0.1 Changelog

0.1.1 Revision 11

As directed by SG1 at the 2019-07 Cologne meeting, we have implemented the following changes suggested by P1658 and P1660 which incorporate "lazy" execution:

• Eliminated all interface-changing properties.
• Introduced set_value, set_error, set_done, execute, submit, and bulk_execute customization point objects.
• Introduced executor, executor_of, receiver, receiver_of, sender, sender_to, typed_sender, and scheduler concepts.
• Renamed polymorphic executor to any_executor.
• Introduced invocable_archetype.
• Eliminated OneWayExecutor and BulkOneWayExecutor requirements.
• Eliminated is_executor, is_oneway_executor, and is_bulk_oneway_executor type traits.
• Eliminated interface-changing properties from any_executor.

0.1.2 Revision 10

As directed by LEWG at the 2018-11 San Diego meeting, we have migrated the property customization mechanism to namespace std and moved all of the details of its specification to a separate paper, P1393. This change also included the introduction of a separate customization point for interface-enforcing properties, require_concept. The generalization also necessitated the introduction of is_applicable_property_v in the properties paper, which in turn led to the introduction of is_executor_v to express the applicability of properties in this paper.

0.1.3 Revision 9

As directed by the SG1/LEWG straw poll taken during the 2018 Bellevue executors meeting, we have separated The Unified Executors programming model proposal into two papers. This paper contains material related to one-way execution which the authors hope to standardize with C++20 as suggested by the Bellevue poll. P1244 contains remaining material related to dependent execution. We expect P1244 to evolve as committee consensus builds around a design for dependent execution.

This revision also contains bug fixes to the allocator_t property which were originally scheduled for Revision 7 but were inadvertently omitted.

0.1.4 Revision 8

Revision 8 of this proposal makes interface-changing properties such as oneway mutually exclusive in order to simplify implementation requirements for executor adaptors such as polymorphic executors. Additionally, this revision clarifies wording regarding execution agent lifetime.

0.1.5 Revision 7

Revision 7 of this proposal corrects wording bugs discovered by the authors after Revision 6's publication.

• Enhanced static_query_v to result in a default property value for executors which do not provide a query function for the property of interest
• Revise then_execute and bulk_then_execute's operational semantics to allow user functions to handle incoming exceptions thrown by preceding execution agents
• Introduce exception_arg to disambiguate the user function's exceptional overload from its nonexceptional overload in then_execute and bulk_then_execute

0.1.6 Revision 6

Revision 6 of this proposal corrects bugs and omissions discovered by the authors after Revision 5's publication, and introduces an enhancement improving the safety of the design.

• Enforce mutual exclusion of behavioral properties via the type system instead of via convention
• Introduce missing execution::require adaptations
• Allow executors to opt-out of invoking factory functions when appropriate
• Various bug fixes and corrections

0.1.7 Revision 5

Revision 5 of this proposal responds to feedback requested during the 2017 Albuquerque ISO C++ Standards Committee meeting and introduces changes which allow properties to better interoperate with polymorphic executor wrappers and also simplify execution::require's behavior.

• Defined general property type requirements
• Elaborated specification of standard property types
• Simplified execution::require's specification
• Enhanced polymorphic executor wrapper
  • Templatized execution::executor<SupportableProperties...>
  • Introduced prefer_only property adaptor
• Responded to Albuquerque feedback
  • From SG1
    • Execution contexts are now optional properties of executors
    • Eliminated ill-specified caller-agent forward progress properties
    • Elaborated Future's requirements to incorporate forward progress
    • Reworded operational semantics of execution functions to use similar language as the blocking properties
    • Elaborated static_thread_pool's specification to guarantee that threads in the bool boost-block their work
    • Elaborated operational semantics of execution functions to note that forward progress guarantees are specific to the concrete executor type
  • From LEWG
    • Eliminated named BaseExecutor concept
    • Simplified general executor requirements
    • Enhanced the OneWayExecutor introductory paragraph
    • Eliminated has_*_member type traits
• Minor changes
  • Renamed TS namespace from concurrency_v2 to executors_v1
  • Introduced static_query_v enabling static queries
  • Eliminated unused property_value trait
  • Eliminated the names allocator_wrapper_t and default_allocator

0.1.8 Revision 4

• Specified the guarantees implied by bulk_sequenced_execution, bulk_parallel_execution, and bulk_unsequenced_execution

0.1.9 Revision 3

• Introduced execution::query() for executor property introspection
• Simplified the design of execution::prefer()
• oneway, twoway, single, and bulk are now require()-only properties
• Introduced properties allowing executors to opt into adaptations that add blocking semantics
• Introduced properties describing the forward progress relationship between caller and agents
• Various minor improvements to existing functionality based on prototyping

0.1.10 Revision 2

• Separated wording from explanatory prose, now contained in paper P0761
• Applied the simplification proposed by paper P0688

0.1.11 Revision 1

• Executor category simplification
• Specified executor customization points in detail
• Introduced new fine-grained executor type traits
  • Detectors for execution functions
  • Traits for introspecting cross-cutting concerns
    • Introspection of mapping of agents to threads
    • Introspection of execution function blocking behavior
• Allocator support for single agent execution functions
• Renamed thread_pool to static_thread_pool
• New introduction

0.1.12 Revision 0

• Initial design

1 Proposed Wording

1.1 Execution Support Library

1.1.1 General

This Clause describes components supporting execution of function objects [function.objects].

(The following definition appears in working draft N4762 [thread.req.lockable.general])

An execution agent is an entity such as a thread that may perform work in parallel with other execution agents. [Note: Implementations or users may introduce other kinds of agents such as processes or thread-pool tasks. --end note] The calling agent is determined by context; e.g., the calling thread that contains the call, and so on.

An execution agent invokes a function object within an execution context such as the calling thread or thread-pool. An executor submits a function object to an execution context to be invoked by an execution agent within that execution context. [Note: Invocation of the function object may be inlined such as when the execution context is the calling thread, or may be scheduled such as when the execution context is a thread-pool with task scheduler. --end note] An executor may submit a function object with execution properties that specify how the submission and invocation of the function object interacts with the submitting thread and execution context, including forward progress guarantees [intro.progress].

For the intent of this library and extensions to this library, the lifetime of an execution agent begins before the function object is invoked and ends after this invocation completes, either normally or having thrown an exception.

### Header <execution> synopsis

namespace std {
namespace execution {

  // Exception types:

  extern runtime_error const invocation-error; // exposition only
  struct receiver_invocation_error : runtime_error, nested_exception {
    receiver_invocation_error() noexcept
      : runtime_error(invocation-error), nested_exception() {}
  };

  // Invocable archetype

  using invocable_archetype = unspecified;

  // Customization points:

  inline namespace unspecified{
    inline constexpr unspecified set_value = unspecified;

    inline constexpr unspecified set_done = unspecified;

    inline constexpr unspecified set_error = unspecified;

    inline constexpr unspecified execute = unspecified;

    inline constexpr unspecified submit = unspecified;

    inline constexpr unspecified schedule = unspecified;

    inline constexpr unspecified bulk_execute = unspecified;
  }

  // Concepts:

  template<class T, class E = exception_ptr>
    concept receiver = see-below;

  template<class T, class... An>
    concept receiver_of = see-below;

  template<class S>
    concept sender = see-below;

  template<class S>
    concept typed_sender = see-below;

  template<class S, class R>
    concept sender_to = see-below;

  template<class S>
    concept scheduler = see-below;

  template<class E>
    concept executor = see-below;

  template<class E, class F>
    concept executor_of = see-below;

  // Sender and receiver utilities type
  class sink_receiver;

  template<class S> struct sender_traits;

  // Associated execution context property:

  struct context_t;

  constexpr context_t context;

  // Blocking properties:

  struct blocking_t;

  constexpr blocking_t blocking;

  // Properties to allow adaptation of blocking and directionality:

  struct blocking_adaptation_t;

  constexpr blocking_adaptation_t blocking_adaptation;

  // Properties to indicate if submitted tasks represent continuations:

  struct relationship_t;

  constexpr relationship_t relationship;

  // Properties to indicate likely task submission in the future:

  struct outstanding_work_t;

  constexpr outstanding_work_t outstanding_work;

  // Properties for bulk execution guarantees:

  struct bulk_guarantee_t;

  constexpr bulk_guarantee_t bulk_guarantee;

  // Properties for mapping of execution on to threads:

  struct mapping_t;

  constexpr mapping_t mapping;

  // Memory allocation properties:

  template <typename ProtoAllocator>
  struct allocator_t;

  constexpr allocator_t<void> allocator;

  // Executor type traits:

  template<class Executor> struct executor_shape;
  template<class Executor> struct executor_index;

  template<class Executor> using executor_shape_t = typename executor_shape<Executor>::type;
  template<class Executor> using executor_index_t = typename executor_index<Executor>::type;

  // Polymorphic executor support:

  class bad_executor;

  template <class... SupportableProperties> class any_executor;

  template<class Property> struct prefer_only;

} // namespace execution
} // namespace std

1.2 Requirements

1.2.1 ProtoAllocator requirements

A type A meets the ProtoAllocator requirements if A is CopyConstructible (C++Std [copyconstructible]), Destructible (C++Std [destructible]), and allocator_traits<A>::rebind_alloc<U> meets the allocator requirements (C++Std [allocator.requirements]), where U is an object type. [Note: For example, std::allocator<void> meets the proto-allocator requirements but not the allocator requirements. --end note] No comparison operator, copy operation, move operation, or swap operation on these types shall exit via an exception.

