33 Execution control library [exec]

33.16 Namespace system_context_replaceability [exec.sysctxrepl]

33.16.1 General [exec.sysctxrepl.general]

Facilities in the system_context_replaceability namespace allow users to replace the default implementation of parallel_scheduler.

33.16.2 query_parallel_scheduler_backend [exec.sysctxrepl.query]

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shared_ptr<parallel_scheduler_backend> query_parallel_scheduler_backend();

query_parallel_scheduler_backend() returns the implementation object for a parallel scheduler.

Returns: A non-null shared pointer to an object that implements the parallel_scheduler_backend interface.

Remarks: This function is replaceable ([dcl.fct.def.replace]).

namespace std::execution::system_context_replaceability { struct receiver_proxy { virtual ~receiver_proxy() = default; protected: virtual bool query-env(unspecified) noexcept = 0; // exposition only public: virtual void set_value() noexcept = 0; virtual void set_error(exception_ptr) noexcept = 0; virtual void set_stopped() noexcept = 0; template<class P, class-type Query> optional<P> try_query(Query q) noexcept; }; struct bulk_item_receiver_proxy : receiver_proxy { virtual void execute(size_t, size_t) noexcept = 0; }; }

receiver_proxy represents a receiver that will be notified by the implementations of parallel_scheduler_backend to trigger the completion operations.

bulk_item_receiver_proxy is derived from receiver_proxy and is used for bulk_chunked and bulk_unchunked customizations that will also receive notifications from implementations of parallel_scheduler_backend corresponding to different iterations.

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template<class P, class-type Query>
  optional<P> try_query(Query q) noexcept;

Mandates: P is a cv-unqualified non-array object type.

Returns: Let env be the environment of the receiver represented by *this.

(6.1)
Query is not a member of an implementation-defined set of supported queries; or
(6.2)
P is not a member of an implementation-defined set of supported result types for Query; or
(6.3)
the expression q(env) is not well-formed or does not have type cv P,

then returns nullopt.

Otherwise, returns q(env).

Remarks: get_stop_token_t is in the implementation-defined set of supported queries, and inplace_stop_token is a member of the implementation-defined set of supported result types for get_stop_token_t.

33.16.3 Class parallel_scheduler_backend [exec.sysctxrepl.psb]

namespace std::execution::system_context_replaceability { struct parallel_scheduler_backend { virtual ~parallel_scheduler_backend() = default; virtual void schedule(receiver_proxy&, span<byte>) noexcept = 0; virtual void schedule_bulk_chunked(size_t, bulk_item_receiver_proxy&, span<byte>) noexcept = 0; virtual void schedule_bulk_unchunked(size_t, bulk_item_receiver_proxy&, span<byte>) noexcept = 0; }; }

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virtual void schedule(receiver_proxy& r, span<byte> s) noexcept = 0;

Preconditions: The ends of the lifetimes of *this, the object referred to by r, and any storage referenced by s all happen after the beginning of the evaluation of one of the expressions below.

Effects: A derived class shall implement this function such that:

(2.1)
One of the following expressions is evaluated:
- (2.1.1)
  r.set_value(), if no error occurs, and the work is successful;
- (2.1.2)
  r.set_error(eptr), if an error occurs, where eptr is an object of type exception_ptr;
- (2.1.3)
  r.set_stopped(), if the work is canceled.
(2.2)
Any call to r.set_value() happens on an execution agent of the execution context represented by *this.

Remarks: The storage referenced by s may be used by *this as temporary storage for the duration of the operation launched by this call.

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virtual void schedule_bulk_chunked(size_t n, bulk_item_receiver_proxy& r,
                                   span<byte> s) noexcept = 0;

Preconditions: The ends of the lifetimes of *this, the object referred to by r, and any storage referenced by s all happen after the beginning of the evaluation of one of the expressions below.

Effects: A derived class shall implement this function such that:

(5.1)
Eventually, one of the following expressions is evaluated:
- (5.1.1)
  r.set_value(), if no error occurs, and the work is successful;
- (5.1.2)
  r.set_error(eptr), if an error occurs, where eptr is an object of type exception_ptr;
- (5.1.3)
  r.set_stopped(), if the work is canceled.
(5.2)
If r.execute(b, e) is called, then b and e are in the range [0, n) and b < e.
(5.3)
For each i in [0, n), there is at most one call to r.execute(b, e) such that i is in the range [b, e).
(5.4)
If r.set_value() is called, then for each i in [0, n), there is exactly one call to r.execute(b, e) such that i is in the range [b, e).
(5.5)
All calls to execute on r happen before the call to either set_value, set_error, or set_stopped on r.
(5.6)
All calls to execute and set_value on r are made on execution agents of the execution context represented by *this.

Remarks: The storage referenced by s may be used by *this as temporary storage for the duration of the operation launched by this call.

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virtual void schedule_bulk_unchunked(size_t n, bulk_item_receiver_proxy& r,
                                     span<byte> s) noexcept = 0;

Preconditions: The ends of the lifetimes of *this, the object referred to by r, and any storage referenced by s all happen after the beginning of the evaluation of one of the expressions below.

Effects: A derived class shall implement this function such that:

(8.1)
Eventually, one of the following expressions is evaluated:
- (8.1.1)
  r.set_value(), if no error occurs, and the work is successful;
- (8.1.2)
  r.set_error(eptr), if an error occurs, where eptr is an object of type exception_ptr;
- (8.1.3)
  r.set_stopped(), if the work is canceled.
(8.2)
If r.execute(b, e) is called, then b is in the range [0, n) and e is equal to b + 1.

For each i in [0, n), there is at most one call to r.execute(i, i + 1).
(8.3)
If r.set_value() is called, then for each i in [0, n), there is exactly one call to r.execute(i, i + 1).
(8.4)
All calls to execute on r happen before the call to either set_value, set_error, or set_stopped on r.
(8.5)
All calls to execute and set_value on r are made on execution agents of the execution context represented by *this.

Remarks: The storage referenced by s may be used by *this as temporary storage for the duration of the operation launched by this call.