doc/requirements/asynchronous_operations.qbk

[/
 / Copyright (c) 2003-2021 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]], `Signature`, `token`, [^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 a ['completion signature], which is a call signature (C++Std
[func.def]) `Signature` that determines the arguments to the completion
handler.

An initiating function determines the type `CompletionHandler` of its
completion handler function object by performing `typename
async_result<decay_t<CompletionToken>, Signature>::completion_handler_type`.
The completion handler object `completion_handler` is initialized with
`forward<CompletionToken>(token)`. [inline_note No other requirements are
placed on the type `CompletionToken`.]

The type `CompletionHandler` must satisfy the requirements of `Destructible`
(C++Std [destructible]) and `MoveConstructible` (C++Std
[moveconstructible]), and be callable with the specified call signature.

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

[heading Automatic deduction of initiating function return type]

The return type of an initiating function is `typename
async_result<decay_t<CompletionToken>, Signature>::return_type`.

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 specifying the return type as
`typename async_result<decay_t<CompletionToken>, Signature>::return_type`.

[heading Production of initiating function return value]

An initiating function produces its return type as follows:

[mdash] constructing an object `result` of type
`async_result<decay_t<CompletionToken>, Signature>`, initialized as
`result(completion_handler)`; and

[mdash] using `result.get()` as the operand of the return statement.

\[['Example:] Given an asynchronous operation with ['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)
  {
    typename async_result<decay_t<CompletionToken>, void(R1, R2)>::completion_handler_type
      completion_handler(forward<CompletionToken>(token));

    async_result<decay_t<CompletionToken>, void(R1, R2)> result(completion_handler);

    // initiate the operation and cause completion_handler to be invoked with
    // the result

    return result.get();
  }

For convenience, initiating functions may be implemented using the
`async_completion` template:

  template<class CompletionToken>
  auto async_xyz(T1 t1, T2 t2, CompletionToken&& token)
  {
    async_completion<CompletionToken, void(R1, R2)> init(token);

    // initiate the operation and cause init.completion_handler to be invoked
    // with the result

    return init.result.get();
  }

'''&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.

[std_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
boost_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 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.

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 boost_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 Outstanding work]

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 boost_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
boost_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.

[std_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 the completion signature `Signature` 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, callable as `f()`, that invokes
`completion_handler` as if by [^completion_handler(forward<T[sub ['0]]>(t[sub
['0]]), ..., forward<T[sub ['N-1]]>(t[sub ['N-1]]))].

If an asynchonous operation completes immediately (that is, 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 `ex2.post(std::move(f), alloc2)`. Otherwise, the completion
handler shall be submitted for execution as if by performing
`ex2.dispatch(std::move(f), alloc2)`.

[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]