0.1 Changelog

0.1.1 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.

0.1.2 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.

0.1.3 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

0.1.4 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

0.1.5 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

0.1.6 Revision 4

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

0.1.7 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

0.1.8 Revision 2

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

0.1.9 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

0.1.10 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.

1.1.2 Header <execution> synopsis

namespace std {
namespace execution {

  // Customization points:

  namespace {
    constexpr unspecified require = unspecified;
    constexpr unspecified prefer = unspecified;
    constexpr unspecified query = unspecified;
  }

  // Customization point type traits:

  template<class Executor, class... Properties> struct can_require;
  template<class Executor, class... Properties> struct can_prefer;
  template<class Executor, class Property> struct can_query;

  template<class Executor, class... Properties>
    constexpr bool can_require_v = can_require<Executor, Properties...>::value;
  template<class Executor, class... Properties>
    constexpr bool can_prefer_v = can_prefer<Executor, Properties...>::value;
  template<class Executor, class Property>
    constexpr bool can_query_v = can_query<Executor, Property>::value;

  // Associated execution context property:

  struct context_t;

  constexpr context_t context;

  // Interface-changing properties:

  struct oneway_t;
  struct bulk_oneway_t;

  constexpr oneway_t oneway;
  constexpr bulk_oneway_t bulk_oneway;

  // 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 is_oneway_executor;
  template<class Executor> struct is_bulk_oneway_executor;

  template<class Executor> constexpr bool is_oneway_executor_v = is_oneway_executor<Executor>::value;
  template<class Executor> constexpr bool is_bulk_oneway_executor_v = is_bulk_oneway_executor<Executor>::value;

  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 InterfaceProperty, class... SupportableProperties>
    using executor = typename InterfaceProperty::template
      polymorphic_executor_type<InterfaceProperty, SupportableProperties...>;
  template<class Property> struct prefer_only;

} // namespace execution
} // namespace std

1.2 Requirements

1.2.1 Customization point objects

(The following text has been adapted from the draft Ranges Technical Specification.)

A customization point object is a function object (C++ Std, [function.objects]) with a literal class type that interacts with user-defined types while enforcing semantic requirements on that interaction.

The type of a customization point object shall satisfy the requirements of CopyConstructible (C++Std [copyconstructible]) and Destructible (C++Std [destructible]).

All instances of a specific customization point object type shall be equal.

Let t be a (possibly const) customization point object of type T, and args... be a parameter pack expansion of some parameter pack Args.... The customization point object t shall be invocable as t(args...) when the types of Args... meet the requirements specified in that customization point object's definition. Otherwise, T shall not have a function call operator that participates in overload resolution.

Each customization point object type constrains its return type to satisfy some particular type requirements.

The library defines several named customization point objects. In every translation unit where such a name is defined, it shall refer to the same instance of the customization point object.

[Note: Many of the customization points objects in the library evaluate function call expressions with an unqualified name which results in invoking a user-defined function found by argument dependent name lookup (C++Std [basic.lookup.argdep]). To preclude such an expression resulting in invoking an unconstrained functions with the same name in namespace std, customization point objects specify that lookup for these expressions is performed in a context that includes deleted overloads matching the signatures of overloads defined in namespace std. When the deleted overloads are viable, user-defined overloads must be more specialized (C++Std [temp.func.order]) to be used by a customization point object. --end note]

1.2.2 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.

1.2.3 General requirements on executors

An executor type shall satisfy the requirements of CopyConstructible (C++Std [copyconstructible]), Destructible (C++Std [destructible]), and EqualityComparable (C++Std [equalitycomparable]).

None of these concepts' operations, nor an executor type's swap operations, shall exit via an exception.

None of these concepts' operations, nor an executor type's associated execution functions, associated query functions, or other member functions defined in executor type requirements, 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 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 executor 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 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]

1.2.4 OneWayExecutor requirements

A type X satisfies the OneWayExecutor requirements if it satisfies the general requirements on executors, as well as the requirements in the Table below.

[Note: OneWayExecutors provides fire-and-forget semantics without a channel for awaiting the completion of a submitted function object and obtaining its result. --end note]

In the Table below,

• x denotes a (possibly const) executor object of type X,
• cf denotes the function object DECAY_COPY(std::forward<F>(f))
• f denotes a function object of type F&& invocable as cf() and where decay_t<F> satisfies the MoveConstructible requirements.

