Document number: P0958R2 Date: 2020-08-21 Project: Programming Language C++ Audience: SG4 - Networking Reply-to: Christopher Kohlhoff <chris@kohlhoff.com>
The purpose of this paper is to illustrate the changes to the Networking TS to conform to the proposed Executors TS in P0443R13.
All changes are relative to N4711.
This paper proposes the following changes to the Networking TS:
executor_work_guard
and make_work_guard
, as these have been superseded by the executors proposal's execution::outstanding_work
property.is_executor
type trait.system_executor
and system_context
to conform to the new executors model.executor
and replace it with a type alias for the executors proposal's execution::any_executor
.dispatch
, post
, and defer
in terms of the new executors model.strand
adapter to conform to the new executors model.use_future
completion token to conform to the new executors model.io_context
to conform to the new executors model.In addition, this paper proposes a minor extension to the executors proposal in P0443R13. This extension enables the use of the polymorphic wrapper execution::any_executor
in the Networking TS.
An implementation of the changes below, including a complete implementation of the executors-related specifications P0443R13, P1348R0, and P1393R0, can be found the Asio library at https://github.com/chriskohlhoff/asio and in Boost 1.74.
This implementation has been used to recompile libraries that depend on Asio, and has been tested in a number of applications. Some library modifications were required, but the majority of the applications needed minor or no modifications.
Add a reference to the executors proposal in -5- Namespaces and headers [namespaces]:
-2- Unless otherwise specified, references to other entities described in this Technical Specification are assumed to be qualified with std::experimental::net::v1::
, references to entities described in the C++ standard are assumed to be qualified with std::
, and references to entities described in C++ Extensions for Library Fundamentals are assumed to be qualified with std::experimental::fundamentals_v2::
, and references to entities described in P1393R0 A General Property Customization Mechanism and in P0443R13 A Unified Executors Proposal for C++ are assumed to be qualified with std::
.
executor_work_guard
and make_work_guard
Remove executor_work_guard
from -12.1- Header <experimental/netfwd> synopsis [fwd.decl.synop]:
template<class Executor> class executor_work_guard;
Remove executor_work_guard
and make_work_guard
from -13.1- Header <experimental/executor> synopsis [async.synop]:
template<class Executor> class executor_work_guard; // 13.17, make_work_guard: template<class Executor> executor_work_guard<Executor> make_work_guard(const Executor& ex); template<class ExecutionContext> executor_work_guard<typename ExecutionContext::executor_type> make_work_guard(ExecutionContext& ctx); template<class T> executor_work_guard<associated_executor_t<T>> make_work_guard(const T& t); template<class T, class U> auto make_work_guard(const T& t, U& u) -> decltype(make_work_guard(get_associated_executor(t, forward<U>(u))));
Remove sections -13.16- Class template executor_work_guard [async.exec.work.guard] and -13.17- Function make_work_guard [async.make.work.guard] in their entirety.
is_executor
type traitModify Table 3 - Template parameters and type requirements [summary] as follows:
Executor |
execution::executor concept (P0443R13) |
Remove is_executor
and is_executor_v
from -13.1- Header <experimental/executor> synopsis [async.synop]:
template<class T> struct is_executor; template<class T> constexpr bool is_executor_v = is_executor<T>::value;
Remove section -13.2.2- Executor requirements [async.reqmts.executor] in its entirety.
Modify Table 5 - ExecutionContext requirements [async.reqmts.executioncontext] as follows:
expression | return type | assertion/note pre/post-condition |
---|---|---|
X::executor_type |
Executor (13.2.2) requirementsA type satisfying the execution::executor concept (P0443R13). |
Modify section -13.2.7.8- I/O executor [async.reqmts.async.io.exec] as follows.
-1- An asynchronous operation has an associated executor satisfying the Executor
(13.2.2) requirementsexecution::executor
concept (P0443R13). If not otherwise specified by the asynchronous operation, this associated executor is an object of type system_executor
.
[...]
-3- 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. can_query_v<Executor1, execution::context_t>
shall be true
, and std::query(ex1, execution::context_t)
shall yield a value of type execution_context&
or of type E&
, where E
satisifies the ExecutionContext
(13.2.3) requirements.
Modify section -13.2.7.9- Completion handler executor [async.reqmts.async.handler.exec] as follows.
-1- A completion handler object of type CompletionHandler
has an associated executor satisfying the Executor requirements (13.2.2)execution::executor
concept (P0443R13). The type of this associated executor is associated_executor_t<CompletionHandler, Executor1>
. Let Executor2
be the type associated_executor_t<CompletionHandler, Executor1>
. Let ex2
be a value of type Executor2
obtained by performing get_associated_executor(completion_handler, ex1)
. can_query_v<Executor2, execution::context_t>
shall be true
, and std::query(ex2, execution::context_t)
shall yield a value of type execution_context&
or of type E&
, where E
satisifies the ExecutionContext
(13.2.3) requirements.
Modify section -13.2.7.14- Composed asynchronous operations [async.reqmts.async.composed] as follows.
