[/
 / Copyright (c) 2003-2023 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:asynchronous_operations Requirements on asynchronous operations]

This section uses the names `Alloc1`, `Alloc2`, `alloc1`, `alloc2`, `Args`,
`CompletionHandler`, `completion_handler`, `Executor1`, `Executor2`, `ex1`,
`ex2`, `f`, [^['i]], [^['N]], `Signatures`, `token`, `initiation`, `Initiation`,
[^T[sub ['i]]], [^t[sub ['i]]], `work1`, and `work2` as placeholders for
specifying the requirements below.

[heading General asynchronous operation concepts]

An ['initiating function] is a function which may be called to start an
asynchronous operation. A ['completion handler] is a function object that will
be invoked, at most once, with the result of the asynchronous operation.

The lifecycle of an asynchronous operation is comprised of the following events
and phases:

[mdash] Event 1: The asynchronous operation is started by a call to the
initiating function.

[mdash] Phase 1: The asynchronous operation is now ['outstanding].

[mdash] Event 2: The externally observable side effects of the asynchronous
operation, if any, are fully established. The completion handler is submitted
to an executor.

[mdash] Phase 2: The asynchronous operation is now ['completed].

[mdash] Event 3: The completion handler is called with the result of the
asynchronous operation.

In this library, all functions with the prefix `async_` are initiating
functions.

[heading Completion tokens and handlers]

Initiating functions:

[mdash] are function templates with template parameter `CompletionToken`;

[mdash] accept, as the final parameter, a ['completion token] object `token`
of type `CompletionToken`;

[mdash] specify one or more ['completion signatures], referred to below as a
variadic pack of call signatures (C++Std [func.def]) `Signatures`, that
determine the possible argument sets to the completion handler.

In this library, initiating functions specify a ['Completion signature] element
that defines the call signatures `Signatures`. The ['Completion signature]
elements in this library may have named parameters, and the results of an
asynchronous operation may be specified in terms of these names.

Completion token behaviour is determined through specialisation of the
`async_result` trait. These specialisations must have the form:

  template <class CompletionToken, class... Signatures>
  struct async_result
  {
    template<
        class Initiation,
        class RawCompletionToken,
        class... Args
      >
    static initiating-fn-return-type initiate(
        Initiation&& initiation,
        RawCompletionToken&& token,
        Args&&... args
      );
  };

An `async_result` specialisation’s implementation of the `initiate()` static
member function must:

* Transform the completion token into an object `completion_handler` of type
  `CompletionHandler`. The type `CompletionHandler` must satisfy the
  requirements of `Destructible` (C++Std [destructible]) and `MoveConstructible`
  (C++Std [moveconstructible]), and be callable with the specified call
  signatures.

* Cause the invocation of the function object `initiation` as if by calling:
  ``
  std::forward<Initiation>(initiation)(
      std::move(completion_handler),
      std::forward<Args>(args)...);
  ``

This invocation of `initiation` may be immediate, or it may be deferred (e.g.
to support lazy evaluation). If `initiation` is deferred, the `initiation` and
`args...` objects must be decay-copied and moved as required.

The `async_result` trait must be specialised for the decayed type of a
`CompletionToken`. A helper function template `async_initiate` is provided to
simplify correct invocation of `async_result<>::initiate` for the appropriate
specialisation:

  template<
      class CompletionToken,
      completion_signature... Signatures,
      class Initiation,
      class... Args
    >
  ``['DEDUCED]`` async_initiate(
      Initiation&& initiation,
      CompletionToken& token,
      Args&&... args
    );

[inline_note No other requirements are placed on the type `CompletionToken`.]

[heading Automatic deduction of initiating function return type]

An initiating function returns [^async_initiate<CompletionToken,
Signatures...>(initiation, token, ['unspecified-args...])], where `initiation`
is a function object of unspecified type `Initiation`, which is defined as:

  class Initiation
  {
  public:
    using executor_type = Executor; // optional
    executor_type get_executor() const noexcept; // optional

    template <class CompletionHandler, ``['unspecified-args...]``>
      void operator()(CompletionHandler&& completion_handler, ``['unspecified-args...]``) const;
  };

For the sake of exposition, this library sometimes annotates functions with a
return type ['[^DEDUCED]]. For every function declaration that returns
['[^DEDUCED]], the meaning is equivalent to an automatically deduced return
type, having the type of the expression [^async_initiate<CompletionToken,
Signatures...>(initiation, token, ['unspecified-args...])].

\[['Example:] Given an asynchronous operation with a single ['Completion
signature] `void(R1 r1, R2 r2)`, an initiating function meeting these
requirements may be implemented as follows:

  template<class CompletionToken>
  auto async_xyz(T1 t1, T2 t2, CompletionToken&& token)
  {
    return async_initiate<CompletionToken, void(R1, R2)>(
        [](auto completion_handler, T1 t1, T2 t2)
        {
          // initiate the operation and cause completion_handler to be invoked
          // with the result
        }, token, std::move(t1), std::move(t2));
  }