1.2.2 Invocable archetype

The name execution::invocable_archetype is an implementation-defined type that, along with any argument pack, models invocable.

A program that creates an instance of execution::invocable_archetype is ill-formed.

1.2.3 Customization points

1.2.3.1 execution::set_value

The name execution::set_value denotes a customization point object. The expression execution::set_value(R, Vs...) for some subexpressions R and Vs... is expression-equivalent to:

• R.set_value(Vs...), if that expression is valid. If the function selected does not send the value(s) Vs... to the receiver R's value channel, the program is ill-formed with no diagnostic required.

• Otherwise, set_value(R, Vs...), if that expression is valid, with overload resolution performed in a context that includes the declaration

  void set_value();

  and that does not include a declaration of execution::set_value. If the function selected by overload resolution does not send the value(s) Vs... to the receiver R's value channel, the program is ill-formed with no diagnostic required.

• Otherwise, execution::set_value(R, Vs...) is ill-formed.

[Editorial note: We should probably define what "send the value(s) Vs... to the receiver R's value channel" means more carefully. --end editorial note]

1.2.3.2 execution::set_done

The name execution::set_done denotes a customization point object. The expression execution::set_done(R) for some subexpression R is expression-equivalent to:

• R.set_done(), if that expression is valid. If the function selected does not signal the receiver R's done channel, the program is ill-formed with no diagnostic required.

• Otherwise, set_done(R), if that expression is valid, with overload resolution performed in a context that includes the declaration

  void set_done();

  and that does not include a declaration of execution::set_done. If the function selected by overload resolution does not signal the receiver R's done channel, the program is ill-formed with no diagnostic required.

• Otherwise, execution::set_done(R) is ill-formed.

[Editorial note: We should probably define what "signal receiver R's done channel" means more carefully. --end editorial note]

1.2.3.3 execution::set_error

The name execution::set_error denotes a customization point object. The expression execution::set_error(R, E) for some subexpressions R and E are expression-equivalent to:

• R.set_error(E), if that expression is valid. If the function selected does not send the error E to the receiver R's error channel, the program is ill-formed with no diagnostic required.

• Otherwise, set_error(R, E), if that expression is valid, with overload resolution performed in a context that includes the declaration

  void set_error();

  and that does not include a declaration of execution::set_error. If the function selected by overload resolution does not send the error E to the receiver R's error channel, the program is ill-formed with no diagnostic required.

• Otherwise, execution::set_error(R, E) is ill-formed.

[Editorial note: We should probably define what "send the error E to the receiver R's error channel" means more carefully. --end editorial note]

1.2.3.4 execution::execute

The name execution::execute denotes a customization point object.

For some subexpressions e and f, let E be a type such that decltype((e)) is E and let F be a type such that decltype((f)) is F. The expression execution::execute(e, f) is ill-formed if F does not model invocable, or if E does not model either executor or sender. Otherwise, it is expression-equivalent to:

• e.execute(f), if that expression is valid. If the function selected does not execute the function object f on the executor e, the program is ill-formed with no diagnostic required.

• Otherwise, execute(e, f), if that expression is valid, with overload resolution performed in a context that includes the declaration

  void execute();

  and that does not include a declaration of execution::execute. If the function selected by overload resolution does not execute the function object f on the executor e, the program is ill-formed with no diagnostic required.

• Otherwise, execution::submit(e,as-receiver<F>(forward<F>(f))) if E and as-receiver<F> model sender_to, where as-receiver is some implementation-defined class template equivalent to:

  template<invocable F>
  struct as-receiver {
  private:
    using invocable_type = std::remove_cvref_t<F>;
    invocable_type f_;
  public:
    explicit as-receiver(invocable_type&& f) : f_(move_if_noexcept(f)) {}
    explicit as-receiver(const invocable_type& f) : f_(f) {}
    as-receiver(as-receiver&& other) = default;
    void set_value() {
      invoke(f_);
    }
    void set_error(std::exception_ptr) {
      terminate();
    }
    void set_done() noexcept {}
  };

[Editorial note: We should probably define what "execute the function object F on the executor E" means more carefully. --end editorial note]

1.2.3.5 execution::submit

The name execution::submit denotes a customization point object.

A receiver object is submitted for execution via a sender by scheduling the eventual evaluation of one of the receiver's value, error, or done channels.

For some subexpressions s and r, let S be a type such that decltype((s)) is S and let R be a type such that decltype((r)) is R. The expression execution::submit(s, r) is ill-formed if R does not model receiver, or if S does not model either sender or executor. Otherwise, it is expression-equivalent to:

• s.submit(r), if that expression is valid and S models sender. If the function selected does not submit the receiver object r via the sender s, the program is ill-formed with no diagnostic required.

• Otherwise, submit(s, r), if that expression is valid and S models sender, with overload resolution performed in a context that includes the declaration

  void submit();

  and that does not include a declaration of execution::submit. If the function selected by overload resolution does not submit the receiver object r via the sender s, the program is ill-formed with no diagnostic required.

• Otherwise, execution::execute(s,as-invocable<R>(forward<R>(r))) if S and as-invocable<R> model executor, where as-invocable is some implementation-defined class template equivalent to:

  template<receiver R>
  struct as-invocable {
  private:
    using receiver_type = std::remove_cvref_t<R>;
    std::optional<receiver_type> r_ {};
    void try_init_(auto&& r) {
      try {
        r_.emplace((decltype(r)&&) r);
      } catch(...) {
        execution::set_error(r, current_exception());
      }
    }
  public:
    explicit as-invocable(receiver_type&& r) {
      try_init_(move_if_noexcept(r));
    }
    explicit as-invocable(const receiver_type& r) {
      try_init_(r);
    }
    as-invocable(as-invocable&& other) {
      if(other.r_) {
        try_init_(move_if_noexcept(*other.r_));
        other.r_.reset();
      }
    }
    ~as-invocable() {
      if(r_)
        execution::set_done(*r_);
    }
    void operator()() {
      try {
        execution::set_value(*r_);
      } catch(...) {
        execution::set_error(*r_, current_exception());
      }
      r_.reset();
    }
  };

• Otherwise, execution::submit(s, r) is ill-formed.

[Editorial note: We should probably define what "submit the receiver object R via the sender S" means more carefully. --end editorial note]

1.2.3.6 execution::schedule

The name execution::schedule denotes a customization point object. The expression execution::schedule(S) for some subexpression S is expression-equivalent to:

• S.schedule(), if that expression is valid and its type N models sender.

• Otherwise, schedule(S), if that expression is valid and its type N models sender with overload resolution performed in a context that includes the declaration

  void schedule();

  and that does not include a declaration of execution::schedule.

• Otherwise, decay-copy(S) if the type S models sender.

• Otherwise, execution::schedule(S) is ill-formed.

1.2.3.7 execution::bulk_execute

The name execution::bulk_execute denotes a customization point object. If is_convertible_v<N, size_t> is true, then the expression execution::bulk_execute(S, F, N) for some subexpressions S, F, and N is expression-equivalent to:

• S.bulk_execute(F, N), if that expression is valid. If the function selected does not execute N invocations of the function object F on the executor S in bulk with forward progress guarantee std::query(S, execution::bulk_guarantee), and the result of that function does not model sender<void>, the program is ill-formed with no diagnostic required.

• Otherwise, bulk_execute(S, F, N), if that expression is valid, with overload resolution performed in a context that includes the declaration

  void bulk_execute();

  and that does not include a declaration of execution::bulk_execute. If the function selected by overload resolution does not execute N invocations of the function object F on the executor S in bulk with forward progress guarantee std::query(E, execution::bulk_guarantee), and the result of that function does not model sender<void>, the program is ill-formed with no diagnostic required.

• Otherwise, if the types F and executor_index_t<remove_cvref_t<S>> model invocable and if std::query(S, execution::bulk_guarantee) equals execution::bulk_guarantee.unsequenced, then
  • Evaluates DECAY_COPY(std::forward<decltype(F)>(F)) on the calling thread to create a function object cf. [Note: Additional copies of cf may subsequently be created. --end note.]
  • For each value of i in N, cf(i) (or copy of cf)) will be invoked at most once by an execution agent that is unique for each value of i.
  • May block pending completion of one or more invocations of cf.
  • Synchronizes with (C++Std [intro.multithread]) the invocations of cf.

• Otherwise, execution::bulk_execute(S, F, N) is ill-formed.

[Editorial note: We should probably define what "execute N invocations of the function object F on the executor S in bulk" means more carefully. --end editorial note]

1.2.4 Concept receiver

XXX TODO The receiver concept...

