOST_ASIO_MOVE_CAST(Function)(f), a);
  }

/// Request the strand to invoke the given function object.
  /**
   * This function is used to ask the executor to execute the given function
   * object. The function object will never be executed inside this function.
   * Instead, it will be scheduled by the underlying executor's defer function.
   *
   * @param f The function object to be called. The executor will make
   * a copy of the handler object as required. The function signature of the
   * function object must be: @code void function(); @endcode
   *
   * @param a An allocator that may be used by the executor to allocate the
   * internal storage needed for function invocation.
   */
  template &lt;typename Function, typename Allocator&gt;
  void post(BOOST_ASIO_MOVE_ARG(Function) f, const Allocator&amp; a) const
  {
    detail::strand_executor_service::post(impl_,
        executor_, BOOST_ASIO_MOVE_CAST(Function)(f), a);
  }

/// Request the strand to invoke the given function object.
  /**
   * This function is used to ask the executor to execute the given function
   * object. The function object will never be executed inside this function.
   * Instead, it will be scheduled by the underlying executor's defer function.
   *
   * @param f The function object to be called. The executor will make
   * a copy of the handler object as required. The function signature of the
   * function object must be: @code void function(); @endcode
   *
   * @param a An allocator that may be used by the executor to allocate the
   * internal storage needed for function invocation.
   */
  template &lt;typename Function, typename Allocator&gt;
  void defer(BOOST_ASIO_MOVE_ARG(Function) f, const Allocator&amp; a) const
  {
    detail::strand_executor_service::defer(impl_,
        executor_, BOOST_ASIO_MOVE_CAST(Function)(f), a);
  }

/// Determine whether the strand is running in the current thread.
  /**
   * @return @c true if the current thread is executing a function that was
   * submitted to the strand using post(), dispatch() or defer(). Otherwise
   * returns @c false.
   */
  bool running_in_this_thread() const BOOST_ASIO_NOEXCEPT
  {
    return detail::strand_executor_service::running_in_this_thread(impl_);
  }

/// Compare two strands for equality.
  /**
   * Two strands are equal if they refer to the same ordered, non-concurrent
   * state.
   */
  friend bool operator==(const strand&amp; a, const strand&amp; b) BOOST_ASIO_NOEXCEPT
  {
    return a.impl_ == b.impl_;
  }

/// Compare two strands for inequality.
  /**
   * Two strands are equal if they refer to the same ordered, non-concurrent
   * state.
   */
  friend bool operator!=(const strand&amp; a, const strand&amp; b) BOOST_ASIO_NOEXCEPT
  {
    return a.impl_ != b.impl_;
  }

private:
  Executor executor_;
  typedef detail::strand_executor_service::implementation_type
    implementation_type;
  implementation_type impl_;
};

} // namespace asio
} // namespace boost

#include &lt;<a href="../../boost/asio/detail/pop_options.hpp">boost/asio/detail/pop_options.hpp</a>&gt;

// If both io_context.hpp and strand.hpp have been included, automatically
// include the header file needed for the io_context::strand class.
#if !defined(BOOST_ASIO_NO_EXTENSIONS)
# if defined(BOOST_ASIO_IO_CONTEXT_HPP)
#  include &lt;<a href="../../boost/asio/io_context_strand.hpp">boost/asio/io_context_strand.hpp</a>&gt;
# endif // defined(BOOST_ASIO_IO_CONTEXT_HPP)
#endif // !defined(BOOST_ASIO_NO_EXTENSIONS)

#endif // BOOST_ASIO_STRAND_HPP
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              <h3>boost/asio/io_context_strand.hpp</h3>
<pre>
//
// io_context_strand.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 <a href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)
//

#ifndef BOOST_ASIO_IO_CONTEXT_STRAND_HPP
#define BOOST_ASIO_IO_CONTEXT_STRAND_HPP

#if defined(_MSC_VER) &amp;&amp; (_MSC_VER &gt;= 1200)
# pragma once
#endif // defined(_MSC_VER) &amp;&amp; (_MSC_VER &gt;= 1200)

#include &lt;<a href="../../boost/asio/detail/config.hpp">boost/asio/detail/config.hpp</a>&gt;

#if !defined(BOOST_ASIO_NO_EXTENSIONS)

