...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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.
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:
— Event 1: The asynchronous operation is started by a call to the initiating function.
— Phase 1: The asynchronous operation is now outstanding.
— Event 2: The externally observable side effects of the asynchronous operation, if any, are fully established. The completion handler is submitted to an executor.
— Phase 2: The asynchronous operation is now completed.
— 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.
Initiating functions:
— are function templates with template parameter CompletionToken
;
— accept, as the final parameter, a completion token object
token
of type CompletionToken
;
— 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)
.
[Note: No other requirements are placed on the type
CompletionToken
. —end note]
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.
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
.
An initiating function produces its return type as follows:
— constructing an object result
of type async_result<decay_t<CompletionToken>,
Signature>
, initialized as result(completion_handler)
;
and
— 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]
Unless otherwise specified, the lifetime of arguments to initiating functions shall be treated as follows:
— 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. [Note: In other words, the program must guarantee the validity of the argument until the completion handler is invoked. —end note]
— Otherwise, the implementation must not assume the validity of the argument after the initiating function completes. [Note: In other words, the program is not required to guarantee the validity of the argument after the initiating function completes. —end note] The implementation may make copies of the argument, and all copies shall be destroyed no later than immediately after invocation of the completion handler.
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.]
Certain objects that participate in asynchronous operations have an associated executor. These are obtained as specified in the sections below.
An asynchronous operation has an associated executor satisfying the 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:
— If the initiating function is a member function, the associated executor
is that returned by the get_executor
member function on the
same object.
— 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.
A completion handler object of type CompletionHandler
has an
associated executor of type Executor2
satisfying the 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)
.
Until the asynchronous operation has completed, the asynchronous operation shall maintain:
— an object work1
of type executor_work_guard<Executor1>
,
initialized as work1(ex1)
, and where work1.owns_work()
== true
; and
— an object work2
of type executor_work_guard<Executor2>
,
initialized as work2(ex2)
, and where work2.owns_work()
== true
.
Asynchronous operations may allocate memory. [Note:
Such as a data structure to store copies of the completion_handler
object and the initiating function's arguments. —end note]
Let Alloc1
be a type, satisfying the ProtoAllocator
requirements, that represents the asynchronous operation's default allocation
strategy. [Note: Typically std::allocator<void>
.
—end note] 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 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:
— If required, allocate memory using only the completion handler's associated allocator.
— 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]).]
Let Args...
be the argument types of the completion signature
Signature
and let N
be sizeof...(Args)
. Let i
be in the 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)
.
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.