[/
 / Copyright (c) 2003-2020 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)
 /]

[section:coroutines_ts Coroutines TS Support]

Support for the Coroutines TS is provided via the [link
boost_asio.reference.awaitable `awaitable`] class template, the [link
boost_asio.reference.use_awaitable_t `use_awaitable`] completion token, and the [link
boost_asio.reference.co_spawn `co_spawn()`] function. These facilities allow programs
to implement asynchronous logic in a synchronous manner, in conjunction with
the `co_await` keyword, as shown in the following example:

  boost::asio::co_spawn(executor,
      [socket = std::move(socket)]() mutable
      {
        return echo(std::move(socket));
      },
      boost::asio::detached);

  // ...

  boost::asio::awaitable<void> echo(tcp::socket socket)
  {
    try
    {
      char data[1024];
      for (;;)
      {
        std::size_t n = co_await socket.async_read_some(boost::asio::buffer(data), boost::asio::use_awaitable);
        co_await async_write(socket, boost::asio::buffer(data, n), boost::asio::use_awaitable);
      }
    }
    catch (std::exception& e)
    {
      std::printf("echo Exception: %s\n", e.what());
    }
  }

The first argument to `co_spawn()` is an [link boost_asio.reference.Executor1
executor] that determines the context in which the coroutine is permitted to
execute. For example, a server's per-client object may consist of multiple
coroutines; they should all run on the same `strand` so that no explicit
synchronisation is required.

The second argument is a nullary function object that returns a [link
boost_asio.reference.awaitable `boost::asio::awaitable<R>`],
where `R` is the type of return value produced by the coroutine. In the above
example, the coroutine returns `void`.

The third argument is a completion token, and this is used by `co_spawn()` to
produce a completion handler with signature `void(std::exception_ptr, R)`. This
completion handler is invoked with the result of the coroutine once it has
finished. In the above example we pass a completion token type, [link
boost_asio.reference.detached `boost::asio::detached`], which is used to explicitly ignore
the result of an asynchronous operation.

In this example the body of the coroutine is implemented in the `echo`
function. When the `use_awaitable` completion token is passed to an
asynchronous operation, the operation's initiating function returns an
`awaitable` that may be used with the `co_await` keyword:

  std::size_t n = co_await socket.async_read_some(boost::asio::buffer(data), boost::asio::use_awaitable);

Where an asynchronous operation's handler signature has the form:

  void handler(boost::system::error_code ec, result_type result);

the resulting type of the `co_await` expression is `result_type`. In the
`async_read_some` example above, this is `size_t`. If the asynchronous
operation fails, the `error_code` is converted into a `system_error` exception
and thrown.

Where a handler signature has the form:

  void handler(boost::system::error_code ec);

the `co_await` expression produces a `void` result. As above, an error is
passed back to the coroutine as a `system_error` exception.

[heading See Also]

[link boost_asio.reference.co_spawn co_spawn],
[link boost_asio.reference.detached detached],
[link boost_asio.reference.redirect_error redirect_error],
[link boost_asio.reference.awaitable awaitable],
[link boost_asio.reference.use_awaitable_t use_awaitable_t],
[link boost_asio.reference.use_awaitable use_awaitable],
[link boost_asio.reference.this_coro__executor this_coro::executor],
[link boost_asio.examples.cpp17_examples.coroutines_ts_support Coroutines TS examples],
[link boost_asio.overview.core.spawn Stackful Coroutines],
[link boost_asio.overview.core.coroutine Stackless Coroutines].

[endsect]