// exposition only:
template<class T>
inline constexpr bool is-nothrow-move-or-copy-constructible =
  is_nothrow_move_constructible<T> ||
   copy_constructible<T>;

template<class T, class E = exception_ptr>
concept receiver =
  move_constructible<remove_cvref_t<T>> &&
  (is-nothrow-move-or-copy-constructible<remove_cvref_t<T>>) &&
  requires(T&& t, E&& e) {
    { execution::set_done((T&&) t) } noexcept;
    { execution::set_error((T&&) t, (E&&) e) } noexcept;
  };

1.2.5 Concept receiver_of

XXX TODO The receiver_of concept...

template<class T, class... An>
concept receiver_of =
  receiver<T> &&
  requires(T&& t, An&&... an) {
    execution::set_value((T&&) t, (An&&) an...);
  };

1.2.6 Concepts sender and sender_to

XXX TODO The sender and sender_to concepts...

Let sender-to-impl be the exposition-only concept

template<class S, class R>
concept sender-to-impl =
  requires(S&& s, R&& r) {
    execution::submit((S&&) s, (R&&) r);
  };

Then,

template<class S>
concept sender =
  move_constructible<remove_cvref_t<S>> &&
  sender-to-impl<S, sink_receiver>;

template<class S, class R>
concept sender_to =
  sender<S> &&
  receiver<R> &&
  sender-to-impl<S, R>;

None of these operations shall introduce data races as a result of concurrent invocations of those functions from different threads.

An sender type's destructor shall not block pending completion of the submitted function objects. [Note: The ability to wait for completion of submitted function objects may be provided by the associated execution context. --end note]

In addition to the above requirements, types S and R model sender_to only if they satisfy the requirements from the Table below.

In the Table below,

• s denotes a (possibly const) sender object of type S,
• r denotes a (possibly const) receiver object of type R.

1.2.7 Concept typed_sender

A sender is typed if it declares what types it sends through a receiver's channels. The typed_sender concept is defined as:

template<template<template<class...> class Tuple, template<class...> class Variant> class>
struct has-value-types; // exposition only

template<template<class...> class Variant>
struct has-error-types; // exposition only

template<class S>
comcept has-sender-types = // exposition only
  requires {
    typename has-value-types<S::template value_types>;
    typename has-error-types<S::template error_types>;
    typename bool_constant<S::sends_done>;
  };

template<class S>
concept typed_sender =
  sender<S> &&
  has-sender-types<sender_traits<S>>;

1.2.8 Concept scheduler

XXX TODO The scheduler concept...

template<class S>
concept scheduler =
  copy_constructible<remove_cvref_t<S>> &&
  equality_comparable<remove_cvref_t<S>> &&
  requires(E&& e) {
    execution::schedule((S&&)s);
  }; // && sender<invoke_result_t<execution::schedule, S>>

None of a scheduler's copy constructor, destructor, equality comparison, or swap operation shall exit via an exception.

None of these operations, nor an scheduler type's schedule function, or associated query functions shall introduce data races as a result of concurrent invocations of those functions from different threads.

For any two (possibly const) values x1 and x2 of some scheduler type X, x1 == x2 shall return true only if x1.query(p) == x2.query(p) for every property p where both x1.query(p) and x2.query(p) are well-formed and result in a non-void type that is EqualityComparable (C++Std [equalitycomparable]). [Note: The above requirements imply that x1 == x2 returns true if x1 and x2 can be interchanged with identical effects. An scheduler may conceptually contain additional properties which are not exposed by a named property type that can be observed via execution::query; in this case, it is up to the concrete scheduler implementation to decide if these properties affect equality. Returning false does not necessarily imply that the effects are not identical. --end note]

An scheduler type's destructor shall not block pending completion of any receivers submitted to the sender objects returned from schedule. [Note: The ability to wait for completion of submitted function objects may be provided by the execution context that produced the scheduler. --end note]

In addition to the above requirements, type S models scheduler only if it satisfies the requirements in the Table below.

In the Table below,

• s denotes a (possibly const) scheduler object of type S,
• N denotes a type that models sender, and
• n denotes a sender object of type N

execution::submit(N, r), for some receiver object r, is required to eagerly submit r for execution on an execution agent that s creates for it. Let rc be r or an object created by copy or move construction from r. The semantic constraints on the sender N returned from a scheduler s's schedule function are as follows:

• If rc's set_error function is called in response to a submission error, scheduling error, or other internal error, let E be an expression that refers to that error if set_error(rc, E) is well-formed; otherwise, let E be an exception_ptr that refers to that error. [ Note: E could be the result of calling current_exception or make_exception_ptr — end note ] The scheduler calls set_error(rc, E) on an unspecified weakly-parallel execution agent ([ Note: An invocation of set_error on a receiver is required to be noexcept — end note]), and

• If rc's set_error function is called in response to an exception that propagates out of the invocation of set_value on rc, let E be make_exception_ptr(receiver_invocation_error{}) invoked from within a catch clause that has caught the exception. The executor calls set_error(rc, E) on an unspecified weakly-parallel execution agent, and

• A call to set_done(rc) is made on an unspecified weakly-parallel execution agent ([ Note: An invocation of a receiver's set_done function is required to be noexcept — end note ]).

[ Note: The senders returned from a scheduler's schedule function have wide discretion when deciding which of the three receiver functions to call upon submission. — end note ]

1.2.9 Concepts executor and executor_of

XXX TODO The executor and executor_of concepts...

Let executor-of-impl be the exposition-only concept

template<class E, class F>
concept executor-of-impl =
  invocable<F> &&
  is_nothrow_copy_constructible_v<E> &&
  is_nothrow_destructible_v<E> &&
  equality_comparable<E> &&
  requires(const E& e, F&& f) {
    execution::execute(e, (F&&)f);
  };

Then,

template<class E>
concept executor = executor-of-impl<E, execution::invocable_archetype>;

template<class E, class F>
concept executor_of = executor-of-impl<E, F>;

Neither of an executor's equality comparison or swap operation shall exit via an exception.

None of an executor type's copy constructor, destructor, equality comparison, swap function, execute function, or associated query functions shall introduce data races as a result of concurrent invocations of those functions from different threads.

For any two (possibly const) values x1 and x2 of some executor type X, x1 == x2 shall return true only if std::query(x1,p) == std::query(x2,p) for every property p where both std::query(x1,p) and std::query(x2,p) are well-formed and result in a non-void type that is equality_comparable (C++Std [equalitycomparable]). [Note: The above requirements imply that x1 == x2 returns true if x1 and x2 can be interchanged with identical effects. An executor may conceptually contain additional properties which are not exposed by a named property type that can be observed via std::query; in this case, it is up to the concrete executor implementation to decide if these properties affect equality. Returning false does not necessarily imply that the effects are not identical. --end note]

An executor type's destructor shall not block pending completion of the submitted function objects. [Note: The ability to wait for completion of submitted function objects may be provided by the associated execution context. --end note]

In addition to the above requirements, types E and F model executor_of only if they satisfy the requirements of the Table below.

In the Table below,

• e denotes a (possibly const) executor object of type E,
• cf denotes the function object DECAY_COPY(std::forward<F>(f))
• f denotes a function of type F&& invocable as cf() and where decay_t<F> models move_constructible.

[Editorial note: The operational semantics of execution::execute should be specified with the execution::execute CPO rather than the executor concept. --end note.]

1.2.10 Sender and receiver traits

1.2.10.1 Class sink_receiver

XXX TODO The class sink_receiver...

    class sink_receiver {
    public:
      void set_value(auto&&...) {}
      [[noreturn]] void set_error(auto&&) noexcept {
        std::terminate();
      }
      void set_done() noexcept {}
    };

1.2.10.2 Class template sender_traits

XXX TODO The class templatesender_traits...

The class template sender_traits can be used to query information about a sender; in particular, what values and errors it sends through a receiver's value and error channel, and whether or not it ever calls set_done on a receiver.

    template<class S>
    struct sender-traits-base {}; // exposition-only

    template<class S>
      requires (!same_as<S, remove_cvref_t<S>>)
    struct sender-traits-base : sender_traits<remove_cvref_t<S>> {};

    template<class S>
      requires same_as<S, remove_cvref_t<S>> &&
      sender<S> && has-sender-traits<S>
    struct sender-traits-base<S> {
      template<template<class...> class Tuple, template<class...> class Variant>
      using value_types = typename S::template value_types<Tuple, Variant>;

      template<template<class...> class Variant>
      using error_types = typename S::template error_types<Variant>;

      static constexpr bool sends_done = S::sends_done;
    };

    template<class S>
    struct sender_traits : sender-traits-base<S> {};

Because a sender may send one set of types or another to a receiver based on some runtime condition, sender_traits may provide a nested value_types template that is parameterized on a tuple-like class template and a variant-like class template that are used to hold the result.

[Example: If a sender type S sends types As... or Bs... to a receiver's value channel, it may specialize sender_traits such that typename sender_traits<S>::value_types<tuple, variant> names the type variant<tuple<As...>, tuple<Bs...>> -- end example]

Because a sender may send one or another type of error types to a receiver, sender_traits may provide a nested error_types template that is parameterized on a variant-like class template that is used to hold the result.

