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- [/
- / Copyright (c) 2003-2019 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.
- \[['Example:] Given an asynchronous operation with ['Completion signature]
- `void(R1 r1, R2 r2)`, an initiating function meeting these requirements may be
- implemented as follows:
- template<class CompletionToken>
- auto async_xyz(T1 t1, T2 t2, CompletionToken&& token)
- {
- typename async_result<decay_t<CompletionToken>, void(R1, R2)>::completion_handler_type
- completion_handler(forward<CompletionToken>(token));
- async_result<decay_t<CompletionToken>, void(R1, R2)> result(completion_handler);
- // initiate the operation and cause completion_handler to be invoked with
- // the result
- return result.get();
- }
- For convenience, initiating functions may be implemented using the
- `async_completion` template:
- template<class CompletionToken>
- auto async_xyz(T1 t1, T2 t2, CompletionToken&& token)
- {
- async_completion<CompletionToken, void(R1, R2)> init(token);
- // initiate the operation and cause init.completion_handler to be invoked
- // with the result
- return init.result.get();
- }
- '''—'''['end example]\]
- [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.
- \[['Example:] The `default_completion_token` trait is specialised for the
- `use_awaitable` completion token so that it may be used as shown in the
- following example:
- auto socket = use_awaitable.as_default_on(tcp::socket(my_context));
- // ...
- co_await socket.async_connect(my_endpoint); // Defaults to use_awaitable.
- In this example, the type of the `socket` object is transformed from
- `tcp::socket` to have an I/O executor with the default completion token set to
- `use_awaitable`.
- Alternatively, the socket type may be computed directly:
- using tcp_socket = use_awaitable_t<>::as_default_on_t<tcp::socket>;
- tcp_socket socket(my_context);
- // ...
- co_await socket.async_connect(my_endpoint); // Defaults to use_awaitable.
- '''—'''['end example]\]
- [endsect]
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