P3045R7 — Quantities and units library

P3045R7 — Quantities and units library (26 items) SG6, SG16, LEWGI

Mateusz Pusz, Dominik Berner, Johel Ernesto Guerrero Peña, Chip Hogg, Nicolas Holthaus, Roth Michaels, Vincent Reverdy

This paper proposes a type-safe, zero-overhead C++ standard library for physical quantities and units, based on the mp-units library and consolidated from earlier proposals P2980R1, P2981R1, and P2982R1. It defines a generic framework including dimensions, quantity specifications, units, quantity and quantity_point class templates, affine space abstractions, symbolic expression simplification, and text formatting facilities. The proposal emphasizes compile-time safety through quantity kinds, unit conversions, truncation prevention, and overflow detection, while supporting extensibility for user-defined systems of quantities and units.

Section 11.4.4.6, AC temperature controller code — Angle brackets used where square brackets are required: delta(0.5) should be delta(0.5), consistent with the paper's own quantity reference syntax using operator[]. [1]
Section 18.13.1, bullet list for point helper — point(21) is described as returning a quantity instead of a quantity_point, and the unit in the result type is si::metre instead of si::degree_Celsius. [2]
Section 12.6, User defined quantities and units — DottedEighthNote is assigned the symbol "q." which is already used by DottedQuarterNote; the symbol must be distinct. [3]
Section 15.1.9, Unicode table, PER MILLE SIGN row — C++ code column shows u8"030" (bare ASCII) instead of u8"\u2030"; the Unicode escape backslash-u prefix is missing. [4]
Section 17.2.6, quantity_from example — Format specifier N[:.2f] contains an extra colon before .2f; all other instances in the paper use N[.2f]. [5]
Section 17.2.6, third quantity_from_zero example — Uses fmt::println; every other example in the paper uses std::println and the fmt namespace is never declared. [6]
Document header — Header says P3045R6 but the paper is identified as P3045R7 in other metadata. [7]
Section 9.2, Performance — "The should be no runtime overhead" — "The" should be "There". [8]
Section 9.5, Easy to extend — Subject-verb disagreement: "It ... allow" should be "It ... allows". [9]
Section 18.5.4, Simplifying the resulting symbolic expressions — "prevents ... to derive" is ungrammatical; should be "prevents ... from deriving". [10]
Section 1.5, Changes since P3045R1 — "St. Lois" should be "St. Louis". [11]
Section 18.12.6.3 — "heve" should be "have". [12]
Section 18.12.6.3 — "possbile" should be "possible". [13]
Section 15.1.7, Symbols of scaled units — "artifical" should be "artificial". [14]
Section 15.1.7, Symbols of scaled units — "denumerator" should be "denominator". [15]
Section 15.1.7, Symbols of scaled units — "suprising" should be "surprising". [16]
Section 15.4.3, Unit formatting — "aternatives" should be "alternatives". [17]
Section 18.6.1, Equality and equivalence — "semanthics" should be "semantics". [18]
Section 18.12.6.3, bikeshedding alternatives list — "coerse" (twice) should be "coerce"; the Au library's actual function is coerce_in. [19]
Section 18.10.1, Bikeshedding quantity_spec — "dictonaries" should be "dictionaries". [20]
Section 18, Design details and rationale (Note) — "familar" should be "familiar". [21]

References — Anthropic Citations API

[1]
"Document #: P3045R6"

[2]
"constexpr auto step_delta = delta>(0.5);"

[3]
"point(5) results with a quantity."

[4]
"point(5) results with a quantity."

[5]
"inline constexpr struct DottedEighthNote final : std::named_unit<"q.", std::mag<3> * SixteenthNote> {} DottedEighthNote;"

[6]
"u8"030""

[7]
"std::println("qp2: {::N[:.2f]}, {}, {}","

[8]
"fmt::println("qp1: {}, {}, {}","

[9]
"The should be no runtime overhead"

[10]
"It is fully generic and allow defining any system abstraction on top of it."

[11]
"After consulting with the LEWGI in St. Lois"

[12]
"we also heve a x.force_numerical_value_in(u)"

[13]
"Here are a few possbile alternatives:"

[14]
"the above outputs are a bit artifical"

[15]
"in case a value of a numerator or denumerator is greater or equal 1000"

[16]
"which may look suprising"

[17]
"However, a few aternatives are possible:"

[18]
"Depending on the desired semanthics of equivalent function"

[19]
"x.coerse_in(u), x.coerse_numerical_value_in(u),"

[20]
"is not provided in ISO or BIPM metrology dictonaries"

[21]
"a reader is expected to be familar with them already"

[22]
"This prevents the users to derive own strong types from them"

Summary: P3045R7 proposes a comprehensive quantities and units library for the C++ standard, covering dimensional analysis, unit conversions, quantity points, formatting, and extensibility for user-defined systems. The paper provides design rationale, API surface, and extensive usage examples across physical and non-physical domains.

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