[Example: If a sender type S sends error types exception_ptr or error_code to a receiver's error channel, it may specialize sender_traits such that typename sender_traits<S>::error_types<variant> names the type variant<exception_ptr, error_code> -- end example]

A sender type can signal that it never calls set_done on a receiver by specializing sender_traits such that sender_traits<S>::sends_done is false; conversely, it may set sender_traits<S>::sends_done to true to indicate that it does call set_done on a receiver.

Users may specialize sender_traits on program-defined types.

1.2.11 Query-only properties

1.2.11.1 Associated execution context property

struct context_t
{
  template <class T>
    static constexpr bool is_applicable_property_v = executor<T>;

  static constexpr bool is_requirable = false;
  static constexpr bool is_preferable = false;

  using polymorphic_query_result_type = any;

  template<class Executor>
    static constexpr decltype(auto) static_query_v
      = Executor::query(context_t());
};

The context_t property can be used only with query, which returns the execution context associated with the executor.

The value returned from std::query(e, context_t), where e is an executor, shall not change between invocations.

1.2.11.2 Polymorphic executor wrappers

The any_executor class template provides polymorphic wrappers for executors.

In several places in this section the operation CONTAINS_PROPERTY(p, pn) is used. All such uses mean std::disjunction_v<std::is_same<p, pn>...>.

In several places in this section the operation FIND_CONVERTIBLE_PROPERTY(p, pn) is used. All such uses mean the first type P in the parameter pack pn for which std::is_convertible_v<p, P> is true. If no such type P exists, the operation FIND_CONVERTIBLE_PROPERTY(p, pn) is ill-formed.

template <class... SupportableProperties>
class any_executor
{
public:
  // construct / copy / destroy:

  any_executor() noexcept;
  any_executor(nullptr_t) noexcept;
  any_executor(const any_executor& e) noexcept;
  any_executor(any_executor&& e) noexcept;
  template<class... OtherSupportableProperties>
    any_executor(any_executor<OtherSupportableProperties...> e);
  template<class... OtherSupportableProperties>
    any_executor(any_executor<OtherSupportableProperties...> e) = delete;
  template<executor Executor>
    any_executor(Executor e);

  any_executor& operator=(const any_executor& e) noexcept;
  any_executor& operator=(any_executor&& e) noexcept;
  any_executor& operator=(nullptr_t) noexcept;
  template<executor Executor>
    any_executor& operator=(Executor e);

  ~any_executor();

  // any_executor modifiers:

  void swap(any_executor& other) noexcept;

  // any_executor operations:

  template <class Property>
  any_executor require(Property) const;

  template <class Property>
  typename Property::polymorphic_query_result_type query(Property) const;

  template<class Function>
    void execute(Function&& f) const;

  // any_executor capacity:

  explicit operator bool() const noexcept;

  // any_executor target access:

  const type_info& target_type() const noexcept;
  template<executor Executor> Executor* target() noexcept;
  template<executor Executor> const Executor* target() const noexcept;
};

// any_executor comparisons:

template <class... SupportableProperties>
bool operator==(const any_executor<SupportableProperties...>& a, const any_executor<SupportableProperties...>& b) noexcept;
template <class... SupportableProperties>
bool operator==(const any_executor<SupportableProperties...>& e, nullptr_t) noexcept;
template <class... SupportableProperties>
bool operator==(nullptr_t, const any_executor<SupportableProperties...>& e) noexcept;
template <class... SupportableProperties>
bool operator!=(const any_executor<SupportableProperties...>& a, const any_executor<SupportableProperties...>& b) noexcept;
template <class... SupportableProperties>
bool operator!=(const any_executor<SupportableProperties...>& e, nullptr_t) noexcept;
template <class... SupportableProperties>
bool operator!=(nullptr_t, const any_executor<SupportableProperties...>& e) noexcept;

// any_executor specialized algorithms:

template <class... SupportableProperties>
void swap(any_executor<SupportableProperties...>& a, any_executor<SupportableProperties...>& b) noexcept;

template <class Property, class... SupportableProperties>
any_executor prefer(const any_executor<SupportableProperties>& e, Property p);

The any_executor class satisfies the executor concept requirements.

[Note: To meet the noexcept requirements for executor copy constructors and move constructors, implementations may share a target between two or more any_executor objects. --end note]

Each property type in the SupportableProperties... pack shall provide a nested type polymorphic_query_result_type.

The target is the executor object that is held by the wrapper.

1.2.11.2.1 any_executor constructors

any_executor() noexcept;

Postconditions: !*this.

any_executor(nullptr_t) noexcept;

Postconditions: !*this.

any_executor(const any_executor& e) noexcept;

Postconditions: !*this if !e; otherwise, *this targets e.target() or a copy of e.target().

any_executor(any_executor&& e) noexcept;

Effects: If !e, *this has no target; otherwise, moves e.target() or move-constructs the target of e into the target of *this, leaving e in a valid state with an unspecified value.

template<class... OtherSupportableProperties>
  any_executor(any_executor<OtherSupportableProperties...> e);

Remarks: This function shall not participate in overload resolution unless: * CONTAINS_PROPERTY(p, OtherSupportableProperties) , where p is each property in SupportableProperties....

Effects: *this targets a copy of e initialized with std::move(e).

template<class... OtherSupportableProperties>
  any_executor(any_executor<OtherSupportableProperties...> e) = delete;

Remarks: This function shall not participate in overload resolution unless CONTAINS_PROPERTY(p, OtherSupportableProperties) is false for some property p in SupportableProperties....

template<executor Executor>
  any_executor(Executor e);

Remarks: This function shall not participate in overload resolution unless:

• can_require_v<Executor, P>, if P::is_requirable, where P is each property in SupportableProperties....
• can_prefer_v<Executor, P>, if P::is_preferable, where P is each property in SupportableProperties....
• and can_query_v<Executor, P>, if P::is_requirable == false and P::is_preferable == false, where P is each property in SupportableProperties....

Effects: *this targets a copy of e.

1.2.11.2.2 any_executor assignment

any_executor& operator=(const any_executor& e) noexcept;

Effects: any_executor(e).swap(*this).

Returns: *this.

any_executor& operator=(any_executor&& e) noexcept;

Effects: Replaces the target of *this with the target of e, leaving e in a valid state with an unspecified value.

Returns: *this.

any_executor& operator=(nullptr_t) noexcept;

Effects: any_executor(nullptr).swap(*this).

Returns: *this.

template<executor Executor>
  any_executor& operator=(Executor e);

Requires: As for template<executor Executor> any_executor(Executor e).

Effects: any_executor(std::move(e)).swap(*this).

Returns: *this.

1.2.11.2.3 any_executor destructor

~any_executor();

Effects: If *this != nullptr, releases shared ownership of, or destroys, the target of *this.

1.2.11.2.4 any_executor modifiers

void swap(any_executor& other) noexcept;

Effects: Interchanges the targets of *this and other.

1.2.11.2.5 any_executor operations

template <class Property>
any_executor require(Property p) const;

Remarks: This function shall not participate in overload resolution unless FIND_CONVERTIBLE_PROPERTY(Property, SupportableProperties)::is_requirable is well-formed and has the value true.

Returns: A polymorphic wrapper whose target is the result of std::require(e, p), where e is the target object of *this.

template <class Property>
typename Property::polymorphic_query_result_type query(Property p) const;

Remarks: This function shall not participate in overload resolution unless FIND_CONVERTIBLE_PROPERTY(Property, SupportableProperties) is well-formed.

Returns: If std::query(e, p) is well-formed, static_cast<Property::polymorphic_query_result_type>(std::query(e, p)), where e is the target object of *this. Otherwise, Property::polymorphic_query_result_type{}.

template<class Function>
  void execute(Function&& f) const;

Effects: Performs execution::execute(e, f2), where:

• e is the target object of *this;
• f1 is the result of DECAY_COPY(std::forward<Function>(f));
• f2 is a function object of unspecified type that, when invoked as f2(), performs f1().

1.2.11.2.6 any_executor capacity

explicit operator bool() const noexcept;

Returns: true if *this has a target, otherwise false.

1.2.11.2.7 any_executor target access

const type_info& target_type() const noexcept;

Returns: If *this has a target of type T, typeid(T); otherwise, typeid(void).

template<executor Executor> Executor* target() noexcept;
template<executor Executor> const Executor* target() const noexcept;

Returns: If target_type() == typeid(Executor) a pointer to the stored executor target; otherwise a null pointer value.

1.2.11.2.8 any_executor comparisons

template<class... SupportableProperties>
bool operator==(const any_executor<SupportableProperties...>& a, const any_executor<SupportableProperties...>& b) noexcept;

Returns:

• true if !a and !b;
• true if a and b share a target;
• true if e and f are the same type and e == f, where e is the target of a and f is the target of b;
• otherwise false.

template<class... SupportableProperties>
bool operator==(const any_executor<SupportableProperties...>& e, nullptr_t) noexcept;
template<class... SupportableProperties>
bool operator==(nullptr_t, const any_executor<SupportableProperties...>& e) noexcept;

Returns: !e.

template<class... SupportableProperties>
bool operator!=(const any_executor<SupportableProperties...>& a, const any_executor<SupportableProperties...>& b) noexcept;

Returns: !(a == b).

template<class... SupportableProperties>
bool operator!=(const any_executor<SupportableProperties...>& e, nullptr_t) noexcept;
template<class... SupportableProperties>
bool operator!=(nullptr_t, const any_executor<SupportableProperties...>& e) noexcept;

Returns: (bool) e.

