P3980R0 — Task's Allocator Use

P3980R0 — Task's Allocator Use (10 items) LEWG, LWG

Dietmar Kühl

This paper addresses several NB comments (US 253-386, US 254-385, US 255-384, US 261-391) concerning the use of allocators in std::execution::task. It proposes that the allocator used for the coroutine frame should be specified via an allocator_arg/allocator pair (preferably as the first arguments), while the allocator forwarded through task's environment to child senders should be obtained from the receiver's environment via get_allocator, separating these two concerns. The paper presents multiple wording alternatives and includes the preferred approach of requiring allocator_arg as the first argument and deriving the environment allocator from the connected receiver.

Section 4.1, new Mandates for connect — The function parameter is named recv but the Mandates clause refers to rcvr, which is not declared in this normative context. One of the two names must be changed for consistency. [1]
Section 3.2, paragraph 18.2 — Paragraphs 17 and 18.1 change allocator_arg_t to const allocator_arg_t& but paragraph 18.2 still conditions on allocator_arg_t without diff markup, creating an inconsistency with the surrounding wording changes. [2]
Section 3, paragraph after the table — body of the allocator is nonsensical; the intended phrase is body of the coroutine. [3]
Section 4.1, editor's note before paragraph 16 — URL contains task.pomise (missing r), producing a broken link; should be task.promise. [4]
Section 3.1, code blocks cb7 and cb9 — ASCII ... and Unicode HORIZONTAL ELLIPSIS are mixed in the same code blocks. Only ASCII ... is valid C++; use it consistently. [5]
Section 3, numbered list item 1 — Missing verb: the use identical should read the use is identical. [6]
Section 3, numbered list item 3 — simplied is misspelled; should be simplified. [7]
Section 3.2, editor's note — pargraph is misspelled; should be paragraph. [8]
Section 3.3, editor's note — pargraph is misspelled; should be paragraph. [9]

References — Anthropic Citations API

[1]
"Change [task.promise] pargraph 17 and 18 to use the correct type"

[2]
"Change [task.promise] pargraph 17 and 18 to use the correct type and don't convert to allocator_type"

[3]
"No allocator support (none): the use identical and just doesn't mention any allocator."

[4]
"the use can be identical but it can also be simplied taking advantage of the flexible location."

[5]
"Note that the allocator used for the coroutine frame is normally not used in the body of the allocator."

[6]
"[task.promise] p16" with href "https://wg21.link/task.pomise#16""

[7]
"Declaration: `state connect(Rcvr&& recv) &&;` / Mandates: `allocator_type(get_allocator(get_env(rcvr)))`"

[8]
"(18.2) `allocator_type(arg_next)` is a valid expression if there is a parameter of type `allocator_arg_t`."

[9]
"cb7 line 9: `class... Args` (ASCII) vs cb7 line 11: `class … Args` (Unicode U+2026)"

Summary: P3980R0 proposes changes to the allocator handling for std::execution::task so that the coroutine frame allocator is consistently passed as the first coroutine function parameter (of type const allocator_arg_t&) and made accessible within the coroutine body via the promise, rather than only being used implicitly for frame allocation. It provides wording changes to [task.promise] to implement this.

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