Belfast 2019-11 LEWGI Minutes

P1935R0 Units Library Rationale: Direction Review

Chair: Bryce Adelstein Lelbach

Champion: Mateusz Pusz

Minute Taker: Marco Foco

Start Review: 11-07 8:41

std::chrono is a good litmus test for this feature, as it's an existing part of the library that has a notion of units.

Design goals starting on slide 10.

Priorities: safety > performance > ease of use

POLL: Quantity kind (width, height, etc) should be a priority for the initial release of this feature.

Attendance: 19

# of Authors: 1

Author Position: A

That has consensus against.

POLL: We should promise more committee time to pursuing adding common units (such as SI, customary, etc) to the standard library, knowing that our time is scarce and this will leave less time for other work.

Attendance: 23

# of Authors: 1

Author Position: SF

A: I think only SI will have too small of an audience; I want something extensible.

That has consensus.

POLL: We should promise more committee time to pursuing a standard library framework for user defined units and unit systems, knowing that our time is scarce and this will leave less time for other work.

Attendance: 24

# of Authors: 1

Author Position: SF

A: Afraid the audience that needs this is too small.

SA: Currency is over my dead body.

That has consensus.

POLL: In a C++ standard units framework, C++ conversions should not vary over runtime.

Attendance: 23

# of Authors: 1

Author Position: SF

POLL: We should plan for versioning of standard library physical constants.

Attendance: 23

# of Authors: 0

That has unanimous consent.

POLL: Compile time diagnostics are a Quality-of-Implementation (QOI) matter and should not influence the standard units library API.

Attendance: 23

# of Authors: 1

Authors Position: SA

That has no consensus.

Lunch: 12:00

Resume: 13:35

Minute Taker: Ben Craig

POLL: A standard library of quantity concepts are a priority for the initial release of a standard library units framework.

Attendance: 21

# of Authors: 1

Author Position: SF

A: I voted against because I don't think they should be a priority.

A: I voted against because of compile-time concerns.

That has no consensus.

POLL: We should promise more committee time to pursuing P1935R0 (A C++ Approach to Physical Units), knowing that our time is scarce and this will leave less time for other work.

Attendance: 23

# of Authors: 1

Author Position: SF

SA: Existing practice seems unsettled, unclear but very large scope, worried it obscures precision from my developers, concerns about how it interacts with vectors and matrices.

End: 15:30

CONSENSUS: Bring a revision of P1935R0 (A C++ Approach to Physical Units), with the guidance below, to LEWGI and SG6 for further direction review.

• Remove the proposed standard library quantity concepts.
• Write a separate paper exploring using rational as a CNTTP.
• Explore approaches for versioning standard library physical constants:
  • Version namespaces.
  • Add the version to the identifiers.
  • Template variables parameterized by a tag.
  • WEB's approach (see minutes).

This is informative only, no action.