1.2.11.2.9 any_executor specialized algorithms

template<class... SupportableProperties>
void swap(any_executor<SupportableProperties...>& a, any_executor<SupportableProperties...>& b) noexcept;

Effects: a.swap(b).

template <class Property, class... SupportableProperties>
any_executor prefer(const any_executor<SupportableProperties...>& e, Property p);

Remarks: This function shall not participate in overload resolution unless FIND_CONVERTIBLE_PROPERTY(Property, SupportableProperties)::is_preferable is well-formed and has the value true.

Returns: A polymorphic wrapper whose target is the result of std::prefer(e, p), where e is the target object of *this.

1.2.12 Behavioral properties

Behavioral properties define a set of mutually-exclusive nested properties describing executor behavior.

Unless otherwise specified, behavioral property types S, their nested property types S::Ni, and nested property objects S::ni conform to the following specification:

struct S
{
  template <class T>
    static constexpr bool is_applicable_property_v = executor<T>;

  static constexpr bool is_requirable = false;
  static constexpr bool is_preferable = false;
  using polymorphic_query_result_type = S;

  template<class Executor>
    static constexpr auto static_query_v
      = see-below;

  template<class Executor>
  friend constexpr S query(const Executor& ex, const Property& p) noexcept(see-below);

  friend constexpr bool operator==(const S& a, const S& b);
  friend constexpr bool operator!=(const S& a, const S& b) { return !operator==(a, b); }

  constexpr S();

  struct N1
  {
    static constexpr bool is_requirable = true;
    static constexpr bool is_preferable = true;
    using polymorphic_query_result_type = S;

    template<class Executor>
      static constexpr auto static_query_v
        = see-below;

    static constexpr S value() { return S(N1()); }
  };

  static constexpr N1 n1;

  constexpr S(const N1);

  ...

  struct NN
  {
    static constexpr bool is_requirable = true;
    static constexpr bool is_preferable = true;
    using polymorphic_query_result_type = S;

    template<class Executor>
      static constexpr auto static_query_v
        = see-below;

    static constexpr S value() { return S(NN()); }
  };

  static constexpr NN nN;

  constexpr S(const NN);
};

Queries for the value of an executor's behavioral property shall not change between invocations unless the executor is assigned another executor with a different value of that behavioral property.

S() and S(S::Ei()) are all distinct values of S. [Note: This means they compare unequal. --end note.]

The value returned from std::query(e1, p1) and a subsequent invocation std::query(e1, p1), where

• p1 is an instance of S or S::Ei, and
• e2 is the result of std::require(e1, p2) or std::prefer(e1, p2),

shall compare equal unless

• p2 is an instance of S::Ei, and
• p1 and p2 are different types.

The value of the expression S::N1::static_query_v<Executor> is

• Executor::query(S::N1()), if that expression is a well-formed expression;
• ill-formed if declval<Executor>().query(S::N1()) is well-formed;
• ill-formed if can_query_v<Executor,S::Ni> is true for any 1 < i <= N;
• otherwise S::N1().

[Note: These rules automatically enable the S::N1 property by default for executors which do not provide a query function for properties S::Ni. --end note]

The value of the expression S::Ni::static_query_v<Executor>, for all 1 < i <= N, is

• Executor::query(S::Ni()), if that expression is a well-formed constant expression;
• otherwise ill-formed.

The value of the expression S::static_query_v<Executor> is

• Executor::query(S()), if that expression is a well-formed constant expression;
• otherwise, ill-formed if declval<Executor>().query(S()) is well-formed;
• otherwise, S::Ni::static_query_v<Executor> for the least i <= N for which this expression is a well-formed constant expression;
• otherwise ill-formed.

[Note: These rules automatically enable the S::N1 property by default for executors which do not provide a query function for properties S or S::Ni. --end note]

Let k be the least value of i for which can_query_v<Executor,S::Ni> is true, if such a value of i exists.

template<class Executor>
  friend constexpr S query(const Executor& ex, const Property& p) noexcept(noexcept(std::query(ex, std::declval<const S::Nk>())));

Returns: std::query(ex, S::Nk()).

Remarks: This function shall not participate in overload resolution unless is_same_v<Property,S> && can_query_v<Executor,S::Ni> is true for at least one S::Ni`.

bool operator==(const S& a, const S& b);

Returns: true if a and b were constructed from the same constructor; false, otherwise.

1.2.12.1 Blocking properties

The blocking_t property describes what guarantees executors provide about the blocking behavior of their execution functions.

blocking_t provides nested property types and objects as described below.

1.2.12.1.1 blocking_t::always_t customization points

In addition to conforming to the above specification, the blocking_t::always_t property provides the following customization:

struct always_t
{
  static constexpr bool is_requirable = true;
  static constexpr bool is_preferable = false;

  template <class T>
    static constexpr bool is_applicable_property_v = executor<T>;

  template<class Executor>
    friend see-below require(Executor ex, blocking_t::always_t);
};

If the executor has the blocking_adaptation_t::allowed_t property, this customization uses an adapter to implement the blocking_t::always_t property.

template<class Executor>
  friend see-below require(Executor ex, blocking_t::always_t);

Returns: A value e1 of type E1 that holds a copy of ex. E1 provides an overload of require such that e1.require(blocking.always) returns a copy of e1, an overload of query such that std::query(e1,blocking) returns blocking.always, and functions execute and bulk_execute shall block the calling thread until the submitted functions have finished execution. e1 has the same executor properties as ex, except for the addition of the blocking_t::always_t property, and removal of blocking_t::never_t and blocking_t::possibly_t properties if present.

Remarks: This function shall not participate in overload resolution unless blocking_adaptation_t::static_query_v<Executor> is blocking_adaptation.allowed.

1.2.12.2 Properties to indicate if blocking and directionality may be adapted

The blocking_adaptation_t property allows or disallows blocking or directionality adaptation via std::require.

blocking_adaptation_t provides nested property types and objects as described below.

1.2.12.2.1 blocking_adaptation_t::allowed_t customization points

In addition to conforming to the above specification, the blocking_adaptation_t::allowed_t property provides the following customization:

struct allowed_t
{
  static constexpr bool is_requirable = true;
  static constexpr bool is_preferable = false;

  template <class T>
    static constexpr bool is_applicable_property_v = executor<T>;

  template<class Executor>
    friend see-below require(Executor ex, blocking_adaptation_t::allowed_t);
};

This customization uses an adapter to implement the blocking_adaptation_t::allowed_t property.

template<class Executor>
  friend see-below require(Executor ex, blocking_adaptation_t::allowed_t);

Returns: A value e1 of type E1 that holds a copy of ex. In addition, blocking_adaptation_t::static_query_v<E1> is blocking_adaptation.allowed, and e1.require(blocking_adaptation.disallowed) yields a copy of ex. e1 has the same executor properties as ex, except for the addition of the blocking_adaptation_t::allowed_t property.

1.2.12.3 Properties to indicate if submitted tasks represent continuations

The relationship_t property allows users of executors to indicate that submitted tasks represent continuations.

relationship_t provides nested property types and objects as indicated below.

1.2.12.4 Properties to indicate likely task submission in the future

The outstanding_work_t property allows users of executors to indicate that task submission is likely in the future.

outstanding_work_t provides nested property types and objects as indicated below.

[Note: The outstanding_work_t::tracked_t and outstanding_work_t::untracked_t properties are used to communicate to the associated execution context intended future work submission on the executor. The intended effect of the properties is the behavior of execution context's facilities for awaiting outstanding work; specifically whether it considers the existance of the executor object with the outstanding_work_t::tracked_t property enabled outstanding work when deciding what to wait on. However this will be largely defined by the execution context implementation. It is intended that the execution context will define its wait facilities and on-destruction behaviour and provide an interface for querying this. An initial work towards this is included in P0737r0. --end note]

1.2.12.5 Properties for bulk execution guarantees

Bulk execution guarantee properties communicate the forward progress and ordering guarantees of execution agents associated with the bulk execution.

bulk_guarantee_t provides nested property types and objects as indicated below.

Execution agents associated with the bulk_guarantee_t::unsequenced_t property may invoke the function object in an unordered fashion. Any such invocations in the same thread of execution are unsequenced with respect to each other. [Note: This means that multiple execution agents may be interleaved on a single thread of execution, which overrides the usual guarantee from [intro.execution] that function executions do not interleave with one another. --end note]

Execution agents associated with the bulk_guarantee_t::sequenced_t property invoke the function object in sequence in lexicographic order of their indices.

