P2846 R6 reserve_hint: Eagerly reserving memory for not-quite-sized lazy ranges

[wg21bot]

P2846R0 size_hint: Eagerly reserving memory for not-quite-sized lazy ranges (Corentin Jabot)

[inbal2l]

SG9 (Ranges) reviewed P846R0 during the Varna meeting on 2023-06-12 (Full Minutes).

Polls

POLL: The standard needs to clearly define where size_hint should be called

SF	F	N	A	SA
0	2	0	3	2

Outcome: Weak consensus against

# Of Authors: 1

Author’s Position: WF

Attendance: 9

WF: We’re unsure what’s the recommendation (both said “very weakly in favor”)

SA: Performance hints should be hints

POLL: Prefer approximate_size over size_hint (for consistency with the concept name, approximately_sized_range)

F	N	A
3	2	2

Outcome: No consensus

# Of Authors: 1

Author’s Position: F

Attendance: 9

POLL: Modify the paper to include suggested guidance, make sized_range to subsume approximately_sized_range (assuming non breaking change), and forward “P2846R0: size_hint: Eagerly reserving memory for not-quite-sized lazy ranges” to LEWG for C++26.

SF	F	N	A	SA
3	3	1	0	0

Attendance: 9

# of Authors: 1

Author Position: SF

Outcome: Strong Consensus in Favour

Summary

The paper needs to apply the following updates:

1. Capture the implementation options (includes: mandating a call to size_hint (taking side effects into consideration), defining loosened requirements without mandating the use, or allowing implementations to ignore size_hint) - to be determined by LEWG
2. rename size_hint to: approximately_sized_range

And move the revised paper to LEWG for C++26

[wg21bot]

P2846R1 size_hint: Eagerly reserving memory for not-quite-sized lazy ranges (Corentin Jabot)

[ben-craig]

2024-01-16 Library Evolution Telecon

P2846R1: size_hint: Eagerly reserving memory for not-quite-sized lazy ranges

2024-01-16 Library Evolution Telecon Minutes

Champion: Corentin Jabot

Chair: Ben Craig

Minute Taker: Mark Hoemmen

Summary

POLL: P2846 (size_hint: Eagerly reserving memory for not-quite-sized lazy ranges) needs max, min and approximate facilities, and not just a single number.

SF	F	N	A	SA
1	0	4	4	7

Attendance: 18

# of Authors: 1

Author's Position: SA

Outcome: Consensus against

Next Steps

Research idea of unconditional size hint on views like take

Discuss some of the names brought up in the meeting, reserve_hint, min_reserve

Next time, look at modifications, consider impl defined or otherwise for vector's use of sized hint, then look at wording.

[wg21bot]

P2846R2 reserve_hint: Eagerly reserving memory for not-quite-sized lazy ranges (Corentin Jabot)

[FabioFracassi]

2024-07-23 Library Evolution Telecon

P2846R2: reserve_hint: Eagerly reserving memory for not-quite-sized lazy ranges

2024-07-23 Library Evolution Telecon Minutes

Champion: Corentin Jabot
Chair: Fabio Fracassi
Minute Taker: Ben Craig

Summary

Papers Forwarding Checklist:

1. Why should this utility go into the standard library? - Customization point for standard facilities / optimization opportunity
2. Examples? - Yes
3. Discussion of prior art? - Yes
4. Implementation experience? - No
5. Usage experience? - No
6. Deployment experience? - No
7. Performance considerations? - Yes, with benchmarks
8. Changes Library Evolution previously requested? - N/A
9. Support in "std::format" and "hash" (if required)? - N/A
10. Feature test macro? - Yes, but needs correction
11. Freestanding? - N/A
12. Constexpr? - Yes
13. Wording? - Partial, not checked

POLL: Require the use of the reserve_hint in the range constructor of vector and string in P2846R2

SF	F	N	A	SA
2	6	2	0	2

Attendance: 18
# of Authors: 1
Author's Position: SA
Outcome: consensus in favour

SA:

• adds a lot of complexity without much benefit
• requires a call, but does not say what is the user observable behaviour

Next Steps

Next revision will be seen by LEWG

[wg21bot]

P2846R3 reserve_hint: Eagerly reserving memory for not-quite-sized lazy ranges (Corentin Jabot)

[wg21bot]

P2846R4 reserve_hint: Eagerly reserving memory for not-quite-sized lazy ranges (Corentin Jabot)

[inbal2l]

2024-11-21 Library Evolution Wroclaw Meeting

P2846R4 reserve_hint: Eagerly reserving memory for not-quite-sized lazy ranges

2024-11-21 Library Evolution Wroclaw Meeting Minutes

Champion: Corentin Jabot
Chair: Inbal Levi / Robert Leahy
Minutes: Eddie Nolan

Summary

POLL: Remove the requirement for “is_nothrow_move_constructible_v” to be true from the second condition for avoiding allocation for a vector in the vector constructor in “P2846R4 reserve_hint: Eagerly reserving memory for not-quite-sized lazy ranges”.

SF	F	N	A	SA
7	2	5	3	0

Attendance: 24 (IP) + 3 (R)
Author's Position: - (didn’t vote)
Outcome: Consensus in favor

POLL: Modify the FTM into “__cpp_lib_ranges_reserve_hint”, and apply the modification from the poll above (Remove the requirement for “is_nothrow_move_constructible_v” to be true from the second condition for avoiding allocation for a vector in the vector constructor) and forward the modified “P2846R5 reserve_hint: Eagerly reserving memory for not-quite-sized lazy ranges” to LWG for C++26.

SF	F	N	A	SA
11	6	0	0	1

Attendance: 24 (IP) + 3 (R)
Author's Position: SF
Outcome: Consensus in favor

SA: There’s a very narrow use case for this paper and it can be done in another way and the complexity is high, anyone who’s making a generic wrapping range is going to want to pass on the size_hint, and the Rust implementation, 10% of all the times they wrap an iterator they do a size_hint thing, so it’s a tremendous cost for a very very narrow use case that could be implemented another way.

Next Steps

The paper with the following modifications:

1. Modify FTM into: “__cpp_lib_ranges_reserve_hint”
2. Remove the requirement for “is_nothrow_move_constructible_v” to be true from the second condition for avoiding allocation for a vector in the vector
3. Fix minor wording bug - add “or”

The updated revision forwarded to LWG for C++26.

[wg21bot]

P2846R5 reserve_hint: Eagerly reserving memory for not-quite-sized lazy ranges (Corentin Jabot)

[bstamour]

LWG looked at this paper on Monday afternoon in Hageneberg.

[bstamour]

LWG polled this in Hagenberg Thursday afternoon:

Put P2846R6 into C++26 with JG and RA to check edits
| F | A | N |
| 9 | 0 | 0 |

[wg21bot]

P2846R6 reserve_hint: Eagerly reserving memory for not-quite-sized lazy ranges (Corentin Jabot)