#include &lt;<a href="../../boost/asio/async_result.hpp">boost/asio/async_result.hpp</a>&gt;
#include &lt;<a href="../../boost/asio/detail/handler_type_requirements.hpp">boost/asio/detail/handler_type_requirements.hpp</a>&gt;
#include &lt;<a href="../../boost/asio/detail/strand_service.hpp">boost/asio/detail/strand_service.hpp</a>&gt;
#include &lt;<a href="../../boost/asio/detail/wrapped_handler.hpp">boost/asio/detail/wrapped_handler.hpp</a>&gt;
#include &lt;<a href="../../boost/asio/io_context.hpp">boost/asio/io_context.hpp</a>&gt;

#include &lt;<a href="../../boost/asio/detail/push_options.hpp">boost/asio/detail/push_options.hpp</a>&gt;

namespace boost {
namespace asio {

/// Provides serialised handler execution.
/**
 * The io_context::strand class provides the ability to post and dispatch
 * handlers with the guarantee that none of those handlers will execute
 * concurrently.
 *
 * @par Order of handler invocation
 * Given:
 *
 * @li a strand object @c s
 *
 * @li an object @c a meeting completion handler requirements
 *
 * @li an object @c a1 which is an arbitrary copy of @c a made by the
 * implementation
 *
 * @li an object @c b meeting completion handler requirements
 *
 * @li an object @c b1 which is an arbitrary copy of @c b made by the
 * implementation
 *
 * if any of the following conditions are true:
 *
 * @li @c s.post(a) happens-before @c s.post(b)
 * 
 * @li @c s.post(a) happens-before @c s.dispatch(b), where the latter is
 * performed outside the strand
 * 
 * @li @c s.dispatch(a) happens-before @c s.post(b), where the former is
 * performed outside the strand
 * 
 * @li @c s.dispatch(a) happens-before @c s.dispatch(b), where both are
 * performed outside the strand
 *   
 * then @c asio_handler_invoke(a1, &amp;a1) happens-before
 * @c asio_handler_invoke(b1, &amp;b1).
 * 
 * Note that in the following case:
 * @code async_op_1(..., s.wrap(a));
 * async_op_2(..., s.wrap(b)); @endcode
 * the completion of the first async operation will perform @c s.dispatch(a),
 * and the second will perform @c s.dispatch(b), but the order in which those
 * are performed is unspecified. That is, you cannot state whether one
 * happens-before the other. Therefore none of the above conditions are met and
 * no ordering guarantee is made.
 *
 * @note The implementation makes no guarantee that handlers posted or
 * dispatched through different @c strand objects will be invoked concurrently.
 *
 * @par Thread Safety
 * @e Distinct @e objects: Safe.@n
 * @e Shared @e objects: Safe.
 *
 * @par Concepts:
 * Dispatcher.
 */
class io_context::strand
{
public:
  /// Constructor.
  /**
   * Constructs the strand.
   *
   * @param io_context The io_context object that the strand will use to
   * dispatch handlers that are ready to be run.
   */
  explicit strand(boost::asio::io_context&amp; io_context)
    : service_(boost::asio::use_service&lt;
        boost::asio::detail::strand_service&gt;(io_context))
  {
    service_.construct(impl_);
  }

/// Destructor.
  /**
   * Destroys a strand.
   *
   * Handlers posted through the strand that have not yet been invoked will
   * still be dispatched in a way that meets the guarantee of non-concurrency.
   */
  ~strand()
  {
  }

#if !defined(BOOST_ASIO_NO_DEPRECATED)
  /// (Deprecated: Use context().) Get the io_context associated with the
  /// strand.
  /**
   * This function may be used to obtain the io_context object that the strand
   * uses to dispatch handlers for asynchronous operations.
   *
   * @return A reference to the io_context object that the strand will use to
   * dispatch handlers. Ownership is not transferred to the caller.
   */
  boost::asio::io_context&amp; get_io_context()
  {
    return service_.get_io_context();
  }

/// (Deprecated: Use context().) Get the io_context associated with the
  /// strand.
  /**
   * This function may be used to obtain the io_context object that the strand
   * uses to dispatch handlers for asynchronous operations.
   *
   * @return A reference to the io_context object that the strand will use to
   * dispatch handlers. Ownership is not transferred to the caller.
   */
  boost::asio::io_context&amp; get_io_service()
  {
    return service_.get_io_context();
  }
#endif // !defined(BOOST_ASIO_NO_DEPRECATED)

