Add Semantics.Cpp: the C++ projection of the quantity vocabulary

[matt-edmondson]

The .NET side of this library has always been generated from dimensions.json. Holotype needs the same vocabulary in C++, and the alternative to generating it is that someone writes 162 classes by hand and they drift — which is the failure this repository exists to avoid, in a second language.

Two layers, because neither can do the other's job

Structural — Quantity<D> over a Dimension of eight integer exponents. Shipped as a prelude rather than generated: no part of it is derived from the metadata. What it buys is that a product nobody declared still has a type.

Nominal — one class per dimension and per named overload. This is what the exponents cannot do: measured against the real metadata, 72 dimensions share 63 exponent vectors, so Area/NuclearCrossSection, Torque/Energy and AbsorbedDose/EquivalentDose are each one vector between two names.

The angle exponent

dimensionalFormula gains angle, carried by AngularDisplacement, AngularVelocity, AngularAcceleration and AngularJerk — and nothing else. Without it an angle is the same type as a ratio and an angular speed the same type as a frequency; with it, 61 distinct exponent vectors become 63.

Nothing on the .NET side depends on it yet; it travels through DimensionInfo, which is why the generated C# moves (4 lines).

A relationship is checked before it is emitted

The operator is written as Result{ lhs.value() * rhs.value() }, so the exponents must agree with the declared result or it doesn't compile. That turns every claim in integrals and derivatives into something checkable rather than asserted. Four are refused, each by name with both dimensions written out:

Torque * AngularDisplacement -> Energy: is not dimensionally true: L² M T⁻² * A is L² M T⁻² A, and Energy is L² M T⁻².
MomentOfInertia * AngularVelocity -> AngularMomentum: ...
MomentOfInertia * AngularAcceleration -> Torque: ...
Sensitivity * Pressure -> ElectricPotential: is not dimensionally true: L⁻¹ M⁻¹ T² I * L⁻¹ M T⁻² is L⁻² I, and ElectricPotential is L² M T⁻³ I⁻¹.

The first three are not fixable by choosing different angle exponents, and that's provable: Torque × AngularDisplacement → Energy forces torque's angle exponent to −1, and Force × Length → Torque forces it to 0. It's the r×F versus τ·θ contradiction, and it's why SI keeps the radian dimensionless.

⚠️ The fourth is a pre-existing metadata bug with nothing to do with angle. Sensitivity is declared as A/Pa (M⁻¹L⁻¹T²I) while the relationship treats it as V/Pa. One of the two is wrong; which is a physics call, so it's reported rather than guessed at. Nothing had ever multiplied the exponents out before, so nothing had noticed.

How the generated code is written is measured, not chosen

The header of CppQuantityGenerator carries the four rules and why. The same vocabulary written two ways measured 0.9896 and 1.4004 against bare floats on MSVC while GCC and clang folded both away — the wrong formulation passes on three compilers of four.

Tests

16, and two of them are the ones that matter:

• TheWholeVocabularyCompiles — all 153 headers through g++ -std=c++20 -Wall -Wextra, clean.
• AProductWithTheWrongDimensionDoesNotCompile — a product whose exponents don't match its result type must be rejected. Without this, the first test would only prove the headers parse, and the "checked" relationships would be checked against nothing.

Existing suite: 1,125 passed, so the metadata change doesn't move the C# side.

Deliberately not here

• The vector forms. 122 dimension-and-form entries against the 72 magnitudes projected here. They're distinct classes too and need componentwise operations, which have a rule of their own — half-doing them would be worse than not starting.
• A runtime guard in the constructor body. It rides in the member initialiser instead, because ktsu.Coder has no expression-statement node — a call made for its effect isn't something the AST can currently say. A comma expression in a constexpr constructor is a legitimate spelling, but the reason it was chosen is that the alternative couldn't be written.

🤖 Generated with Claude Code

https://claude.ai/code/session_01UGHDsYaaTQdzVR4XBR6miu

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
97.9% Coverage on New Code
0.0% Duplication on New Code

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