1.2.5 BulkOneWayExecutor requirements

The BulkOneWayExecutor requirements specify requirements for executors which submit a function object to be invoked multiple times without a channel for awaiting the completion of the submitted function object invocations and obtaining their result. [Note: That is, the executor provides fire-and-forget semantics. --end note]

A type X satisfies the BulkOneWayExecutor requirements if it satisfies the general requirements on executors, as well as the requirements in the Table below.

In the Table below,

• x denotes a (possibly const) executor object of type X,
• n denotes a shape object whose type is executor_shape_t<X>,
• sf denotes a CopyConstructible function object with zero arguments whose result type is S,
• i denotes a (possibly const) object whose type is executor_index_t<X>,
• s denotes an object whose type is S,
• cf denotes the function object DECAY_COPY(std::forward<F>(f)),
• f denotes a function object of type F&& invocable as cf(i, s) and where decay_t<F> satisfies the MoveConstructible requirements,

1.3 Executor customization points

Executor customization points are functions which adapt an executor's properties. Executor customization points enable uniform use of executors in generic contexts.

When an executor customization point named NAME invokes a free execution function of the same name, overload resolution is performed in a context that includes the declaration void NAME(auto&... args) = delete;, where sizeof...(args) is the arity of the free execution function. This context also does not include a declaration of the executor customization point.

[Note: This provision allows executor customization points to invoke the executor's free, non-member execution function of the same name without recursion. --end note]

Whenever std::execution::NAME(ARGS) is a valid expression, that expression satisfies the syntactic requirements for the free execution function named NAME with arity sizeof...(ARGS) with that free execution function's semantics.

1.3.1 require

namespace {
  constexpr unspecified require = unspecified;
}

The name require denotes a customization point. The effect of the expression std::execution::require(E, P0, Pn...) for some expressions E and P0, and where Pn... represents N expressions (where N is 0 or more), is equivalent to:

• If N == 0, P0::is_requirable is true, and the expression decay_t<decltype(P0)>::template static_query_v<decay_t<decltype(E)>> == decay_t<decltype(P0)>::value() is a well-formed constant expression with value true, E.

• If N == 0, P0::is_requirable is true, and the expression (E).require(P0) is well-formed, (E).require(P0).

• If N == 0, P0::is_requirable is true, and the expression require(E, P0) is well-formed, require(E, P0).

• If N > 0 and the expression std::execution::require( std::execution::require(E, P0), Pn...) is well formed, std::execution::require( std::execution::require(E, P0), Pn...).

• Otherwise, std::execution::require(E, P0, Pn...) is ill-formed.

1.3.2 prefer

namespace {
  constexpr unspecified prefer = unspecified;
}

The name prefer denotes a customization point. The effect of the expression std::execution::prefer(E, P0, Pn...) for some expressions E and P0, and where Pn... represents N expressions (where N is 0 or more), is equivalent to:

• If N == 0, P0::is_preferable is true, and the expression decay_t<decltype(P0)>::template static_query_v<decay_t<decltype(E)>> == decay_t<decltype(P0)>::value() is a well-formed constant expression with value true, E.

• If N == 0, P0::is_preferable is true, and the expression (E).require(P0) is well-formed, (E).require(P0).

• If N == 0, P0::is_preferable is true, and the expression prefer(E, P0) is well-formed, prefer(E, P0).

• If N == 0 and P0::is_preferable is true, E.

• If N > 0 and the expression std::execution::prefer( std::execution::prefer(E, P0), Pn...) is well formed, std::execution::prefer( std::execution::prefer(E, P0), Pn...).

• Otherwise, std::execution::require(E, P0, Pn...) is ill-formed.

1.3.3 query

namespace {
  constexpr unspecified query = unspecified;
}

The name query denotes a customization point. The effect of the expression std::execution::query(E, P) for some expressions E and P is equivalent to:

• If the expression decay_t<decltype(P)>::template static_query_v<decay_t<decltype(E)>> is a well-formed constant expression, decay_t<decltype(P)>::template static_query_v<decay_t<decltype(E)>>.