/// Obtain the underlying execution context.
  boost::asio::io_context&amp; context() const BOOST_ASIO_NOEXCEPT
  {
    return service_.get_io_context();
  }

/// Inform the strand that it has some outstanding work to do.
  /**
   * The strand delegates this call to its underlying io_context.
   */
  void on_work_started() const BOOST_ASIO_NOEXCEPT
  {
    context().get_executor().on_work_started();
  }

/// Inform the strand that some work is no longer outstanding.
  /**
   * The strand delegates this call to its underlying io_context.
   */
  void on_work_finished() const BOOST_ASIO_NOEXCEPT
  {
    context().get_executor().on_work_finished();
  }

/// Request the strand to invoke the given function object.
  /**
   * This function is used to ask the strand to execute the given function
   * object on its underlying io_context. The function object will be executed
   * inside this function if the strand is not otherwise busy and if the
   * underlying io_context's executor's @c dispatch() function is also able to
   * execute the function before returning.
   *
   * @param f The function object to be called. The executor will make
   * a copy of the handler object as required. The function signature of the
   * function object must be: @code void function(); @endcode
   *
   * @param a An allocator that may be used by the executor to allocate the
   * internal storage needed for function invocation.
   */
  template &lt;typename Function, typename Allocator&gt;
  void dispatch(BOOST_ASIO_MOVE_ARG(Function) f, const Allocator&amp; a) const
  {
    typename decay&lt;Function&gt;::type tmp(BOOST_ASIO_MOVE_CAST(Function)(f));
    service_.dispatch(impl_, tmp);
    (void)a;
  }

#if !defined(BOOST_ASIO_NO_DEPRECATED)
  /// (Deprecated: Use boost::asio::dispatch().) Request the strand to invoke
  /// the given handler.
  /**
   * This function is used to ask the strand to execute the given handler.
   *
   * The strand object guarantees that handlers posted or dispatched through
   * the strand will not be executed concurrently. The handler may be executed
   * inside this function if the guarantee can be met. If this function is
   * called from within a handler that was posted or dispatched through the same
   * strand, then the new handler will be executed immediately.
   *
   * The strand's guarantee is in addition to the guarantee provided by the
   * underlying io_context. The io_context guarantees that the handler will only
   * be called in a thread in which the io_context's run member function is
   * currently being invoked.
   *
   * @param handler The handler to be called. The strand will make a copy of the
   * handler object as required. The function signature of the handler must be:
   * @code void handler(); @endcode
   */
  template &lt;typename LegacyCompletionHandler&gt;
  BOOST_ASIO_INITFN_RESULT_TYPE(LegacyCompletionHandler, void ())
  dispatch(BOOST_ASIO_MOVE_ARG(LegacyCompletionHandler) handler)
  {
    // If you get an error on the following line it means that your handler does
    // not meet the documented type requirements for a LegacyCompletionHandler.
    BOOST_ASIO_LEGACY_COMPLETION_HANDLER_CHECK(
        LegacyCompletionHandler, handler) type_check;

async_completion&lt;LegacyCompletionHandler, void ()&gt; init(handler);

service_.dispatch(impl_, init.completion_handler);

return init.result.get();
  }
#endif // !defined(BOOST_ASIO_NO_DEPRECATED)

/// Request the strand to invoke the given function object.
  /**
   * This function is used to ask the executor to execute the given function
   * object. The function object will never be executed inside this function.
   * Instead, it will be scheduled to run by the underlying io_context.
   *
   * @param f The function object to be called. The executor will make
   * a copy of the handler object as required. The function signature of the
   * function object must be: @code void function(); @endcode
   *
   * @param a An allocator that may be used by the executor to allocate the
   * internal storage needed for function invocation.
   */
  template &lt;typename Function, typename Allocator&gt;
  void post(BOOST_ASIO_MOVE_ARG(Function) f, const Allocator&amp; a) const
  {
    typename decay&lt;Function&gt;::type tmp(BOOST_ASIO_MOVE_CAST(Function)(f));
    service_.post(impl_, tmp);
    (void)a;
  }

