[/
 / 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:spawn Stackful Coroutines]

The [link boost_asio.reference.spawn `spawn()`] function is a high-level wrapper for
running stackful coroutines. It is based on the Boost.Coroutine library. The
`spawn()` function enables programs to implement asynchronous logic in a
synchronous manner, as shown in the following example:

  boost::asio::spawn(my_strand, do_echo);

  // ...

  void do_echo(boost::asio::yield_context yield)
  {
    try
    {
      char data[128];
      for (;;)
      {
        std::size_t length =
          my_socket.async_read_some(
            boost::asio::buffer(data), yield);

        boost::asio::async_write(my_socket,
            boost::asio::buffer(data, length), yield);
      }
    }
    catch (std::exception& e)
    {
      // ...
    }
  }

The first argument to `spawn()` may be a
[link boost_asio.reference.io_context__strand `strand`],
[link boost_asio.reference.io_context `io_context`], or
[link boost_asio.reference.CompletionHandler completion handler].
This argument 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 function object with signature:

  void coroutine(boost::asio::yield_context yield);

that specifies the code to be run as part of the coroutine. The parameter
`yield` may be passed to an asynchronous operation in place of the completion
handler, as in:

  std::size_t length =
    my_socket.async_read_some(
      boost::asio::buffer(data), yield);

This starts the asynchronous operation and suspends the coroutine. The
coroutine will be resumed automatically when the asynchronous operation
completes.

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

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

the initiating function returns the 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 initiating function returns `void`. As above, an error is passed back to
the coroutine as a `system_error` exception.

To collect the `error_code` from an operation, rather than have it throw an
exception, associate the output variable with the `yield_context` as follows:

  boost::system::error_code ec;
  std::size_t length =
    my_socket.async_read_some(
      boost::asio::buffer(data), yield[ec]);
  
[*Note:] if `spawn()` is used with a custom completion handler of type
`Handler`, the function object signature is actually:
  
  void coroutine(boost::asio::basic_yield_context<Handler> yield);

[heading See Also]

[link boost_asio.reference.spawn spawn],
[link boost_asio.reference.yield_context yield_context],
[link boost_asio.reference.basic_yield_context basic_yield_context],
[link boost_asio.examples.cpp03_examples.spawn Spawn example (C++03)],
[link boost_asio.examples.cpp11_examples.spawn Spawn example (C++11)],
[link boost_asio.overview.core.coroutine Stackless Coroutines].

[endsect]