Support Quantity non-type template parameters #318
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Technically, we can't get non-type template parameters of custom class type until C++20. But for a `Quantity<U, R>` whose rep `R` is integral, we can get the next best thing! This PR introduces a public member typedef, `Quantity<U, R>::NTTP`, which _can_ be used as a template parameter, because it's an enumeration. We support _bidirectional, implicit_ conversion between `Quantity<U, R>` and `Quantity<U, R>::NTTP`, as long as there's an exact match for `U` and `R`. In all other cases, users must use the usual `Quantity` conversion operators to get it into the right type. They can also explicitly request conversion from the NTTP to the `Quantity` by passing the former to `from_nttp(...)`. It's a niche use case, but it's finally possible to use a `Quantity` as a template parameter in a unit-safe way! There just was no other comparable solution before this. Compile time measurements are ongoing, but so far they suggest a small but nonzero penalty. It appears that it varies from file to file, but is generally less than 50 ms. I think that's an OK price to pay for this feature. Fixes #316.
chiphogg
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release notes: ✨ lib (enhancement)
geoffviola
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Oct 31, 2024
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Cool. Safe, compile-time operations are great 😎
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Technically, we can't get non-type template parameters of custom class
type until C++20. But for a
Quantity<U, R>whose repRis integral,we can get the next best thing!
This PR introduces a public member typedef,
Quantity<U, R>::NTTP,which can be used as a template parameter, because it's an
enumeration. We support bidirectional, implicit conversion between
Quantity<U, R>andQuantity<U, R>::NTTP, as long as there's an exactmatch for
UandR. In all other cases, users must use the usualQuantityconversion operators to get it into the right type. They canalso explicitly request conversion from the NTTP to the
Quantitybypassing the former to
from_nttp(...).It's a niche use case, but it's finally possible to use a
Quantityasa template parameter in a unit-safe way! There just was no other
comparable solution before this.
Compile time measurements are ongoing, but so far they suggest a small
but nonzero penalty. It appears that it varies from file to file, but
is generally less than 50 ms. I think that's an OK price to pay for
this feature.
Fixes #316.