Execution agents associated with the bulk_guarantee_t::parallel_t property invoke the function object with a parallel forward progress guarantee. Any such invocations in the same thread of execution are indeterminately sequenced with respect to each other. [Note: It is the caller's responsibility to ensure that the invocation does not introduce data races or deadlocks. --end note]

[Editorial note: The descriptions of these properties were ported from [algorithms.parallel.user]. The intention is that a future standard will specify execution policy behavior in terms of the fundamental properties of their associated executors. We did not include the accompanying code examples from [algorithms.parallel.user] because the examples seem easier to understand when illustrated by std::for_each. --end editorial note]

1.2.12.6 Properties for mapping of execution on to threads

The mapping_t property describes what guarantees executors provide about the mapping of execution agents onto threads of execution.

mapping_t provides nested property types and objects as indicated below.

[Note: A mapping of an execution agent onto a thread of execution implies the execution agent runs as-if on a std::thread. Therefore, the facilities provided by std::thread, such as thread-local storage, are available. mapping_t::new_thread_t provides stronger guarantees, in particular that thread-local storage will not be shared between execution agents. --end note]

1.2.13 Properties for customizing memory allocation

template <typename ProtoAllocator>
struct allocator_t;

The allocator_t property conforms to the following specification:

template <typename ProtoAllocator>
struct allocator_t
{
    template <class T>
      static constexpr bool is_applicable_property_v = executor<T>;

    static constexpr bool is_requirable = true;
    static constexpr bool is_preferable = true;

    template<class Executor>
    static constexpr auto static_query_v
      = Executor::query(allocator_t);

    template <typename OtherProtoAllocator>
    allocator_t<OtherProtoAllocator> operator()(const OtherProtoAllocator &a) const;

    static constexpr ProtoAllocator value() const;

private:
    ProtoAllocator a_; // exposition only
};

If the expression std::query(E, P) is well formed, where P is an object of type allocator_t<ProtoAllocator>, then: * the type of the expression std::query(E, P) shall satisfy the ProtoAllocator requirements; * the result of the expression std::query(E, P) shall be the allocator currently established in the executor E; and * the expression std::query(E, allocator_t<void>{}) shall also be well formed and have the same result as std::query(E, P).

1.2.13.1 allocator_t members

template <typename OtherProtoAllocator>
allocator_t<OtherProtoAllocator> operator()(const OtherProtoAllocator &a) const;

Returns: An allocator object whose exposition-only member a_ is initialized as a_(a).

Remarks: This function shall not participate in overload resolution unless ProtoAllocator is void.

[Note: It is permitted for a to be an executor's implementation-defined default allocator and, if so, the default allocator may also be established within an executor by passing the result of this function to require. --end note]

static constexpr ProtoAllocator value() const;

Returns: The exposition-only member a_.

Remarks: This function shall not participate in overload resolution unless ProtoAllocator is not void.

1.3 Executor type traits

1.3.1 Associated shape type

template<class Executor>
struct executor_shape
{
  private:
    // exposition only
    template<class T>
    using helper = typename T::shape_type;

  public:
    using type = std::experimental::detected_or_t<
      size_t, helper, decltype(std::require(declval<const Executor&>(), execution::bulk))
    >;

    // exposition only
    static_assert(std::is_integral_v<type>, "shape type must be an integral type");
};

1.3.2 Associated index type

template<class Executor>
struct executor_index
{
  private:
    // exposition only
    template<class T>
    using helper = typename T::index_type;

  public:
    using type = std::experimental::detected_or_t<
      executor_shape_t<Executor>, helper, decltype(std::require(declval<const Executor&>(), execution::bulk))
    >;

    // exposition only
    static_assert(std::is_integral_v<type>, "index type must be an integral type");
};

1.4 Polymorphic executor support

1.4.1 Class bad_executor

An exception of type bad_executor is thrown by polymorphic executor member functions execute and bulk_execute when the executor object has no target.

class bad_executor : public exception
{
public:
  // constructor:
  bad_executor() noexcept;
};

1.4.1.1 bad_executor constructors

bad_executor() noexcept;

Effects: Constructs a bad_executor object.

Postconditions: what() returns an implementation-defined NTBS.

1.4.2 Struct prefer_only

The prefer_only struct is a property adapter that disables the is_requirable value.

[Example:

Consider a generic function that performs some task immediately if it can, and otherwise asynchronously in the background.

template<class Executor, class Callback>
void do_async_work(
    Executor ex,
    Callback callback)
{
  if (try_work() == done)
  {
    // Work completed immediately, invoke callback.
    std::require(ex,
        execution::single,
        execution::oneway,
      ).execute(callback);
  }
  else
  {
    // Perform work in background. Track outstanding work.
    start_background_work(
        std::prefer(ex,
          execution::outstanding_work.tracked),
        callback);
  }
}

This function can be used with an inline executor which is defined as follows:

struct inline_executor
{
  constexpr bool operator==(const inline_executor&) const noexcept
  {
    return true;
  }

  constexpr bool operator!=(const inline_executor&) const noexcept
  {
    return false;
  }

  template<class Function> void execute(Function f) const noexcept
  {
    f();
  }
};

as, in the case of an unsupported property, invocation of std::prefer will fall back to an identity operation.

The polymorphic executor wrapper should be able to simply swap in, so that we could change do_async_work to the non-template function:

void do_async_work(
    executor<
      execution::single,
      execution::oneway,
      execution::outstanding_work_t::tracked_t> ex,
    std::function<void()> callback)
{
  if (try_work() == done)
  {
    // Work completed immediately, invoke callback.
    std::require(ex,
        execution::single,
        execution::oneway,
      ).execute(callback);
  }
  else
  {
    // Perform work in background. Track outstanding work.
    start_background_work(
        std::prefer(ex,
          execution::outstanding_work.tracked),
        callback);
  }
}

with no change in behavior or semantics.

However, if we simply specify execution::outstanding_work.tracked in the executor template parameter list, we will get a compile error due to the executor template not knowing that execution::outstanding_work.tracked is intended for use with prefer only. At the point of construction from an inline_executor called ex, executor will try to instantiate implementation templates that perform the ill-formed std::require(ex, execution::outstanding_work.tracked).

The prefer_only adapter addresses this by turning off the is_requirable attribute for a specific property. It would be used in the above example as follows:

void do_async_work(
    executor<
      execution::single,
      execution::oneway,
      prefer_only<execution::outstanding_work_t::tracked_t>> ex,
    std::function<void()> callback)
{
  ...
}

-- end example]

template<class InnerProperty>
struct prefer_only
{
  InnerProperty property;

  static constexpr bool is_requirable = false;
  static constexpr bool is_preferable = InnerProperty::is_preferable;

  using polymorphic_query_result_type = see-below; // not always defined

  template<class Executor>
    static constexpr auto static_query_v = see-below; // not always defined

  constexpr prefer_only(const InnerProperty& p);

  constexpr auto value() const
    noexcept(noexcept(std::declval<const InnerProperty>().value()))
      -> decltype(std::declval<const InnerProperty>().value());

  template<class Executor, class Property>
  friend auto prefer(Executor ex, const Property& p)
    noexcept(noexcept(std::prefer(std::move(ex), std::declval<const InnerProperty>())))
      -> decltype(std::prefer(std::move(ex), std::declval<const InnerProperty>()));

  template<class Executor, class Property>
  friend constexpr auto query(const Executor& ex, const Property& p)
    noexcept(noexcept(std::query(ex, std::declval<const InnerProperty>())))
      -> decltype(std::query(ex, std::declval<const InnerProperty>()));
};

If InnerProperty::polymorphic_query_result_type is valid and denotes a type, the template instantiation prefer_only<InnerProperty> defines a nested type polymorphic_query_result_type as a synonym for InnerProperty::polymorphic_query_result_type.

If InnerProperty::static_query_v is a variable template and InnerProperty::static_query_v<E> is well formed for some executor type E, the template instantiation prefer_only<InnerProperty> defines a nested variable template static_query_v as a synonym for InnerProperty::static_query_v.

constexpr prefer_only(const InnerProperty& p);

Effects: Initializes property with p.

constexpr auto value() const
  noexcept(noexcept(std::declval<const InnerProperty>().value()))
    -> decltype(std::declval<const InnerProperty>().value());

Returns: property.value().

Remarks: Shall not participate in overload resolution unless the expression property.value() is well-formed.

template<class Executor, class Property>
friend auto prefer(Executor ex, const Property& p)
  noexcept(noexcept(std::prefer(std::move(ex), std::declval<const InnerProperty>())))
    -> decltype(std::prefer(std::move(ex), std::declval<const InnerProperty>()));

Returns: std::prefer(std::move(ex), p.property).

Remarks: Shall not participate in overload resolution unless std::is_same_v<Property, prefer_only> is true, and the expression std::prefer(std::move(ex), p.property) is well-formed.

template<class Executor, class Property>
friend constexpr auto query(const Executor& ex, const Property& p)
  noexcept(noexcept(std::query(ex, std::declval<const InnerProperty>())))
    -> decltype(std::query(ex, std::declval<const InnerProperty>()));

Returns: std::query(ex, p.property).

Remarks: Shall not participate in overload resolution unless std::is_same_v<Property, prefer_only> is true, and the expression std::query(ex, p.property) is well-formed.

1.5 Thread pools

Thread pools manage execution agents which run on threads without incurring the overhead of thread creation and destruction whenever such agents are needed.

