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nholthaus / units

a compile-time, header-only, dimensional analysis and unit conversion library built on c++14 with no dependencies.
http://nholthaus.github.io/units/
MIT License
933 stars 134 forks source link

Reverting #150, "strong" units #219

Open JohelEGP opened 5 years ago

JohelEGP commented 5 years ago

I believe merging #150 was an error.

For starters, its title is wrong. The units already were strong types, so there was nothing to "strengthen." The README is very clear on the advantages implied by this type safety, unlike using double or a type alias like using meters = double; for representing meters. I should have used some other term.

The purposes of the changes in #150 were to allow using CTAD and improved error messages. Out of this came the idea manifested in #167, which greatly took care of the latter purpose. And CTAD became the only purpose (personally) for #150.

Since then, we've discovered a few problems with the merged #150, as discussed in #200. In exchange for CTAD, we're paying N * M more in some ways. What used to be a single partial specialization of std::common_type for units, became M specializations for each of the N "strong" units.

Hopefully one day alias templates can support CTAD, as is being proposed in https://wg21.link/P1021. For now, I'd rather settle with being explicit about the underlying type. Naturally, I propose myself for reverting #150. And I am sorry for pushing for it without fully realizing what it entailed.

JohelEGP commented 5 years ago

Luckily, it seems that EWG approved the CTAD patches, says https://botondballo.wordpress.com/2018/11/30/trip-report-c-standards-meeting-in-san-diego-november-2018/.

JohelEGP commented 5 years ago

There's also https://github.com/nholthaus/units/pull/217#issuecomment-436797746.

The next committee meeting (http://wg21.link/N4783, July 15 – 20, 2019) is the last opportunity to put into the WD the CTAD wording for alias templates for C++20. It looks like the wording's difficult, so it might not make it.

JohelEGP commented 4 years ago

Class Template Argument Deduction for Alias Templates made it in for the C++20 CD (https://github.com/cplusplus/papers/issues/566).

nholthaus commented 3 years ago

@johelegp I think resolving this issue is the crux of the 3.0 release. Do you think you can revert the strong types?

JohelEGP commented 3 years ago

Yes. I'll get to it right away.