[/
    Copyright (c) 2013-2017 Vinnie Falco (vinnie dot falco at gmail 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:async Asynchronous I/O]

Asynchronous operations are started by calling a free function or member
function known as an ['asynchronous initiation function]. The initiation
function accepts parameters specific to the operation as well as a "completion
token." This token is either a completion handler, or another type allowing for
customization of how the result of the asynchronous operation is conveyed to
callers. __Asio__ allows the special completion tokens __use_future__ and
objects of type __yield_context__ to allow callers to specify the use of futures
and coroutines respectively. This system, where the return value and method of
indicating completion may be customize at the call site of the asynchronous
initiation function, is known as the ['Extensible Asynchronous Model] described
in __N3747__, and built-in to __N4588__.

[note
    A full explanation of completion handlers, the Extensible Asynchronous
    Model and how these asynchronous interfaces are used is beyond the
    scope of this document. Interested readers should consult the
    __Asio__ documentation.
]

Since the interfaces provided here are low level, authors of libraries
may wish to create higher level interfaces using the primitives found
in this library. Non-trivial applications will want to provide their own
asynchronous initiation functions which perform a series of other,
intermediate asynchronous operations before invoking the final completion
handler. The set of intermediate actions produced by calling an initiation
function is known as a ['composed operation]. To ensure full interoperability
and well-defined behavior, __Asio__ imposes requirements on the implementation
of composed operations. A number of useful classes and macros to facilitate
the development of composed operations and the associated asynchronous
initiation functions used to launch them are available:

[table Asynchronous Helpers
[[Name][Description]]
[[
    [link beast.ref.async_completion `async_completion`]
][
    This class aggregates the completion handler customization point and
    the asynchronous initiation function return value customization point
    into a single object which exposes the appropriate output types for the
    given input types, and also contains boilerplate that is necessary to
    implement an initiation function using the Extensible Model.
]]
[[
    [link beast.ref.async_return_type `async_return_type`]
][
    This template alias determines the return value of an asynchronous
    initiation function given the completion token and signature. It is used
    by asynchronous initiation functions to meet the requirements of the
    Extensible Asynchronous Model.
]]
[[
    [link beast.ref.bind_handler `bind_handler`]
][
    This function returns a new, nullary completion handler which when
    invoked with no arguments invokes the original completion handler with a
    list of bound arguments. The invocation is made from the same implicit
    or explicit strand as that which would be used to invoke the original
    handler. This is accomplished by using the correct overload of
    `asio_handler_invoke` associated with the original completion handler.

]]
[[
    [link beast.ref.handler_alloc `handler_alloc`]
][
    This class meets the requirements of [*Allocator], and uses any custom
    memory allocation and deallocation hooks associated with a given handler.
    It is useful for when a composed operation requires temporary dynamic
    allocations to achieve its result. Memory allocated using this allocator
    must be freed before the final completion handler is invoked.
]]
[[
    [link beast.ref.handler_ptr `handler_ptr`]
][
    This is a smart pointer container used to manage the internal state of a
    composed operation. It is useful when the state is non trivial. For example
    when the state has non-copyable or expensive to copy types. The container
    takes ownership of the final completion handler, and provides boilerplate
    to invoke the final handler in a way that also deletes the internal state.
    The internal state is allocated using the final completion handler's
    associated allocator, benefiting from all handler memory management
    optimizations transparently.
]]
[[
    [link beast.ref.handler_type `handler_type`]
][
    This template alias converts a completion token and signature to the
    correct completion handler type. It is used in the implementation of
    asynchronous initiation functions to meet the requirements of the
    Extensible Asynchronous Model.
]]
]

[endsect]