1.5.1 Header <thread_pool> synopsis

namespace std {

  class static_thread_pool;

} // namespace std

1.5.2 Class static_thread_pool

static_thread_pool is a statically-sized thread pool which may be explicitly grown via thread attachment. The static_thread_pool is expected to be created with the use case clearly in mind with the number of threads known by the creator. As a result, no default constructor is considered correct for arbitrary use cases and static_thread_pool does not support any form of automatic resizing.

static_thread_pool presents an effectively unbounded input queue and the execution functions of static_thread_pool's associated executors do not block on this input queue.

[Note: Because static_thread_pool represents work as parallel execution agents, situations which require concurrent execution properties are not guaranteed correctness. --end note.]

class static_thread_pool
{
  public:
    using scheduler_type = see-below;
    using executor_type = see-below;
    
    // construction/destruction
    explicit static_thread_pool(std::size_t num_threads);
    
    // nocopy
    static_thread_pool(const static_thread_pool&) = delete;
    static_thread_pool& operator=(const static_thread_pool&) = delete;

    // stop accepting incoming work and wait for work to drain
    ~static_thread_pool();

    // attach current thread to the thread pools list of worker threads
    void attach();

    // signal all work to complete
    void stop();

    // wait for all threads in the thread pool to complete
    void wait();

    // placeholder for a general approach to getting schedulers from 
    // standard contexts.
    scheduler_type scheduler() noexcept;

    // placeholder for a general approach to getting executors from 
    // standard contexts.
    executor_type executor() noexcept;
};

For an object of type static_thread_pool, outstanding work is defined as the sum of:

• the number of existing executor objects associated with the static_thread_pool for which the execution::outstanding_work.tracked property is established;

• the number of function objects that have been added to the static_thread_pool via the static_thread_pool executor, scheduler and sender, but not yet invoked; and

• the number of function objects that are currently being invoked within the static_thread_pool.

The static_thread_pool member functions scheduler, executor, attach, wait, and stop, and the associated schedulers', senders` and executors' copy constructors and member functions, do not introduce data races as a result of concurrent invocations of those functions from different threads of execution.

A static_thread_pool's threads run execution agents with forward progress guarantee delegation. [Note: Forward progress is delegated to an execution agent for its lifetime. Because static_thread_pool guarantees only parallel forward progress to running execution agents; i.e., execution agents which have run the first step of the function object. --end note]

1.5.2.1 Types

using scheduler_type = see-below;

A scheduler type conforming to the specification for static_thread_pool scheduler types described below.

using executor_type = see-below;

An executor type conforming to the specification for static_thread_pool executor types described below.

1.5.2.2 Construction and destruction

static_thread_pool(std::size_t num_threads);

Effects: Constructs a static_thread_pool object with num_threads threads of execution, as if by creating objects of type std::thread.

~static_thread_pool();

Effects: Destroys an object of class static_thread_pool. Performs stop() followed by wait().

1.5.2.3 Worker management

void attach();

Effects: Adds the calling thread to the pool such that this thread is used to execute submitted function objects. [Note: Threads created during thread pool construction, or previously attached to the pool, will continue to be used for function object execution. --end note] Blocks the calling thread until signalled to complete by stop() or wait(), and then blocks until all the threads created during static_thread_pool object construction have completed. (NAMING: a possible alternate name for this function is join().)

void stop();

Effects: Signals the threads in the pool to complete as soon as possible. If a thread is currently executing a function object, the thread will exit only after completion of that function object. Invocation of stop() returns without waiting for the threads to complete. Subsequent invocations to attach complete immediately.

void wait();

Effects: If not already stopped, signals the threads in the pool to complete once the outstanding work is 0. Blocks the calling thread (C++Std [defns.block]) until all threads in the pool have completed, without executing submitted function objects in the calling thread. Subsequent invocations of attach() complete immediately.

Synchronization: The completion of each thread in the pool synchronizes with (C++Std [intro.multithread]) the corresponding successful wait() return.

1.5.2.4 Scheduler creation

scheduler_type scheduler() noexcept;

Returns: A scheduler that may be used to create sender objects that may be used to submit receiver objects to the thread pool. The returned scheduler has the following properties already established:

• execution::allocator
• execution::allocator(std::allocator<void>())

1.5.2.5 Executor creation

executor_type executor() noexcept;

Returns: An executor that may be used to submit function objects to the thread pool. The returned executor has the following properties already established:

• execution::oneway
• execution::blocking.possibly
• execution::relationship.fork
• execution::outstanding_work.untracked
• execution::allocator
• execution::allocator(std::allocator<void>())

1.5.3 static_thread_pool scheduler types

All scheduler types accessible through static_thread_pool::scheduler(), and subsequent invocations of the member function require, conform to the following specification.

class C
{
  public:

    // types:

    using sender_type = see-below;

    // construct / copy / destroy:

    C(const C& other) noexcept;
    C(C&& other) noexcept;

    C& operator=(const C& other) noexcept;
    C& operator=(C&& other) noexcept;

    // scheduler operations:

    see-below require(const execution::allocator_t<void>& a) const;
    template<class ProtoAllocator>
    see-below require(const execution::allocator_t<ProtoAllocator>& a) const;

    see-below query(execution::context_t) const noexcept;
    see-below query(execution::allocator_t<void>) const noexcept;
    template<class ProtoAllocator>
    see-below query(execution::allocator_t<ProtoAllocator>) const noexcept;

    bool running_in_this_thread() const noexcept;
};

bool operator==(const C& a, const C& b) noexcept;
bool operator!=(const C& a, const C& b) noexcept;

Objects of type C are associated with a static_thread_pool.

1.5.3.1 Constructors

C(const C& other) noexcept;

Postconditions: *this == other.

C(C&& other) noexcept;

Postconditions: *this is equal to the prior value of other.

1.5.3.2 Assignment

C& operator=(const C& other) noexcept;

Postconditions: *this == other.

Returns: *this.

C& operator=(C&& other) noexcept;

Postconditions: *this is equal to the prior value of other.

Returns: *this.

1.5.3.3 Operations

see-below require(const execution::allocator_t<void>& a) const;

Returns: require(execution::allocator(x)), where x is an implementation-defined default allocator.

template<class ProtoAllocator>
  see-below require(const execution::allocator_t<ProtoAllocator>& a) const;

Returns: An scheduler object of an unspecified type conforming to these specifications, associated with the same thread pool 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.alloc. All other properties of the returned scheduler object are identical to those of *this.

static_thread_pool& query(execution::context_t) const noexcept;

Returns: A reference to the associated static_thread_pool object.

see-below query(execution::allocator_t<void>) const noexcept;
see-below query(execution::allocator_t<ProtoAllocator>) const noexcept;

Returns: The allocator object associated with the executor, with type and value as either previously established by the execution::allocator_t<ProtoAllocator> property or the implementation defined default allocator established by the execution::allocator_t<void> property.

bool running_in_this_thread() const noexcept;

Returns: true if the current thread of execution is a thread that was created by or attached to the associated static_thread_pool object.

1.5.3.4 Comparisons

bool operator==(const C& a, const C& b) noexcept;

Returns: true if &a.query(execution::context) == &b.query(execution::context) and a and b have identical properties, otherwise false.

bool operator!=(const C& a, const C& b) noexcept;

Returns: !(a == b).

1.5.3.5 static_thread_pool scheduler functions

In addition to conforming to the above specification, static_thread_pool schedulers shall conform to the following specification.

class C
{
  public:
    sender_type schedule() noexcept;
};

C is a type satisfying the scheduler requirements.

1.5.3.6 Sender creation

  sender_type schedule() noexcept;

Returns: A sender that may be used to submit function objects to the thread pool. The returned sender has the following properties already established:

• execution::oneway
• execution::blocking.possibly
• execution::relationship.fork
• execution::outstanding_work.untracked
• execution::allocator
• execution::allocator(std::allocator<void>())

1.5.4 static_thread_pool sender types

All sender types accessible through static_thread_pool::scheduler().schedule(), and subsequent invocations of the member function require, conform to the following specification.

class C
{
  public:

    // construct / copy / destroy:

    C(const C& other) noexcept;
    C(C&& other) noexcept;

    C& operator=(const C& other) noexcept;
    C& operator=(C&& other) noexcept;

    // sender operations:

    see-below require(execution::blocking_t::never_t) const;
    see-below require(execution::blocking_t::possibly_t) const;
    see-below require(execution::blocking_t::always_t) const;
    see-below require(execution::relationship_t::continuation_t) const;
    see-below require(execution::relationship_t::fork_t) const;
    see-below require(execution::outstanding_work_t::tracked_t) const;
    see-below require(execution::outstanding_work_t::untracked_t) const;
    see-below require(const execution::allocator_t<void>& a) const;
    template<class ProtoAllocator>
    see-below require(const execution::allocator_t<ProtoAllocator>& a) const;

    static constexpr execution::bulk_guarantee_t query(execution::bulk_guarantee_t::parallel_t) const;
    static constexpr execution::mapping_t query(execution::mapping_t::thread_t) const;
    execution::blocking_t query(execution::blocking_t) const;
    execution::relationship_t query(execution::relationship_t) const;
    execution::outstanding_work_t query(execution::outstanding_work_t) const;
    see-below query(execution::context_t) const noexcept;
    see-below query(execution::allocator_t<void>) const noexcept;
    template<class ProtoAllocator>
    see-below query(execution::allocator_t<ProtoAllocator>) const noexcept;

    bool running_in_this_thread() const noexcept;
};

bool operator==(const C& a, const C& b) noexcept;
bool operator!=(const C& a, const C& b) noexcept;

Objects of type C are associated with a static_thread_pool.