#if !defined(BOOST_ASIO_NO_DEPRECATED)
  /// (Deprecated: Use boost::asio::post().) Request the strand to invoke the
  /// given handler and return immediately.
  /**
   * This function is used to ask the strand to execute the given handler, but
   * without allowing the strand to call the handler from inside this function.
   *
   * The strand object guarantees that handlers posted or dispatched through
   * the strand will not be executed concurrently. The strand's guarantee is in
   * addition to the guarantee provided by the underlying io_context. The
   * io_context guarantees that the handler will only be called in a thread in
   * which the io_context's run member function is currently being invoked.
   *
   * @param handler The handler to be called. The strand will make a copy of the
   * handler object as required. The function signature of the handler must be:
   * @code void handler(); @endcode
   */
  template &lt;typename LegacyCompletionHandler&gt;
  BOOST_ASIO_INITFN_RESULT_TYPE(LegacyCompletionHandler, void ())
  post(BOOST_ASIO_MOVE_ARG(LegacyCompletionHandler) handler)
  {
    // If you get an error on the following line it means that your handler does
    // not meet the documented type requirements for a LegacyCompletionHandler.
    BOOST_ASIO_LEGACY_COMPLETION_HANDLER_CHECK(
        LegacyCompletionHandler, handler) type_check;

async_completion&lt;LegacyCompletionHandler, void ()&gt; init(handler);

service_.post(impl_, init.completion_handler);

return init.result.get();
  }
#endif // !defined(BOOST_ASIO_NO_DEPRECATED)

/// Request the strand to invoke the given function object.
  /**
   * This function is used to ask the executor to execute the given function
   * object. The function object will never be executed inside this function.
   * Instead, it will be scheduled to run by the underlying io_context.
   *
   * @param f The function object to be called. The executor will make
   * a copy of the handler object as required. The function signature of the
   * function object must be: @code void function(); @endcode
   *
   * @param a An allocator that may be used by the executor to allocate the
   * internal storage needed for function invocation.
   */
  template &lt;typename Function, typename Allocator&gt;
  void defer(BOOST_ASIO_MOVE_ARG(Function) f, const Allocator&amp; a) const
  {
    typename decay&lt;Function&gt;::type tmp(BOOST_ASIO_MOVE_CAST(Function)(f));
    service_.post(impl_, tmp);
    (void)a;
  }

#if !defined(BOOST_ASIO_NO_DEPRECATED)
  /// (Deprecated: Use boost::asio::bind_executor().) Create a new handler that
  /// automatically dispatches the wrapped handler on the strand.
  /**
   * This function is used to create a new handler function object that, when
   * invoked, will automatically pass the wrapped handler to the strand's
   * dispatch function.
   *
   * @param handler The handler to be wrapped. The strand will make a copy of
   * the handler object as required. The function signature of the handler must
   * be: @code void handler(A1 a1, ... An an); @endcode
   *
   * @return A function object that, when invoked, passes the wrapped handler to
   * the strand's dispatch function. Given a function object with the signature:
   * @code R f(A1 a1, ... An an); @endcode
   * If this function object is passed to the wrap function like so:
   * @code strand.wrap(f); @endcode
   * then the return value is a function object with the signature
   * @code void g(A1 a1, ... An an); @endcode
   * that, when invoked, executes code equivalent to:
   * @code strand.dispatch(boost::bind(f, a1, ... an)); @endcode
   */
  template &lt;typename Handler&gt;
#if defined(GENERATING_DOCUMENTATION)
  unspecified
#else
  detail::wrapped_handler&lt;strand, Handler, detail::is_continuation_if_running&gt;
#endif
  wrap(Handler handler)
  {
    return detail::wrapped_handler&lt;io_context::strand, Handler,
        detail::is_continuation_if_running&gt;(*this, handler);
  }
#endif // !defined(BOOST_ASIO_NO_DEPRECATED)

/// Determine whether the strand is running in the current thread.
  /**
   * @return @c true if the current thread is executing a handler that was
   * submitted to the strand using post(), dispatch() or wrap(). Otherwise
   * returns @c false.
   */
  bool running_in_this_thread() const BOOST_ASIO_NOEXCEPT
  {
    return service_.running_in_this_thread(impl_);
  }

private:
  boost::asio::detail::strand_service&amp; service_;
  mutable boost::asio::detail::strand_service::implementation_type impl_;
};

} // namespace asio
} // namespace boost

#include &lt;<a href="../../boost/asio/detail/pop_options.hpp">boost/asio/detail/pop_options.hpp</a>&gt;

#endif // !defined(BOOST_ASIO_NO_EXTENSIONS)

#endif // BOOST_ASIO_IO_CONTEXT_STRAND_HPP
</pre>
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