An intermediate operation's completion handler shall have an associated executor that is either:
Executor2
and object ex2
obtained from the completion handler type CompletionHandler
and object completion_handler
; orexecution::executor
concept (P0443R13), that delegates executor operations to the type Executor2
and object ex2
.Remove section -13.9- Class template is_executor [async.is.exec] in its entirety.
Modify section -13.10- Executor argument tag [async.executor.arg] as follows.
Theexecutor_arg_t
struct is an empty structure type used as a unique type to disambiguate constructor and function overloading. Specifically, types may have constructors with executor_arg_t
as the first argument, immediately followed by an argument of a type that satisfies the execution::executor
concept (P0443R13).
Modify section -13.12- Class template associated_executor [async.assoc.exec] as follows.
-1- Class template associated_executor
is an associator (13.2.6) for the executors, with default candidate type Executor
(13.2.2) type requirementssystem_executor
and default candidate object system_executor()
.
Modify Table 9 - associated_executor specialization requirements as follows:
Expression | Return type | Note |
---|---|---|
typename X::type |
Executor requirements (13.2.2)A type satisfying the execution::executor concept (P0443R13). |
Modify section -13.13- Function get_associated_executor [async.assoc.exec.get] as follows.
-3- Remarks: This function shall not participate in overload resolution unless
is_executor_v<Executor>
is true
is_convertible<Executor&, execution_context&>::value
is false
.
Modify section -13.14- Class template executor_binder [async.exec.binder] as follows.
-1- The class template executor_binder
binds executors to objects. A specialization executor_binder<T, Executor>
binds an executor of type Executor
satisfying the Executor requirements (13.2.2)execution::executor
concept (P0443R13) to an object or function of type T
.
Modify section -13.15- Function bind_executor [async.bind.executor] as follows.
-2- Remarks: This function shall not participate in overload resolution unless
is_executor_v<Executor>
is true
is_convertible<Executor&, execution_context&>::value
is false
.
Modify section -13.22- Function dispatch [async.dispatch] as follows.
-8- Remarks: This function shall not participate in overload resolution unless
is_executor_v<Executor>
is true
is_convertible<Executor&, execution_context&>::value
is false
.
Modify section -13.23- Function post [async.post] as follows.
-8- Remarks: This function shall not participate in overload resolution unless
is_executor_v<Executor>
is true
is_convertible<Executor&, execution_context&>::value
is false
.
Modify section -13.24- Function defer [async.defer] as follows.
-8- Remarks: This function shall not participate in overload resolution unless
is_executor_v<Executor>
is true
is_convertible<Executor&, execution_context&>::value
is false
.
Modify section -13.25- Class template strand [async.strand] as follows.
-1- The class template strand
is a wrapper around an object of type Executor
satisfying the execution::executor
concept (13.2.2P0443R13).
[...]
-2- strand<Executor>
satisfies the Executor
(13.2.2) requirementsexecution::executor
concept (P0443R13).
Modify Table 17 - AsyncReadStream requirements [buffer.stream.reqmts.asyncreadstream] as follows:
operation | type | semantics, pre/post-conditions |
---|---|---|
a.get_executor() |
A type satisfying the Executor requirements (13.2.2)execution::executor concept (P0443R13). |
Returns the associated I/O executor. |
Modify Table 19 - AsyncWriteStream requirements [buffer.stream.reqmts.asyncwritestream] as follows:
operation | type | semantics, pre/post-conditions |
---|---|---|
a.get_executor() |
A type satisfying the Executor requirements (13.2.2)execution::executor concept (P0443R13). |
Returns the associated I/O executor. |
Modify section -13.2.7.10- Outstanding work [async.reqmts.async.work] as follows.
-1- Until the asynchronous operation has completed, the asynchronous operation shall maintain:
work1
of type executor_work_guard<Executor1>
, initialized as work1(ex1)
, and where work1.owns_work() == true
; andwork2
of type executor_work_guard<Executor2>
, initialized as work2(ex2)
, and where work2.owns_work() == true
.work1
, initialized as std::prefer(ex1, execution::outstanding_work.tracked)
; andwork2
, initialized as std::prefer(ex2, execution::outstanding_work.tracked)
.Modify section -13.2.7.12- Execution of completion handler on completion of asynchronous operation [async.reqmts.async.completion] as follows:
-3- If an asynchronous 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)
.
execution::execute( std::prefer( std::require(ex2, execution::blocking.never), execution::allocator(alloc2)), std::move(f));
Otherwise, the completion handler shall be submitted for execution as if by performing:
execution::execute( std::prefer(work2, execution::blocking.possibly, execution::allocator(alloc2)), std::move(f));
system_executor
and system_context
to conform to the new executors modelRemove system_executor
from, and add system_context
to, -12.1- Header <experimental/netfwd> synopsis [fwd.decl.synop]:
class system_executor;class system_context;
Update system_executor
to be a type alias in -13.1- Header <experimental/executor> synopsis [async.synop]:
class system_executor;class system_context; using system_executor = system_context::executor_type;bool operator==(const system_executor&, const system_executor&); bool operator!=(const system_executor&, const system_executor&);
Remove section -13.18- Class system_executor [async.system.exec] in its entirety.
