boost/asio/coroutine.hpp
//
// coroutine.hpp
// ~~~~~~~~~~~~~
//
// Copyright (c) 2003-2022 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 BOOST_ASIO_COROUTINE_HPP
#define BOOST_ASIO_COROUTINE_HPP
namespace boost {
namespace asio {
namespace detail {
class coroutine_ref;
} // namespace detail
/// Provides support for implementing stackless coroutines.
/**
* The @c coroutine class may be used to implement stackless coroutines. The
* class itself is used to store the current state of the coroutine.
*
* Coroutines are copy-constructible and assignable, and the space overhead is
* a single int. They can be used as a base class:
*
* @code class session : coroutine
* {
* ...
* }; @endcode
*
* or as a data member:
*
* @code class session
* {
* ...
* coroutine coro_;
* }; @endcode
*
* or even bound in as a function argument using lambdas or @c bind(). The
* important thing is that as the application maintains a copy of the object
* for as long as the coroutine must be kept alive.
*
* @par Pseudo-keywords
*
* A coroutine is used in conjunction with certain "pseudo-keywords", which
* are implemented as macros. These macros are defined by a header file:
*
* @code #include <boost/asio/yield.hpp>@endcode
*
* and may conversely be undefined as follows:
*
* @code #include <boost/asio/unyield.hpp>@endcode
*
* <b>reenter</b>
*
* The @c reenter macro is used to define the body of a coroutine. It takes a
* single argument: a pointer or reference to a coroutine object. For example,
* if the base class is a coroutine object you may write:
*
* @code reenter (this)
* {
* ... coroutine body ...
* } @endcode
*
* and if a data member or other variable you can write:
*
* @code reenter (coro_)
* {
* ... coroutine body ...
* } @endcode
*
* When @c reenter is executed at runtime, control jumps to the location of the
* last @c yield or @c fork.
*
* The coroutine body may also be a single statement, such as:
*
* @code reenter (this) for (;;)
* {
* ...
* } @endcode
*
* @b Limitation: The @c reenter macro is implemented using a switch. This
* means that you must take care when using local variables within the
* coroutine body. The local variable is not allowed in a position where
* reentering the coroutine could bypass the variable definition.
*
* <b>yield <em>statement</em></b>
*
* This form of the @c yield keyword is often used with asynchronous operations:
*
* @code yield socket_->async_read_some(buffer(*buffer_), *this); @endcode
*
* This divides into four logical steps:
*
* @li @c yield saves the current state of the coroutine.
* @li The statement initiates the asynchronous operation.
* @li The resume point is defined immediately following the statement.
* @li Control is transferred to the end of the coroutine body.
*
* When the asynchronous operation completes, the function object is invoked
* and @c reenter causes control to transfer to the resume point. It is
* important to remember to carry the coroutine state forward with the
* asynchronous operation. In the above snippet, the current class is a
* function object object with a coroutine object as base class or data member.
*
* The statement may also be a compound statement, and this permits us to
* define local variables with limited scope:
*
* @code yield
* {
* mutable_buffers_1 b = buffer(*buffer_);
* socket_->async_read_some(b, *this);
* } @endcode
*
* <b>yield return <em>expression</em> ;</b>
*
* This form of @c yield is often used in generators or coroutine-based parsers.
* For example, the function object:
*
* @code struct interleave : coroutine
* {
* istream& is1;
* istream& is2;
* char operator()(char c)
* {
* reenter (this) for (;;)
* {
* yield return is1.get();
* yield return is2.get();
* }
* }
* }; @endcode
*
* defines a trivial coroutine that interleaves the characters from two input
* streams.
*
* This type of @c yield divides into three logical steps:
*
* @li @c yield saves the current state of the coroutine.
* @li The resume point is defined immediately following the semicolon.
* @li The value of the expression is returned from the function.
*
* <b>yield ;</b>
*
* This form of @c yield is equivalent to the following steps:
*
* @li @c yield saves the current state of the coroutine.
* @li The resume point is defined immediately following the semicolon.
* @li Control is transferred to the end of the coroutine body.
*
* This form might be applied when coroutines are used for cooperative
* threading and scheduling is explicitly managed. For example:
*
* @code struct task : coroutine
* {
* ...
* void operator()()
* {
* reenter (this)
* {
* while (... not finished ...)
* {
* ... do something ...
* yield;
* ... do some more ...
* yield;
* }
* }
* }
* ...
* };
* ...
* task t1, t2;
* for (;;)
* {
* t1();
* t2();
* } @endcode
*
* <b>yield break ;</b>
*
* The final form of @c yield is used to explicitly terminate the coroutine.
* This form is comprised of two steps:
*
* @li @c yield sets the coroutine state to indicate termination.