The concepts `completion_token_for` and `completion_handler_for` may also be
used to improve compile-time diagnostics:

  template<completion_token_for<void(R1, R2)> CompletionToken>
  auto async_xyz(T1 t1, T2 t2, CompletionToken&& token)
  {
    return async_initiate<CompletionToken, void(R1, R2)>(
        [](completion_handler_for<void(R1, R2)> auto completion_handler, T1 t1, T2 t2)
        {
          // initiate the operation and cause completion_handler to be invoked
          // with the result
        }, token, std::move(t1), std::move(t2));
  }

Initiation functions may also be implemented using the `async_result` trait
directly:

  template<class CompletionToken>
  auto async_xyz(T1 t1, T2 t2, CompletionToken&& token)
  {
    return async_result<decay_t<CompletionToken>, void(R1, R2)>::initiate(
        [](auto completion_handler, T1 t1, T2 t2)
        {
          // initiate the operation and cause completion_handler to be invoked
          // with the result
        }, forward<CompletionToken>(token), std::move(t1), std::move(t2));
  }

Note the use of `decay_t` and `forward` applied to the `CompletionToken` type.
However, the first form is preferred as it preserves compatibility with legacy
completion token requirements.

'''&mdash;'''['end example]\]

[heading Lifetime of initiating function arguments]

Unless otherwise specified, the lifetime of arguments to initiating functions
shall be treated as follows:

[mdash] If the parameter has a pointer type or has a type of lvalue reference
to non-const, the implementation may assume the validity of the pointee or
referent, respectively, until the completion handler is invoked. [inline_note
In other words, the program must guarantee the validity of the argument until
the completion handler is invoked.]

[mdash] Otherwise, the implementation must not assume the validity of the
argument after the initiating function completes. [inline_note In other words,
the program is not required to guarantee the validity of the argument after the
initiating function completes.] The implementation may make copies of the
argument, and all copies shall be destroyed no later than immediately after
invocation of the completion handler.

[heading Non-blocking requirements on initiating functions]

An initiating function shall not block (C++Std [defns.block]) the calling
thread pending completion of the outstanding operation.

[inline_note Initiating functions may still block the calling thread for other
reasons. For example, an initiating function may lock a mutex in order to
synchronize access to shared data.]

[heading Associated executor]

Certain objects that participate in asynchronous operations have an
['associated executor]. These are obtained as specified in the sections below.

[heading Associated I/O executor]

An asynchronous operation has an associated executor satisfying the [link
asio.reference.Executor1 `Executor`] requirements. If not otherwise specified by
the asynchronous operation, this associated executor is an object of type
`system_executor`.

All asynchronous operations in this library have an associated I/O executor
object that is determined as follows:

[mdash] If the initiating function is a member function, the associated
executor is that returned by the `get_executor` member function on the same
object.

[mdash] If the initiating function is not a member function, the associated
executor is that returned by the `get_executor` member function of the first
argument to the initiating function.

The operation's associated I/O executor may be exposed via the `Initiation`
function object's `executor_type` type alias and `get_executor()` member
function.

Let `Executor1` be the type of the associated executor. Let `ex1` be a value of
type `Executor1`, representing the associated executor object obtained as
described above.

[heading Associated completion handler executor]

A completion handler object of type `CompletionHandler` has an associated
executor of type `Executor2` satisfying the [link asio.reference.Executor1
Executor requirements]. The type `Executor2` is
`associated_executor_t<CompletionHandler, Executor1>`. Let `ex2` be a value of
type `Executor2` obtained by performing
`get_associated_executor(completion_handler, ex1)`.

[heading Associated immediate completion executor]

A completion handler object of type `CompletionHandler` has an associated
immediate executor of type `Executor3` satisfying the
[link asio.reference.Executor1 Executor requirements]. The type `Executor3` is
`associated_immediate_executor_t<CompletionHandler, Executor1>`. Let `ex3` be a
value of type `Executor3` obtained by performing
`get_associated_immediate_executor(completion_handler, ex1)`.

[heading Outstanding work]

If the operation does not complete immediately (that is, the operation does not
complete within the thread of execution calling the initiating function, before
the initiating function returns) then, until the asynchronous operation has
completed, the asynchronous operation shall maintain:

[mdash] an object `work1` of type `executor_work_guard<Executor1>`, initialized
as `work1(ex1)`, and where `work1.owns_work() == true`; and

[mdash] an object `work2` of type `executor_work_guard<Executor2>`, initialized
as `work2(ex2)`, and where `work2.owns_work() == true`.

[heading Allocation of intermediate storage]

Asynchronous operations may allocate memory. [inline_note Such as a data
structure to store copies of the `completion_handler` object and the initiating
function's arguments.]