• If the expression (E).query(P) is well-formed, (E).query(P).

• If the expression query(E, P) is well-formed, query(E, P).

• Otherwise, std::execution::query(E, P) is ill-formed.

1.3.4 Customization point type traits

template<class Executor, class... Properties> struct can_require;
template<class Executor, class... Properties> struct can_prefer;
template<class Executor, class Property> struct can_query;

This sub-clause contains templates that may be used to query the properties of a type at compile time. Each of these templates is a UnaryTypeTrait (C++Std [meta.rqmts]) with a BaseCharacteristic of true_type if the corresponding condition is true, otherwise false_type.

1.4 Executor properties

1.4.1 In general

An executor's behavior in generic contexts is determined by a set of executor properties, and each executor property imposes certain requirements on the executor.

Given an existing executor, a related executor with different properties may be created by invoking the require member or non-member functions. These functions behave according the Table below. In the Table below, x denotes a (possibly const) executor object of type X, and p denotes a (possibly const) property object.

[Note: As a general design note properties which define a mutually exclusive pair, that describe an enabled or non-enabled behaviour follow the convention of having the same property name for both with the not_ prefix to the property for the non-enabled behaviour. --end note]

The current value of an executor's properties can be queried by invoking the query function. This function behaves according the Table below. In the Table below, x denotes a (possibly const) executor object of type X, and p denotes a (possibly const) property object.

1.4.2 Requirements on properties

A property type P shall provide:

• A nested constant expression named is_requirable of type bool, usable as P::is_requirable.
• A nested constant expression named is_preferable of type bool, usable as P::is_preferable.

[Note: These constants are used to determine whether the property can be used with the require and prefer customization points, respectively. --end note]

A property type P may provide a nested type polymorphic_query_result_type that satisfies the CopyConstructible and Destructible requirements. If P::is_requirable == true or P::is_preferable == true, polymorphic_query_result_type shall also satisfy the DefaultConstructible requirements. [Note: When present, this type allows the property to be used with a polymorphic executor wrapper. --end note]

A property type P may provide:

• A nested variable template static_query_v, usable as P::static_query_v<Executor>. This may be conditionally present.
• A member function value().

If both static_query_v and value() are present, they shall return the same type and this type shall satisfy the EqualityComparable requirements.

[Note: These are used to determine whether invoking require would result in an identity transformation. --end note]

1.4.3 Query-only properties

1.4.3.1 Associated execution context property

struct context_t
{
  static constexpr bool is_requirable = false;
  static constexpr bool is_preferable = false;

  using polymorphic_query_result_type = any; // TODO: alternatively consider void*, or simply omitting the type.

  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 execution::query(e, context_t), where e is an executor, shall not change between invocations.

1.4.4 Interface-changing properties

struct oneway_t;
struct bulk_oneway_t;

The interface-changing properties conform to the following specification:

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

  template<class... SupportableProperties> 
    class polymorphic_executor_type;

  using polymorphic_query_result_type = bool;

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

  static constexpr bool value() const { return true; }
};

S::static_query_v<Executor> is true if and only if Executor fulfills S's requirements.

The oneway_t and bulk_oneway_t properties are mutually exclusive.

1.4.4.1 Polymorphic wrappers

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.

The nested class template S::polymorphic_executor_type conforms to the following specification.

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

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

  polymorphic_executor_type& operator=(const polymorphic_executor_type& e) noexcept;
  polymorphic_executor_type& operator=(polymorphic_executor_type&& e) noexcept;
  polymorphic_executor_type& operator=(nullptr_t) noexcept;

  ~polymorphic_executor_type();

  // polymorphic_executor_type modifiers:

  void swap(polymorphic_executor_type& other) noexcept;

  // polymorphic_executor_type operations:

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

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

  // polymorphic_executor_type capacity:

  explicit operator bool() const noexcept;

  // polymorphic_executor_type target access:

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

  // polymorphic_executor_type casts:

  template<class... OtherSupportableProperties>
    polymorphic_executor_type<OtherSupportableProperties...> static_executor_cast() const;
};

// polymorphic_executor_type comparisons:

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

// polymorphic_executor_type specialized algorithms:

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

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

The polymorphic_executor_type class satisfies the general requirements on executors.