Modify section -13.19- Class system_context [async.system.context] as follows:
-1- Class system_context
implements the execution context associated with an execution context that represents the ability to run a submitted function object on any thread.
system_executor
objects
namespace std { namespace experimental { namespace net { inline namespace v1 { class system_context : public execution_context { public: // types: using executor_type =system_executorsee below;
[...]
-2- The class system_context
satisfies the ExecutionContext
(13.2.3) type requirements.
-?- executor_type
is an executor type conforming to the specification for system_context
executor types described below. Executor objects of type executor_type
have the following properties established:
execution::blocking.possibly
execution::relationship.fork
execution::mapping.thread
execution::allocator(std::allocator<void>())
-?- system_context
executors having a different set of established properties may be represented by distinct, unspecified types. Function objects submitted via a system_context
executor object are permitted to execute on any thread. To satisfy the requirements for the execution::blocking.never
property, a system_context
executor may create thread
objects to run the submitted function objects. These thread
objects are collectively referred to as system threads.
[...]
executor_type get_executor() noexcept;
-5- Returns: system_executor()
executor_type()
.
After section -13.19- Class system_context [async.system.context] insert a new section as follows:
system_context
executor types-1- All executor types accessible through system_context::executor_type()
, system_context::get_executor()
, and subsequent calls to the member function require
, conform to the following specification.
namespace std { namespace experimental { namespace net { inline namespace v1 { class system-context-executor { public: // construct / copy / destroy: system-context-executor() {} // executor operations: see below require(execution::blocking_t::possibly_t) const; see below require(execution::blocking_t::never_t) const; see below require(execution::blocking_t::always_t) const; see below require(execution::relationship_t::fork_t) const; see below require(execution::relationship_t::continuation_t) const; see below require(execution::allocator_t<void>) const; template<class ProtoAllocator> see below require(const execution::allocator_t<ProtoAllocator>& a) const; static constexpr execution::mapping_t query(execution::mapping_t) noexcept; system_context& query(execution::context_t) const noexcept; execution::blocking_t query(execution::blocking_t) const noexcept; execution::relationship_t query(execution::relationship_t) const noexcept; see below query(execution::allocator_t<void>) const noexcept; template<class ProtoAllocator> see below query(const execution::allocator_t<ProtoAllocator>&) const noexcept; template<class Function> void execute(Function&& f) const; }; bool operator==(const system-context-executor& a, const system-context-executor& b) noexcept; bool operator!=(const system-context-executor& a, const system-context-executor& b) noexcept; } // inline namespace v1 } // namespace net } // namespace experimental } // namespace std
-2- system-context-executor
is a type satisfying the execution::executor
concept (P0443R13).
system_context
executor operationssee below require(execution::blocking_t::possibly_t) const; see below require(execution::blocking_t::never_t) const; see below require(execution::blocking_t::always_t) const; see below require(execution::relationship_t::fork_t) const; see below require(execution::relationship_t::continuation_t) const;
-1- Returns: A system_context
executor object of an unspecified type conforming to these specifications, with the requested property established. When the requested property is part of a group that is defined as a mutually exclusive set, any other properties in the group are removed from the returned executor object. All other properties of the returned executor object are identical to those of *this
.
see below require(execution::allocator_t<void>) const;
-2- Returns: require(execution::allocator(std::allocator<void>()))
.
template<class ProtoAllocator> see below require(const execution::allocator_t<ProtoAllocator>& a) const;
-3- Returns: A system_context
executor object of an unspecified type conforming to these specifications, with the execution::allocator_t<ProtoAllocator>
property established such that allocation and deallocation associated with function submission will be performed using a copy of a.value()
. All other properties of the returned executor object are identical to those of *this
.
static constexpr execution::mapping_t query(execution::mapping_t) noexcept;
-4- Returns: true
.
system_context& query(execution::context_t) const;
-5- Returns: A reference to the system_context
object.
execution::blocking_t query(execution::blocking_t) const noexcept; execution::relationship_t query(execution::relationship_t) const noexcept;
-6- Returns: The established value of the property for the executor object *this
.
see below query(execution::allocator_t<void>) const noexcept; template<class ProtoAllocator> see below query(const execution::allocator_t<ProtoAllocator>&) const noexcept;
-7- Returns: The allocator object associated with the executor, with type and value as previously established by the execution::allocator_t<ProtoAllocator>
property.
template<class Function> void execute(Function&& f) const
-8- Effects: Submits the function f
for execution according to the execution::executor
concept and the properties established for *this
. If f
exits via an exception, calls std::terminate()
.
system_context
executor comparisonsbool operator==(const system-context-executor& a, const system-context-executor& b) noexcept;
-1- Returns: true
if a
and b
have identical properties, otherwise false
.
bool operator!=(const system-context-executor& a, const system-context-executor& b) noexcept;
-2- Returns: !(a == b)
.