* @li Control is transferred to the end of the coroutine body.
*
* Once terminated, calls to is_complete() return true and the coroutine cannot
* be reentered.
*
* Note that a coroutine may also be implicitly terminated if the coroutine
* body is exited without a yield, e.g. by return, throw or by running to the
* end of the body.
*
* <b>fork <em>statement</em></b>
*
* The @c fork pseudo-keyword is used when "forking" a coroutine, i.e. splitting
* it into two (or more) copies. One use of @c fork is in a server, where a new
* coroutine is created to handle each client connection:
*
* @code reenter (this)
* {
* do
* {
* socket_.reset(new tcp::socket(my_context_));
* yield acceptor->async_accept(*socket_, *this);
* fork server(*this)();
* } while (is_parent());
* ... client-specific handling follows ...
* } @endcode
*
* The logical steps involved in a @c fork are:
*
* @li @c fork saves the current state of the coroutine.
* @li The statement creates a copy of the coroutine and either executes it
* immediately or schedules it for later execution.
* @li The resume point is defined immediately following the semicolon.
* @li For the "parent", control immediately continues from the next line.
*
* The functions is_parent() and is_child() can be used to differentiate
* between parent and child. You would use these functions to alter subsequent
* control flow.
*
* Note that @c fork doesn't do the actual forking by itself. It is the
* application's responsibility to create a clone of the coroutine and call it.
* The clone can be called immediately, as above, or scheduled for delayed
* execution using something like boost::asio::post().
*
* @par Alternate macro names
*
* If preferred, an application can use macro names that follow a more typical
* naming convention, rather than the pseudo-keywords. These are:
*
* @li @c BOOST_ASIO_CORO_REENTER instead of @c reenter
* @li @c BOOST_ASIO_CORO_YIELD instead of @c yield
* @li @c BOOST_ASIO_CORO_FORK instead of @c fork
*/
class coroutine
{
public:
/// Constructs a coroutine in its initial state.
coroutine() : value_(0) {}
/// Returns true if the coroutine is the child of a fork.
bool is_child() const { return value_ < 0; }
/// Returns true if the coroutine is the parent of a fork.
bool is_parent() const { return !is_child(); }
/// Returns true if the coroutine has reached its terminal state.
bool is_complete() const { return value_ == -1; }
private:
friend class detail::coroutine_ref;
int value_;
};
namespace detail {
class coroutine_ref
{
public:
coroutine_ref(coroutine& c) : value_(c.value_), modified_(false) {}
coroutine_ref(coroutine* c) : value_(c->value_), modified_(false) {}
~coroutine_ref() { if (!modified_) value_ = -1; }
operator int() const { return value_; }
int& operator=(int v) { modified_ = true; return value_ = v; }
private:
void operator=(const coroutine_ref&);
int& value_;
bool modified_;
};
} // namespace detail
} // namespace asio
} // namespace boost
#define BOOST_ASIO_CORO_REENTER(c) \
switch (::boost::asio::detail::coroutine_ref _coro_value = c) \
case -1: if (_coro_value) \
{ \
goto terminate_coroutine; \
terminate_coroutine: \
_coro_value = -1; \
goto bail_out_of_coroutine; \
bail_out_of_coroutine: \
break; \
} \
else /* fall-through */ case 0:
#define BOOST_ASIO_CORO_YIELD_IMPL(n) \
for (_coro_value = (n);;) \
if (_coro_value == 0) \
{ \
case (n): ; \
break; \
} \
else \
switch (_coro_value ? 0 : 1) \
for (;;) \
/* fall-through */ case -1: if (_coro_value) \
goto terminate_coroutine; \
else for (;;) \
/* fall-through */ case 1: if (_coro_value) \
goto bail_out_of_coroutine; \
else /* fall-through */ case 0:
#define BOOST_ASIO_CORO_FORK_IMPL(n) \
for (_coro_value = -(n);; _coro_value = (n)) \
if (_coro_value == (n)) \
{ \
case -(n): ; \
break; \
} \
else
#if defined(_MSC_VER)
# define BOOST_ASIO_CORO_YIELD BOOST_ASIO_CORO_YIELD_IMPL(__COUNTER__ + 1)
# define BOOST_ASIO_CORO_FORK BOOST_ASIO_CORO_FORK_IMPL(__COUNTER__ + 1)
#else // defined(_MSC_VER)
# define BOOST_ASIO_CORO_YIELD BOOST_ASIO_CORO_YIELD_IMPL(__LINE__)
# define BOOST_ASIO_CORO_FORK BOOST_ASIO_CORO_FORK_IMPL(__LINE__)
#endif // defined(_MSC_VER)
#endif // BOOST_ASIO_COROUTINE_HPP