[Note: To meet the noexcept requirements for executor copy constructors and move constructors, implementations may share a target between two or more polymorphic_executor_type 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.

1.4.4.1.1 polymorphic_executor_type constructors

polymorphic_executor_type() noexcept;

Postconditions: !*this.

polymorphic_executor_type(nullptr_t) noexcept;

Postconditions: !*this.

polymorphic_executor_type(const polymorphic_executor_type& e) noexcept;

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

polymorphic_executor_type(polymorphic_executor_type&& 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>
  polymorphic_executor_type(polymorphic_executor_type<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>
  polymorphic_executor_type(polymorphic_executor_type<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....

1.4.4.1.2 polymorphic_executor_type assignment

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

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

Returns: *this.

polymorphic_executor_type& operator=(polymorphic_executor_type&& 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.

polymorphic_executor_type& operator=(nullptr_t) noexcept;

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

Returns: *this.

1.4.4.1.3 polymorphic_executor_type destructor

~polymorphic_executor_type();

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

1.4.4.1.4 polymorphic_executor_type modifiers

void swap(polymorphic_executor_type& other) noexcept;

Effects: Interchanges the targets of *this and other.

1.4.4.1.5 polymorphic_executor_type operations

template <class Property>
polymorphic_executor_type 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 execution::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 polymorphic_executor_type::query(e, p) is well-formed, static_cast<Property::polymorphic_query_result_type>(polymorphic_executor_type::query(e, p)), where e is the target object of *this. Otherwise, Property::polymorphic_query_result_type{}.

1.4.4.1.6 polymorphic_executor_type capacity

explicit operator bool() const noexcept;

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

1.4.4.1.7 polymorphic_executor_type target access

const type_info& target_type() const noexcept;

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

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

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

1.4.4.1.8 polymorphic_executor_type comparisons

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

Returns: !e.

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

Returns: !(a == b).

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

Returns: (bool) e.

1.4.4.1.9 polymorphic_executor_type specialized algorithms

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

Effects: a.swap(b).

template <class Property, class... SupportableProperties>
polymorphic_executor_type prefer(const polymorphic_executor_type<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 execution::prefer(e, p), where e is the target object of *this.

1.4.4.1.10 polymorphic_executor_type casts

template<class... OtherSupportableProperties>
  polymorphic_executor_type<OtherSupportableProperties...> static_executor_cast() const;

Requires: The target object was first inserted into a polymorphic wrapper (whether via the wrapper's constructor or assignment operator) whose template parameters included the parameters in OtherSupportableProperties.

Returns: A polymorphic wrapper whose target is e.

1.4.4.2 oneway_t customization points

In addition to conforming to the above specification for interface-changing properties, the oneway_t property provides the following customization:

struct oneway_t
{
  template<class Executor>
    friend see-below require(Executor ex, oneway_t);
};

This customization point returns an executor that satisfies the oneway_t requirements by adapting the native functionality of an executor that does not satisfy the oneway_t requirements.

template<class Executor>
  friend see-below require(Executor ex, oneway_t);

Returns: A value e1 of type E1 that holds a copy of ex. E1 has member functions require and query that forward to the corresponding members of the copy of ex, if present. e1 has the same properties as ex, except for the addition of the oneway_t property and the exclusion of other interface-changing properties. The type E1 satisfies the OneWayExecutor requirements by implementing member function execute in terms of the member function bulk_execute of the object ex, and E1 has a member function Executor require(bulk_oneway_t) const that returns a copy of ex.

Remarks: This function shall not participate in overload resolution unless oneway_t::static_query_v<Executor> is false and bulk_oneway_t::static_query_v<Executor> is true.

1.4.4.3 oneway_t polymorphic wrapper

In addition to conforming to the above specification for polymorphic wrappers, the nested class template oneway_t::polymorphic_executor_type provides the following member functions:

template <class... SupportableProperties>
class polymorphic_executor_type
{
public:
  template<class Executor>
    polymorphic_executor_type(Executor e);

  template<class Executor>
    polymorphic_executor_type& operator=(Executor e);

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

oneway_t::polymorphic_executor_type satisfies the OneWayExecutor requirements.

template<class Executor>
  polymorphic_executor_type(Executor e);

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

• can_require_v<Executor, oneway_t>.
• 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 e1, where e1 is the result of execution::require(e, oneway).

template<class Executor>
  polymorphic_executor_type& operator=(Executor e);

Requires: As for template<class Executor> polymorphic_executor_type(Executor e).