executor
and replace it with a type aliasRemove executor
, and add a new type alias any_io_executor
, in -12.1- Header <experimental/netfwd> synopsis [fwd.decl.synop]:
class executor;namespace execution { template<class... SupportableProperties> class any_executor; template<class T> struct context_as_t; template<class T> struct prefer_only; } // namespace execution using any_io_executor = execution::any_executor< execution::context_as_t<execution_context&>, execution::blocking_t::never_t, execution::prefer_only<execution::blocking_t::possibly_t>, execution::prefer_only<execution::outstanding_work::untracked_t>, execution::prefer_only<execution::outstanding_work::tracked_t>, execution::prefer_only<execution::relationship_t::fork_t>, execution::prefer_only<execution::relationship_t::continuation_t>>;
Assuming the changes in P1322 have been applied, use any_io_executor
as the default executor in -12.1- Header <experimental/netfwd> synopsis [fwd.decl.synop]:
template<class Clock> struct wait_traits; template<class Clock, class WaitTraits = wait_traits<Clock>, class Executor = any_io_executor> class basic_waitable_timer; using system_timer = basic_waitable_timer<chrono::system_clock>; using steady_timer = basic_waitable_timer<chrono::steady_clock>; using high_resolution_timer = basic_waitable_timer<chrono::high_resolution_clock>; template<class Protocol, class Executor = any_io_executor> class basic_socket; template<class Protocol, class Executor = any_io_executor> class basic_datagram_socket; template<class Protocol, class Executor = any_io_executor> class basic_stream_socket; template<class Protocol, class Executor = any_io_executor> class basic_socket_acceptor; template<class Protocol, class Clock = chrono::steady_clock, class WaitTraits = wait_traits<Clock>, class Executor = any_io_executor> class basic_socket_streambuf; template<class Protocol, class Clock = chrono::steady_clock, class WaitTraits = wait_traits<Clock>, class Executor = any_io_executor> class basic_socket_iostream; namespace ip {
[...]
template<class InternetProtocol, class Executor = any_io_executor> class basic_resolver;
Remove classes bad_executor
and executor
, and add a new type alias any_io_executor
, in -13.1- Header <experimental/executor> synopsis [async.synop]:
class bad_executor; class executor; bool operator==(const executor& a, const executor& b) noexcept; bool operator==(const executor& e, nullptr_t) noexcept; bool operator==(nullptr_t, const executor& e) noexcept; bool operator!=(const executor& a, const executor& b) noexcept; bool operator!=(const executor& e, nullptr_t) noexcept; bool operator!=(nullptr_t, const executor& e) noexcept;using any_io_executor = execution::any_executor< execution::context_as_t<execution_context&>, execution::blocking_t::never_t, execution::prefer_only<execution::blocking_t::possibly_t>, execution::prefer_only<execution::outstanding_work::untracked_t>, execution::prefer_only<execution::outstanding_work::tracked_t>, execution::prefer_only<execution::relationship_t::fork_t>, execution::prefer_only<execution::relationship_t::continuation_t>>;
[...]
} // inline namespace v1 } // namespace net } // namespace experimentaltemplate<class Allocator> struct uses_allocator<experimental::net::v1::executor, Allocator> : true_type {};} // namespace std
Remove sections -13.20- Class bad_executor [async.bad.exec] and -13.21- Class executor [async.executor] in their entirety.
Assuming the changes in P1322 have been applied, use any_io_executor
as the default executor in -15.1- Header <experimental/timer> synopsis [timer.synop]:
template<class Clock, class WaitTraits = wait_traits<Clock>, class Executor = any_io_executor> class basic_waitable_timer;
Assuming the changes in P1322 have been applied, use any_io_executor
as the default executor in -18.1- Header <experimental/socket> synopsis [socket.synop]:
// Sockets: class socket_base; template<class Protocol, class Executor = any_io_executor> class basic_socket; template<class Protocol, class Executor = any_io_executor> class basic_datagram_socket; template<class Protocol, class Executor = any_io_executor> class basic_stream_socket; template<class Protocol, class Executor = any_io_executor> class basic_socket_acceptor; // [socket.iostreams], Socket streams: template<class Protocol, class Clock = chrono::steady_clock, class WaitTraits = wait_traits<Clock>, class Executor = any_io_executor> class basic_socket_streambuf; template<class Protocol, class Clock = chrono::steady_clock, class WaitTraits = wait_traits<Clock>, class Executor = any_io_executor> class basic_socket_iostream;
Assuming the changes in P1322 have been applied, use any_io_executor
as the default executor in -21.1- Header <experimental/internet> synopsis [internet.synop]:
template<class InternetProtocol, class Executor = any_io_executor> class basic_resolver;
dispatch
, post
, and defer
in terms of the new executors modelModify section -13.22- Function dispatch [async.dispatch] as follows:
-1- [Note: The function dispatch
satisfies the requirements for an asynchronous operation (13.2.7), except for the requirement that the operation uses the post
execution::blocking.never
property if it completes immediately. --end note]
[...]
