More constexpr in C++20

The C++ Kona meeting starts tomorrow or if you prefer on February the 18th, 2019, since you’re probably not reading this on the day I’m writing it. Apart from looking forward to C++20 in general, it matters to me because I’ve got two closely related papers under consideration.  TL;DR:


Except since they’re C++ committee papers, it’s actually more like:


Since the C++ committee doesn’t accept memes as proposals, what we actually submitted is:

  1. P0533R4, a proposal to add constexpr to the elementary cmath functions. Elementary is in this case as on the same level as +,-,* and /, which we defined formally as closed over the rationals.Floating point numbers aren’t rationals of course, but arithmetic operations have the property that if you apply them to a rational number you get a rational number (unlike, say, sqrt) and importantly, there are no domain errors when fed with a finite number. Our property seemed to be the simplest one that fitted. Looking at the other functions, we found things like floor, ceil also fitted neatly.

    We though this paper would be straightforward since those functions really aren’t fundamentally different from arithmetic, so  the same rules should apply.

  2. P1383R0 a proposal to constexpr most of the rest of cmath. The LEWG pretty much liked P0533 and asked if we could submit a further paper to add constexpr to the non elementary functions.This paper adds most of the rest (sin, cos, exp, log, lgamma, etc). After some useful discussion, we excluded the cmath special functions, on the grounds that there are very few people in the world who know how to implement them so having them in could be a serious burden on compiler vendors.

If they get in then this post will finally be legal C++.

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