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

Returns: *this.

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

Effects: Performs e.execute(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().

1.4.4.4 bulk_oneway_t customization points

In addition to conforming to the above specification for interface-changing properties, the oneway_t property provides the following customization:

struct bulk_oneway_t
{
  template<class Executor>
    friend see-below require(Executor ex, bulk_oneway_t);
};

This customization point returns an executor that satisfies the bulk_oneway_t requirements by adapting the native functionality of an executor that does not satisfy the bulk_oneway_t requirements.

template<class Executor>
  friend see-below require(Executor ex, bulk_oneway_t);

Returns: A value e1 of type E1 that holds a copy of ex. E1 has member functions require and query that forward to the corresponding members of the copy of ex, if present. e1 has the same properties as ex, except for the addition of the bulk_oneway_t property and the exclusion of other interface-changing properties. The type E1 satisfies the BulkOneWayExecutor requirements by implementing member function bulk_execute in terms of the member function execute of the object ex, and E1 has a member function Executor require(oneway_t) const that returns a copy of ex.

Remarks: This function shall not participate in overload resolution unless bulk_oneway_t::static_query_v<Executor> is false and oneway_t::static_query_v<Executor> is true.

1.4.4.5 bulk_oneway_t polymorphic wrapper

In addition to conforming to the above specification for polymorphic wrappers, the nested class template bulk_oneway_t::polymorphic_executor_type has the following member functions:

template <class... SupportableProperties>
class polymorphic_executor_type
{
public:
  template<class Executor>
    polymorphic_executor_type(Executor e);

  template<class Executor>
    polymorphic_executor_type& operator=(Executor e);

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

bulk_oneway_t::polymorphic_executor_type satisfies the BulkOneWayExecutor requirements.

template<class Executor>
  polymorphic_executor_type(Executor e);

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

• can_require_v<Executor, bulk_oneway_t>.
• 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 e1, where e1 is the result of execution::require(e, bulk_oneway).

template<class Executor>
  polymorphic_executor_type& operator=(Executor e);

Requires: As for template<class Executor> polymorphic_executor_type(Executor e).

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

Returns: *this.

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

Effects: Performs e.bulk_execute(f2, n, sf2), where:

• e is the target object of *this;
• sf1 is the result of DECAY_COPY(std::forward<SharedFactory>(sf));
• sf2 is a function object of unspecified type that, when invoked as sf2(), performs sf1();
• s1 is the result of sf1();
• s2 is the result of sf2();
• f1 is the result of DECAY_COPY(std::forward<Function>(f));
• f2 is a function object of unspecified type that, when invoked as f2(i, s2), performs f1(i, s1), where i is a value of type size_t.

1.4.5 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
{
  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;

  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;

  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 execution::query(e1, p1) and a subsequent invocation execution::query(e1, p1), where

• p1 is an instance of S or S::Ei, and
• e2 is the result of execution::require(e1, p2) or execution::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(execution::query(ex, std::declval<const S::Nk>())));

Returns: execution::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.

1.4.5.1 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.

1.4.5.1.1 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
{
  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. If Executor satisfies the OneWayExecutor requirements, E1 shall satisfy the OneWayExecutor requirements by providing member functions require, query, and execute that forward to the corresponding member functions of the copy of ex. If Executor satisfies the BulkOneWayExecutor requirements, E1 shall satisfy the BulkOneWayExecutor requirements by providing member functions require, query, and bulk_execute that forward to the corresponding member functions of the copy of ex. In addition, E1 provides an overload of require such that e1.require(blocking.always) returns a copy of e1, an overload of query such that e1.query(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.

1.4.5.2 Properties to indicate if blocking and directionality may be adapted

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

blocking_adaptation_t provides nested property types and objects as described below.