-3- Effects:
completion
of type async_completion<CompletionToken, void()>
, initialized with token
.ex.dispatch(std::move(completion.completion_handler), alloc)
, where ex
is the result of get_associated_executor(completion.completion_handler)
, and alloc
is the result of get_associated_allocator(completion.completion_handler)
.execution::execute( std::prefer(ex, execution::blocking.possibly, execution::allocator(alloc)), std::move(completion.completion_handler));where
ex
is the result of get_associated_executor(completion.completion_handler)
, and alloc
is the result of get_associated_allocator(completion.completion_handler)
.[...]
-6- Effects:
completion
of type async_completion<CompletionToken, void()>
, initialized with token
.f
containing as members:
w
for the completion handler's associated executor, initialized with std::prefer(get_associated_executor(h), execution::outstanding_work)
,h
, initialized with std::move(completion.completion_handler)
,executor_work_guard
object w
for the completion handler's associated executor, initialized with make_work_guard(h)
,f()
is:
w.get_executor().dispatch(std::move(h), alloc)
, where alloc
is the result of get_associated_allocator(h)
, followed byw.reset()
.execution::execute( std::prefer(w, execution::blocking.possibly, execution::allocator(alloc)), std::move(h));where
alloc
is the result of get_associated_allocator(h)
.ex.dispatch(std::move(f), alloc)
, where alloc
is the result of get_associated_allocator(completion.completion_handler)
prior to the construction of f
.execution::execute( std::prefer(ex, execution::blocking.possibly, execution::allocator(alloc)), std::move(f));
Modify section -13.23- Function post [async.post] as follows:
-3- Effects:
completion
of type async_completion<CompletionToken, void()>
, initialized with token
.ex.post(std::move(completion.completion_handler), alloc)
, where ex
is the result of get_associated_executor(completion.completion_handler)
, and alloc
is the result of get_associated_allocator(completion.completion_handler)
.execution::execute( std::prefer( std::require(ex, execution::blocking.never), execution::relationship.fork, execution::allocator(alloc)), std::move(completion.completion_handler));where
ex
is the result of get_associated_executor(completion.completion_handler)
, and alloc
is the result of get_associated_allocator(completion.completion_handler)
.[...]
-6- Effects:
completion
of type async_completion<CompletionToken, void()>
, initialized with token
.f
containing as members:
w
for the completion handler's associated executor, initialized with std::prefer(get_associated_executor(h), execution::outstanding_work)
,h
, initialized with std::move(completion.completion_handler)
,executor_work_guard
object w
for the completion handler's associated executor, initialized with make_work_guard(h)
,f()
is:
w.get_executor().dispatch(std::move(h), alloc)
, where alloc
is the result of get_associated_allocator(h)
, followed byw.reset()
.execution::execute( std::prefer(w, execution::blocking.possibly, execution::allocator(alloc)), std::move(h));where
alloc
is the result of get_associated_allocator(h)
.ex.dispatch(std::move(f), alloc)
, where alloc
is the result of get_associated_allocator(completion.completion_handler)
prior to the construction of f
.execution::execute( std::prefer( std::require(ex, execution::blocking.never), execution::relationship.fork, execution::allocator(alloc)), std::move(f));
Modify section -13.24- Function defer [async.defer] as follows:
-1- [Note: The function defer
satisfies the requirements for an asynchronous operation (13.2.7), except for the requirement that the operation uses . --end note]post
if it completes immediately
[...]
-3- Effects:
completion
of type async_completion<CompletionToken, void()>
, initialized with token
.ex.defer(std::move(completion.completion_handler), alloc)
, where ex
is the result of get_associated_executor(completion.completion_handler)
, and alloc
is the result of get_associated_allocator(completion.completion_handler)
.execution::execute( std::prefer( std::require(ex, execution::blocking.never), execution::relationship.continuation, execution::allocator(alloc)), std::move(completion.completion_handler));where
ex
is the result of get_associated_executor(completion.completion_handler)
, and alloc
is the result of get_associated_allocator(completion.completion_handler)
.[...]
-6- Effects:
completion
of type async_completion<CompletionToken, void()>
, initialized with token
.f
containing as members:
w
for the completion handler's associated executor, initialized with std::prefer(get_associated_executor(h), execution::outstanding_work)
,h
, initialized with std::move(completion.completion_handler)
,executor_work_guard
object w
for the completion handler's associated executor, initialized with make_work_guard(h)
,f()
is:
w.get_executor().dispatch(std::move(h), alloc)
, where alloc
is the result of get_associated_allocator(h)
, followed byw.reset()
.execution::execute( std::prefer(w, execution::blocking.possibly, execution::allocator(alloc)), std::move(h));where
alloc
is the result of get_associated_allocator(h)
.ex.dispatch(std::move(f), alloc)
, where alloc
is the result of get_associated_allocator(completion.completion_handler)
prior to the construction of f
.execution::execute( std::prefer( std::require(ex, execution::blocking.never), execution::relationship.continuation, execution::allocator(alloc)), std::move(f));
strand
adapter to conform to the new executors modelModify section -13.25- Class template strand [async.strand] as follows:
namespace std { namespace experimental { namespace net { inline namespace v1 { template<class Executor> class strand { public:
[...]