1.5.4.1 Constructors

C(const C& other) noexcept;

Postconditions: *this == other.

C(C&& other) noexcept;

Postconditions: *this is equal to the prior value of other.

1.5.4.2 Assignment

C& operator=(const C& other) noexcept;

Postconditions: *this == other.

Returns: *this.

C& operator=(C&& other) noexcept;

Postconditions: *this is equal to the prior value of other.

Returns: *this.

1.5.4.3 Operations

see-below require(execution::blocking_t::never_t) const;
see-below require(execution::blocking_t::possibly_t) const;
see-below require(execution::blocking_t::always_t) const;
see-below require(execution::relationship_t::continuation_t) const;
see-below require(execution::relationship_t::fork_t) const;
see-below require(execution::outstanding_work_t::tracked_t) const;
see-below require(execution::outstanding_work_t::untracked_t) const;

Returns: An sender object of an unspecified type conforming to these specifications, associated with the same thread pool 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 sender object. All other properties of the returned sender object are identical to those of *this.

see-below require(const execution::allocator_t<void>& a) const;

Returns: require(execution::allocator(x)), where x is an implementation-defined default allocator.

template<class ProtoAllocator>
  see-below require(const execution::allocator_t<ProtoAllocator>& a) const;

Returns: An sender object of an unspecified type conforming to these specifications, associated with the same thread pool 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.alloc. All other properties of the returned sender object are identical to those of *this.

static constexpr execution::bulk_guarantee_t query(execution::bulk_guarantee_t) const;

Returns: execution::bulk_guarantee.parallel

static constexpr execution::mapping_t query(execution::mapping_t) const;

Returns: execution::mapping.thread.

execution::blocking_t query(execution::blocking_t) const;
execution::relationship_t query(execution::relationship_t) const;
execution::outstanding_work_t query(execution::outstanding_work_t) const;

Returns: The value of the given property of *this.

static_thread_pool& query(execution::context_t) const noexcept;

Returns: A reference to the associated static_thread_pool object.

see-below query(execution::allocator_t<void>) const noexcept;
see-below query(execution::allocator_t<ProtoAllocator>) const noexcept;

Returns: The allocator object associated with the sender, with type and value as either previously established by the execution::allocator_t<ProtoAllocator> property or the implementation defined default allocator established by the execution::allocator_t<void> property.

bool running_in_this_thread() const noexcept;

Returns: true if the current thread of execution is a thread that was created by or attached to the associated static_thread_pool object.

1.5.4.4 Comparisons

bool operator==(const C& a, const C& b) noexcept;

Returns: true if &a.query(execution::context) == &b.query(execution::context) and a and b have identical properties, otherwise false.

bool operator!=(const C& a, const C& b) noexcept;

Returns: !(a == b).

1.5.4.5 static_thread_pool sender execution functions

In addition to conforming to the above specification, static_thread_pool executors shall conform to the following specification.

class C
{
  public:
    template<class Receiver>
      void submit(Receiver&& r) const;
};

C is a type satisfying the sender requirements.

template<class Receiver>
  void submit(Receiver&& r) const;

Effects: Submits the receiver r for execution on the static_thread_pool according to the the properties established for *this. let e be an object of type exception_ptr, then static_thread_pool will evaluate one of set_value(r), set_error(r, e), or set_done(r).

1.5.5 static_thread_pool executor types

All executor types accessible through static_thread_pool::executor(), and subsequent invocations of the member function require, conform to the following specification.

class C
{
  public:

    // types:

    using shape_type = size_t;
    using index_type = size_t;

    // construct / copy / destroy:

    C(const C& other) noexcept;
    C(C&& other) noexcept;

    C& operator=(const C& other) noexcept;
    C& operator=(C&& other) noexcept;

    // executor operations:

    see-below require(execution::blocking_t::never_t) const;
    see-below require(execution::blocking_t::possibly_t) const;
    see-below require(execution::blocking_t::always_t) const;
    see-below require(execution::relationship_t::continuation_t) const;
    see-below require(execution::relationship_t::fork_t) const;
    see-below require(execution::outstanding_work_t::tracked_t) const;
    see-below require(execution::outstanding_work_t::untracked_t) const;
    see-below require(const execution::allocator_t<void>& a) const;
    template<class ProtoAllocator>
    see-below require(const execution::allocator_t<ProtoAllocator>& a) const;

    static constexpr execution::bulk_guarantee_t query(execution::bulk_guarantee_t::parallel_t) const;
    static constexpr execution::mapping_t query(execution::mapping_t::thread_t) const;
    execution::blocking_t query(execution::blocking_t) const;
    execution::relationship_t query(execution::relationship_t) const;
    execution::outstanding_work_t query(execution::outstanding_work_t) const;
    see-below query(execution::context_t) const noexcept;
    see-below query(execution::allocator_t<void>) const noexcept;
    template<class ProtoAllocator>
    see-below query(execution::allocator_t<ProtoAllocator>) const noexcept;

    bool running_in_this_thread() const noexcept;
};

bool operator==(const C& a, const C& b) noexcept;
bool operator!=(const C& a, const C& b) noexcept;

Objects of type C are associated with a static_thread_pool.

1.5.5.1 Constructors

C(const C& other) noexcept;

Postconditions: *this == other.

C(C&& other) noexcept;

Postconditions: *this is equal to the prior value of other.

1.5.5.2 Assignment

C& operator=(const C& other) noexcept;

Postconditions: *this == other.

Returns: *this.

C& operator=(C&& other) noexcept;

Postconditions: *this is equal to the prior value of other.

Returns: *this.

1.5.5.3 Operations

see-below require(execution::blocking_t::never_t) const;
see-below require(execution::blocking_t::possibly_t) const;
see-below require(execution::blocking_t::always_t) const;
see-below require(execution::relationship_t::continuation_t) const;
see-below require(execution::relationship_t::fork_t) const;
see-below require(execution::outstanding_work_t::tracked_t) const;
see-below require(execution::outstanding_work_t::untracked_t) const;

Returns: An executor object of an unspecified type conforming to these specifications, associated with the same thread pool 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(const execution::allocator_t<void>& a) const;

Returns: require(execution::allocator(x)), where x is an implementation-defined default allocator.

template<class ProtoAllocator>
  see-below require(const execution::allocator_t<ProtoAllocator>& a) const;

Returns: An executor object of an unspecified type conforming to these specifications, associated with the same thread pool 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.alloc. All other properties of the returned executor object are identical to those of *this.

static constexpr execution::bulk_guarantee_t query(execution::bulk_guarantee_t) const;

Returns: execution::bulk_guarantee.parallel

static constexpr execution::mapping_t query(execution::mapping_t) const;

Returns: execution::mapping.thread.

execution::blocking_t query(execution::blocking_t) const;
execution::relationship_t query(execution::relationship_t) const;
execution::outstanding_work_t query(execution::outstanding_work_t) const;

Returns: The value of the given property of *this.

static_thread_pool& query(execution::context_t) const noexcept;

Returns: A reference to the associated static_thread_pool object.

see-below query(execution::allocator_t<void>) const noexcept;
see-below query(execution::allocator_t<ProtoAllocator>) const noexcept;

Returns: The allocator object associated with the executor, with type and value as either previously established by the execution::allocator_t<ProtoAllocator> property or the implementation defined default allocator established by the execution::allocator_t<void> property.

bool running_in_this_thread() const noexcept;

Returns: true if the current thread of execution is a thread that was created by or attached to the associated static_thread_pool object.

1.5.5.4 Comparisons

bool operator==(const C& a, const C& b) noexcept;

Returns: true if &a.query(execution::context) == &b.query(execution::context) and a and b have identical properties, otherwise false.

bool operator!=(const C& a, const C& b) noexcept;

Returns: !(a == b).

1.5.5.5 static_thread_pool executor execution functions

In addition to conforming to the above specification, static_thread_pool executors shall conform to the following specification.

class C
{
  public:
    template<class Function>
      void execute(Function&& f) const;

    template<class Function>
      void bulk_execute(Function&& f, size_t n) const;
};

C is a type satisfying the Executor requirements.

template<class Function>
  void execute(Function&& f) const;

Effects: Submits the function f for execution on the static_thread_pool according to the the properties established for *this. If the submitted function f exits via an exception, the static_thread_pool invokes std::terminate().

template<class Function>
  void bulk_execute(Function&& f, size_t n) const;

Effects: Submits the function f for bulk execution on the static_thread_pool according to properties established for *this. If the submitted function f exits via an exception, the static_thread_pool invokes std::terminate().