1.4.5.2.1 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
{
  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. If Executor satisfies the OneWayExecutor requirements, E1 shall satisfy the OneWayExecutor requirements by providing member functions require, query, and execute that forward to the corresponding member functions of the copy of ex. If Executor satisfies the BulkOneWayExecutor requirements, E1 shall satisfy the BulkOneWayExecutor requirements by providing member functions require, query, and bulk_execute that forward to the corresponding member functions of the 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.

1.4.5.3 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.

1.4.5.4 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]

1.4.5.5 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]

1.4.5.6 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]

1.4.6 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
{
    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 execution::query(E, P) is well formed, where P is an object of type allocator_t<ProtoAllocator>, then: * the type of the expression execution::query(E, P) shall satisfy the ProtoAllocator requirements; * the result of the expression execution::query(E, P) shall be the allocator currently established in the executor E; and * the expression execution::query(E, allocator_t<void>{}) shall also be well formed and have the same result as execution::query(E, P).

1.4.6.1 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.

1.5 Executor type traits

1.5.1 Determining that a type satisfies executor type requirements

template<class T> struct is_oneway_executor;
template<class T> struct is_bulk_oneway_executor;

This sub-clause contains templates that may be used to query the properties of a type at compile time. Each of these templates is a UnaryTypeTrait (C++Std [meta.rqmts]) with a BaseCharacteristic of true_type if the corresponding condition is true, otherwise false_type.

1.5.2 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(execution::require(declval<const Executor&>(), execution::bulk))
    >;

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

1.5.3 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(execution::require(declval<const Executor&>(), execution::bulk))
    >;

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

1.6 Polymorphic executor support

1.6.1 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;
};

1.6.1.1 bad_executor constructors

bad_executor() noexcept;

Effects: Constructs a bad_executor object.

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

1.6.2 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>
void do_async_work(
    Executor ex,
    Callback callback)
{
  if (try_work() == done)
  {
    // Work completed immediately, invoke callback.
    execution::require(ex,
        execution::single,
        execution::oneway,
      ).execute(callback);
  }
  else
  {
    // Perform work in background. Track outstanding work.
    start_background_work(
        execution::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 execution::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.
    execution::require(ex,
        execution::single,
        execution::oneway,
      ).execute(callback);
  }
  else
  {
    // Perform work in background. Track outstanding work.
    start_background_work(
        execution::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 execution::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(execution::prefer(std::move(ex), std::declval<const InnerProperty>())))
      -> decltype(execution::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(execution::query(ex, std::declval<const InnerProperty>())))
      -> decltype(execution::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(execution::prefer(std::move(ex), std::declval<const InnerProperty>())))
    -> decltype(execution::prefer(std::move(ex), std::declval<const InnerProperty>()));

Returns: execution::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 execution::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(execution::query(ex, std::declval<const InnerProperty>())))
    -> decltype(execution::query(ex, std::declval<const InnerProperty>()));

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

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

1.7 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.

1.7.1 Header <thread_pool> synopsis

namespace std {

  class static_thread_pool;

} // namespace std

1.7.2 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 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 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, 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 executor, attach, wait, and stop, and the associated 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]

1.7.2.1 Types

using executor_type = see-below;

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

1.7.2.2 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().

1.7.2.3 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.

1.7.2.4 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>())

1.7.3 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:

    typedef std::size_t shape_type;
    typedef std::size_t index_type;

    // 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.

1.7.3.1 Constructors

C(const C& other) noexcept;

Postconditions: *this == other.

C(C&& other) noexcept;

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

1.7.3.2 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.

1.7.3.3 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.

1.7.3.4 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).

1.7.3.5 static_thread_pool executor types with the execution::oneway property

In addition to conforming to the above specification, static_thread_pool executors having the execution::oneway property established shall conform to the following specification.

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

C is a type satisfying the OneWayExecutor requirements.

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

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

1.7.3.6 static_thread_pool executor types with the execution::bulk_oneway property

In addition to conforming to the above specification, static_thread_pool executors having the execution::bulk_oneway property established shall conform to the following specification.

class C
{
  public:
    template<class Function, class SharedFactory>
      void bulk_execute(Function&& f, size_t n, SharedFactory&& sf) const;
};

C is a type satisfying the BulkOneWayExecutor requirements.

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

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