execution_context& context() const noexcept; void on_work_started() const noexcept; void on_work_finished() const noexcept; template<class Func, class ProtoAllocator> void dispatch(Func&& f, const ProtoAllocator& a) const; template<class Func, class ProtoAllocator> void post(Func&& f, const ProtoAllocator& a) const; template<class Func, class ProtoAllocator> void defer(Func&& f, const ProtoAllocator& a) const;template<class Property> see below query(const Property& p) const; template<class Property> see below require(const Property& p) const; template<class Property> see below prefer(const Property& p) const; template<class Function> void execute(Function&& f) const;
[...]
-3- A strand provides guarantees of ordering and non-concurrency. Given:
s1
and s2
such that s1 == s2
f1
s1
post
or defer
, or using dispatch
execution::blocking.never
property is established in s1
, or when s1.running_in_this_thread()
is false
f2
s2
post
or defer
, or using dispatch
execution::blocking.never
property is established in s2
, or when s2.running_in_this_thread()
is false
Modify section -13.25.4- strand operations [async.strand.ops] as follows:
bool running_in_this_thread() const noexcept;
-2- Returns: true
if the current thread of execution is running a function that was submitted to the strand, or to any other strand object that shares the same ordered, non-concurrent state, using s
such that s == *this
, using dispatch
, post
or defer
execute
; otherwise false
. [Note: That is, the current thread of execution's call chain includes a function that was submitted to the strand. --end note]
void on_work_started() const noexcept;
-4- Effects: Calls inner_ex_.on_work_started()
.
void on_work_finished() const noexcept;
-5- Effects: Calls inner_ex_.on_work_finished()
.
template<class Func, class ProtoAllocator> void dispatch(Func&& f, const ProtoAllocator& a) const;
-6- Effects: If running_in_this_thread()
is true, calls DECAY_COPY(forward<Func>(f))()
(C++ 2014 [thread.decaycopy]). [Note: If f
exits via an exception, the exception propagates to the caller of dispatch()
. --end note] Otherwise, requests invocation of f
, as if by forwarding the function object f
and allocator a
to the executor inner_ex_
, such that the guarantees of ordering and non-concurrency are met.
template<class Func, class ProtoAllocator> void post(Func&& f, const ProtoAllocator& a) const;
-7- Effects: Requests invocation of f
, as if by forwarding the function object f
and allocator a
to the executor inner_ex_
, such that the guarantees of ordering and non-concurrency are met.
-8- Effects: Requests invocation of f
, as if by forwarding the function object f
and allocator a
to the executor inner_ex_
, such that the guarantees of ordering and non-concurrency are met.
template<class Property> see below query(const Property& p) const;
-?- Returns: std::query(inner_ex_, p)
.
-?- Remarks: Shall not participate in overload resolution unless can_query_v<const Executor&, const Property&>
is true
.
template<class Property> see below require(const Property& p) const;
-?- Returns: A strand s
of type strand<decay_t<decltype(std::require(inner_ex_, p))>>
, where s.inner_ex_
is initialized with std::require(inner_ex_, p)
, and sharing the same ordered, non-concurrent state as *this
.
-?- Remarks: Shall not participate in overload resolution unless can_require_v<const Executor&, const Property&>
is true
.
template<class Property> see below prefer(const Property& p) const;
-?- Returns: A strand s
of type strand<decay_t<decltype(std::prefer(inner_ex_, p))>>
, where s.inner_ex_
is initialized with std::prefer(inner_ex_, p)
, and sharing the same ordered, non-concurrent state as *this
.
-?- Remarks: Shall not participate in overload resolution unless can_prefer_v<const Executor&, const Property&>
is true
.
template<class Function> void execute(Function&& f) const;
-?- Effects: Submits f
to the executor inner_ex_
, such that the guarantees of ordering and non-concurrency are met.
use_future
completion token to conform to the new executors modelModify section -13.26.2- use_future_t members [async.use.future.members] as follows:
-8- For any executor type E
, the associated object for the associator associated_executor<H, E>
is an executor where, for function objects executed using the executor's dispatch()
, post()
or defer()
functionsexecute()
function, any exception thrown is caught by a function object and stored in the associated shared state.
Modify section -13.26.3- Partial class template specialization async_result for use_future_t [async.use.future.result] as follows:
-3- The implementation specializes associated_executor
for F
. For function objects executed using the associated executor's dispatch()
, post()
or defer()
functionsexecute()
function, any exception thrown is caught by the executor and stored in the associated shared state.
-4- For any executor type E
, the associated object for the associator associated_executor<F, E>
is an executor where, for function objects executed using the executor's dispatch()
, post()
or defer()
functionsexecute()
function, any exception thrown by a function object is caught by the executor and stored in the associated shared state.
io_context
to conform to the new executors modelModify section -14.2- Class io_context [io_context.io_context] as follows:
namespace std { namespace experimental { namespace net { inline namespace v1 { class io_context : public execution_context { public: // types:class executor_type;using executor_type = see below;
[...]
