[/
 / Copyright (c) 2003-2025 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:promises Promises]

[note This is an experimental feature.]

The [link asio.reference.experimental__promise `experimental::promise`] type
and [link asio.reference.experimental__use_promise `experimental::use_promise`]
completion token allow eager execution and synchronisation of asynchronous
operations. For example:

  auto promise = async_read(
      stream, asio::buffer(my_buffer),
      asio::experimental::use_promise);

  ... do other stuff while the read is going on ...

  promise.async_wait( // completion the operation
      [](error_code ec, std::size_t bytes_read)
      {
        ...
      });

Promises can be safely disregarded if the result is no longer required.
Different operations can be combined to either wait for all to complete or for
one to complete (and cancel the rest). For example, to wait for one to
complete:

  auto timeout_promise =
    timer.async_wait(
      asio::experimental::use_promise);

  auto read_promise = async_read(
      stream, asio::buffer(my_buffer),
      asio::experimental::use_promise);

  auto promise =
    asio::experimental::make_parallel_group(
      timeout_promise.async_wait(asio::deferred),
      read_promise.async_wait(asio::deferred)
    ).async_wait(
      asio::experimental::wait_for_one(),
      asio::experimental::use_promise);

  promise.async_wait(
      [](std::array<std::size_t, 2> order,
         error_code timer_ec,
         error_code read_ec, std::size_t read_size)
      {
        if (order[0] == 0)
        {
          // timed out
        }
        else
        {
          // completed in time
        }
      });

or to wait for all to complete:

  auto write_promise = async_write(
      stream, asio::buffer(my_write_buffer),
      asio::experimental::use_promise);

  auto read_promise = async_read(
      stream, asio::buffer(my_buffer),
      asio::experimental::use_promise);

  auto promise =
    asio::experimental::make_parallel_group(
      timeout_promise.async_wait(asio::deferred),
      read_promise.async_wait(asio::deferred)
    ).async_wait(
      asio::experimental::wait_for_all(),
      asio::experimental::use_promise);

  promise.async_wait(
      [](std::array<std::size_t, 2> order,
         error_code timer_ec,
         error_code read_ec, std::size_t read_size)
      {
        // ...
      });

[heading See Also]

[link asio.reference.experimental__promise promise],
[link asio.reference.experimental__use_promise use_promise].

[endsect]
