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-[/
- / Copyright (c) 2003-2016 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.
-
-[section 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.
-
-[endsect]
-
-[section:completion_token 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.
-
-[endsect]
-
-[section 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`.
-
-[endsect]
-
-[section 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.
-
-\[['Example:] Given an asynchronous operation with ['Completion signature]
-`void(R1 r1, R2 r2)`, an initiating function meeting these requirements may be
-implemented as follows:
-
- template<class CompletionToken>
- auto async_xyz(T1 t1, T2 t2, CompletionToken&& token)
- {
- typename async_result<decay_t<CompletionToken>, void(R1, R2)>::completion_handler_type
- completion_handler(forward<CompletionToken>(token));
-
- async_result<decay_t<CompletionToken>, void(R1, R2)> result(completion_handler);
-
- // initiate the operation and cause completion_handler to be invoked with
- // the result
-
- return result.get();
- }
-
-For convenience, initiating functions may be implemented using the
-`async_completion` template:
-
- template<class CompletionToken>
- auto async_xyz(T1 t1, T2 t2, CompletionToken&& token)
- {
- async_completion<CompletionToken, void(R1, R2)> init(token);
-
- // initiate the operation and cause init.completion_handler to be invoked
- // with the result
-
- return init.result.get();
- }
-
-'''&mdash;'''['end example]\]
-
-[endsect]
-
-[section 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.
-
-[endsect]
-
-[section 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.]
-
-[endsect]
-
-[section Associated executor]
-
-Certain objects that participate in asynchronous operations have an
-['associated executor]. These are obtained as specified below.
-
-[endsect]
-
-[section 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 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.
-
-[endsect]
-
-[section 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)`.
-
-[endsect]
-
-[section 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`.
-
-[endsect]
-
-[section 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.
-
-[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
-\[res.on.data.races\]).]
-
-[endsect]
-
-[section 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)`.
-
-[endsect]
-
-[section 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.
-
-[endsect]
-
-[endsect]