-1- The class io_context
satisfies the ExecutionContext type requirements (13.2.3).
-?- executor_type
is an executor type conforming to the specification for io_context
executor types described below.
[...]
-4- For an object of type io_context
, outstanding work is defined as the sum of:
on_work_started
function, less the total number of calls to the on_work_finished
function, to any executor of the io_context
.io_context
for which the execution::outstanding_work.tracked
property is established;io_context
via any executor of the io_context
, but not yet executed; andio_context
.[...]
executor_type get_executor() noexcept;
-3- Returns: An executor that may be used for submitting function objects to the io_context
. The returned executor has the following properties already established:
execution::blocking.possibly
execution::relationship.fork
execution::outstanding_work.untracked
execution::mapping.thread
execution::allocator(std::allocator<void>())
-?- io_context
executors having a different set of established properties may be represented by distinct, unspecified types.
[...]
-13- Remarks: This function may invoke additional function objects through nested calls to the io_context
executor's dispatch
execute
member function. These do not count towards the return value.
[...]
-19- Remarks: This function may invoke additional function objects through nested calls to the io_context
executor's dispatch
execute
member function. These do not count towards the return value.
[...]
-25- Remarks: This function may invoke additional function objects through nested calls to the io_context
executor's dispatch
execute
member function. These do not count towards the return value.
Remove section -14.3- Class io_context::executor_type [io_context.exec] in its entirety.
After section -14.2- Class io_context [io_context.io_context] insert a new section as follows:
io_context
executor typesAll executor types accessible through io_context::get_executor()
, and subsequent calls to the member function require
, conform to the following specification.
namespace std { namespace experimental { namespace net { inline namespace v1 { class io-context-executor { public: // construct / copy / destroy: io-context-executor(const io-context-executor& other) noexcept; io-context-executor(io-context-executor&& other) noexcept; io-context-executor& operator=(const io-context-executor& other) noexcept; io-context-executor& operator=(io-context-executor&& other) noexcept; // executor operations: see below require(execution::blocking_t::possibly_t) const; see below require(execution::blocking_t::never_t) const; see below require(execution::relationship_t::fork_t) const; see below require(execution::relationship_t::continuation_t) const; see below require(execution::outstanding_work_t::untracked_t) const; see below require(execution::outstanding_work_t::tracked_t) const; see below require(execution::allocator_t<void>) const; template<class ProtoAllocator> see below require(const execution::allocator_t<ProtoAllocator>& a) const; static constexpr execution::mapping_t query(execution::mapping_t) noexcept; io_context& query(execution::context_t) const noexcept; execution::blocking_t query(execution::blocking_t) const noexcept; execution::relationship_t query(execution::relationship_t) const noexcept; execution::outstanding_work_t query(execution::outstanding_work_t) const noexcept; see below query(execution::allocator_t<void>) const noexcept; template<class ProtoAllocator> see below query(const execution::allocator_t<ProtoAllocator>&) const noexcept; bool running_in_this_thread() const noexcept; template<class Function> void execute(Function&& f) const; }; bool operator==(const io-context-executor& a, const io-context-executor& b) noexcept; bool operator!=(const io-context-executor& a, const io-context-executor& b) noexcept; } // inline namespace v1 } // namespace net } // namespace experimental } // namespace std
-1- io-context-executor
is a type satisfying the execution::executor
concept (P0443R13). Objects of type io-context-executor
are associated with an io_context
, and function objects submitted using the execute
member function will be executed by the io_context
from within a run function.
io_context
executor constructorsio-context-executor(const io-context-executor& other) noexcept;
-1- Postconditions: *this == other
.
io-context-executor(io-context-executor&& other) noexcept;
-2- Postconditions: *this
is equal to the prior value of other
.
io_context
executor assignmentio-context-executor& operator=(const io-context-executor& other) noexcept;
-1- Postconditions: *this == other
.
-2- Returns: *this
.
io-context-executor& operator=(io-context-executor&& other) noexcept;
-3- Postconditions: *this
is equal to the prior value of other
.
-4- Returns: *this
.
io_context
executor operationssee below require(execution::blocking_t::possibly_t) const; see below require(execution::blocking_t::never_t) const; see below require(execution::relationship_t::fork_t) const; see below require(execution::relationship_t::continuation_t) const; see below require(execution::outstanding_work_t::untracked_t) const; see below require(execution::outstanding_work_t::tracked_t) const;
-1- Returns: An executor object of an unspecified type conforming to these specifications, associated with the same io_context
as *this
, and having the requested property established. When the requested property is part of a group that is defined as a mutually exclusive set, any other properties in the group are removed from the returned executor object. All other properties of the returned executor object are identical to those of *this
.
see below require(execution::allocator_t<void>) const;
-2- Returns: require(execution::allocator(std::allocator<void>()))
.