Let `Alloc1` be a type, satisfying the [link asio.reference.ProtoAllocator
`ProtoAllocator`] requirements, that represents the asynchronous operation's
default allocation strategy. [inline_note Typically `std::allocator<void>`.]
Let `alloc1` be a value of type `Alloc1`.

A completion handler object of type `CompletionHandler` has an associated
allocator object `alloc2` of type `Alloc2` satisfying the [link
asio.reference.ProtoAllocator `ProtoAllocator`] requirements. The type `Alloc2`
is `associated_allocator_t<CompletionHandler, Alloc1>`. Let `alloc2` be a value
of type `Alloc2` obtained by performing
`get_associated_allocator(completion_handler, alloc1)`.

The asynchronous operations defined in this library:

[mdash] If required, allocate memory using only the completion handler's
associated allocator.

[mdash] Prior to completion handler execution, deallocate any memory allocated
using the completion handler's associated allocator.

[inline_note The implementation may perform operating system or underlying API
calls that perform memory allocations not using the associated allocator.
Invocations of the allocator functions may not introduce data races (See C++Std
\[res.on.data.races\]).]

[heading Execution of completion handler on completion of asynchronous operation]

Let `Args...` be the argument types of a completion signature in
`Signatures...` and let [^['N]] be `sizeof...(Args)`. Let [^['i]] be in the
range [half_open_range `0`,[^['N]]]. Let [^T[sub ['i]]] be the [^['i]]th type
in `Args...` and let [^t[sub ['i]]] be the [^['i]]th completion handler
argument associated with [^T[sub ['i]]].

Let `f` be a function object that:

[mdash] is callable as `f()`, and when so called invokes `completion_handler`
as if by [^std::move(completion_handler)(forward<T[sub ['0]]>(t[sub ['0]]), ...,
forward<T[sub ['N-1]]>(t[sub ['N-1]]))];

[mdash] has an associated executor such that `get_associated_executor(f, ex1)`
returns an executor that is equal to `ex2`; and

[mdash] has an associated allocator such that `get_associated_allocator(f,
alloc1)` returns an allocator that is equal to `alloc2`.

[inline_note These associated executor and allocator requirements on `f` are
typically implemented by specialising the associator traits for `f` so that they
forward to the associator traits for `completion_handler`.]

If an asynchonous operation completes immediately (that is, the operation
completes within the thread of execution calling the initiating function, and
before the initiating function returns), the completion handler shall be
submitted for execution as if by performing `post(ex1, std::move(f))`.

Otherwise, when the operation completes, the completion handler shall be
submitted for execution as if by performing `dispatch(ex2, std::move(f))`.

[heading Optimisation of immediate completion]

If an asynchronous operation completes immediately then, as an optimisation,
the operation may either:

[mdash] obtain the associated immediate completion executor `ex3` by performing
`get_associated_immediate_executor(completion_handler, ex1)`, and then
submit the completion handler for execution as if by performing `dispatch(ex3,
std::move(f))`; or

[mdash] submit the completion handler for execution by performing the expression
`post(ex2, std::move(f))`, if that expression is well-formed.

[inline_note If `completion_handler` does not customise the associated immediate
executor, the behaviour of the first optimisation is equivalent to `post(ex1,
std::move(f))`.]

[heading Completion handlers and exceptions]

Completion handlers are permitted to throw exceptions. The effect of any
exception propagated from the execution of a completion handler is determined
by the executor which is executing the completion handler.

[heading Default completion tokens]

Every I/O executor type has an associated default completion token type. This is
specified via the `default_completion_token` trait. This trait may be used in
asynchronous operation declarations as follows:

  template <
      typename IoObject,
      typename CompletionToken =
        typename default_completion_token<
          typename IoObject::executor_type
        >::type
    >
  auto async_xyz(
      IoObject& io_object,
      CompletionToken&& token =
        typename default_completion_token<
          typename IoObject::executor_type
        >::type{}
    );

If not specialised, this trait type is `void`, meaning no default completion
token type is available for the given I/O executor.

\[['Example:] The `default_completion_token` trait is specialised for the
`use_awaitable` completion token so that it may be used as shown in the
following example:

  auto socket = use_awaitable.as_default_on(tcp::socket(my_context));
  // ...
  co_await socket.async_connect(my_endpoint); // Defaults to use_awaitable.

In this example, the type of the `socket` object is transformed from
`tcp::socket` to have an I/O executor with the default completion token set to
`use_awaitable`.

Alternatively, the socket type may be computed directly:

  using tcp_socket = use_awaitable_t<>::as_default_on_t<tcp::socket>;
  tcp_socket socket(my_context);
  // ...
  co_await socket.async_connect(my_endpoint); // Defaults to use_awaitable.

'''&mdash;'''['end example]\]

[endsect]