template<class ProtoAllocator> see below require(const execution::allocator_t<ProtoAllocator>& a) const;
-3- Returns: An executor object of an unspecified type conforming to these specifications, associated with the same io_context
as *this
, with the execution::allocator_t<ProtoAllocator>
property established such that allocation and deallocation associated with function submission will be performed using a copy of a.value()
. All other properties of the returned executor object are identical to those of *this
.
static constexpr execution::mapping_t query(execution::execution::mapping_t) noexcept;
-4- Returns: true
.
io_context& query(execution::context_t) const noexcept;
-5- Returns: A reference to the associated io_context
object.
execution::blocking_t query(execution::blocking_t) const noexcept; execution::relationship_t query(execution::relationship_t) const noexcept; execution::outstanding_work_t query(execution::outstanding_work_t) const noexcept;
-6- Returns: The established value of the property for the executor object *this.
see below query(execution::allocator_t<void>) const noexcept; template<class ProtoAllocator> see below query(const execution::allocator_t<ProtoAllocator>&) const noexcept;
-7- Returns: The allocator object associated with the executor, with type and value as previously established by the execution::allocator_t<ProtoAllocator>
property.
bool running_in_this_thread() const noexcept;
-8- Returns: true
if the current thread of execution is calling a run function of the associated io_context
object. [Note: That is, the current thread of execution's call chain includes a run function. --end note]
template<class Function> void execute(Function&& f) const
-9- Effects: Submits the function f
for execution on the io_context
according to the execution::executor
concept and the properties established for *this
. If f
exits via an exception, the exception does not propagate to the caller of execute()
, but is instead subsequently propagated to a caller of a run function for the io_context
object.
io_context
executor comparisonsbool operator==(const io-context-executor& a, const io-context-executor& b) noexcept;
-1- Returns: true
if addressof(a.query(execution::context_t)) == addressof(b.query(execution::context_t))
, and a
and b
have identical properties, otherwise false
.
bool operator!=(const io-context-executor& a, const io-context-executor& b) noexcept;
-2- Returns: !(a == b)
.
The asynchronous operation requirements specified above stipulate that querying the execution::context_t
property for Networking TS executors returns a type of either execution_context&
, or of E&
where E
is a type that is unambiguously derived from execution_context
.
However, the execution::context_t
property as currently specified in P0443 uses std::any
as its polymorphic_query_result_type
. This prevents the polymorphic wrapper execution::any_io_executor
from satisfying the above requirements.
For this reason, this paper proposes the following extension to P0443R13.
Add the following context_as_t<>
property adapter. This adapter allows us to expose the execution::context_t
property through the polymorphic wrapper using a polymorphic query type of our choosing.
context_as_t
-?- The context_as_t
struct is a property adapter for the context_t
property, to specify a polymorphic_query_result_type
.
-?- [Example: context_as_t
may be used with the polymorphic wrapper executor
to expose the query-only property context_t
using a suitable polymorphic type:
static_thread_pool pool; execution::any_executor< execution::context_as_t<static_thread_pool&> > my_executor(pool.executor()); // ... static_thread_pool& ctx = std::query(my_executor, execution::context);
--end example]
template<class T> struct context_as_t { template <typename U> static constexpr bool is_applicable_property_v = executor<U> || sender<U> || scheduler<U>; static constexpr bool is_requirable = false; static constexpr bool is_preferable = false; using polymorphic_query_result_type = T; template<class Executor> static constexpr T static_query_v = context_t::static_query_v<Executor>; constexpr context_as_t() {} constexpr context_as_t(execution::context_t) {} template<class Executor, class Property> friend constexpr T query(const Executor& ex, const Property& p) noexcept(noexcept(std::query(ex, execution::context))) -> decltype(std::query(ex, execution::context)); }; template<class T> constexpr context_as_tcontext_as;
-?- The expression context_as_t<T>::static_query_v<E>
is well-formed for some executor type E
if and only if the expression context_t::static_query_v<E>
is well-formed and can be used to initialize a constant of type T
.
template<class Executor, class Property> friend constexpr T query(const Executor& ex, const Property& p) noexcept(noexcept(std::query(ex, execution::context)))
-?- Returns: std::query(ex, p.property)
.
-?- Remarks: Shall not participate in overload resolution unless std::is_same_v<Property, context_as_t>
is true
, and the expression std::query(ex, execution::context)
is well-formed.
The following changes were made in revision 2 of this paper:
std
rather than std::execution
.context()
member function to I/O objects.any_io_executor
.any_io_executor
to the forward declaration header <netfwd>
.any_io_executor
as the default, assuming P1322 changes have been applied.context_as
to context_as_t
, and added template variable context_as
.system_context
executor specification based on implementation experience.io_context
executor specification based on implementation experience.The following changes were made in revision 1 of this paper:
system_context
executors use distinct types when different properties are established.system_context
executors.strand
to conditionally forward static query()
calls to the adapted executor.io_context
executors use distinct types when different properties are established.io_context
executors.execution::context_as<>
property adapter.