Re: Another Boost.Decimal Review

From: Jeff Garland (azswdude_at_[hidden])
Date: 2025-01-23 16:10:21

On Thu, Jan 23, 2025 at 8:53 AM Matt Borland <matt_at_[hidden]> wrote:

>
> > Standardization:
> >
>
>
> We would certainly appreciate your help with standardization. Since C23
> added decimal types I don't know why C++ would not have them.
>

I expect c++26 will be standardized on top of C23 so will inherit those
types. But of course having a native C++ type that does c++ things is
different from _Decimal32 C code. I'll contact you offline on standardizing.

> > C23 and _Decimal*
> >
>
> > Should this library consider being a layer over C23 types where that's
> > possible? GCC supports them at least in a limited fashion.
> >
>
> > https://gcc.gnu.org/onlinedocs/gcc/Decimal-Float.html
> >
>
>
> It would be easier to add a wrapper over those than the Intel lib because
> they already come with GCC.
>
>
Yes -- noting that the current gcc implementation doesn't implement any of
the cmath functions.

>
> > > Does the API match with current best practices?
> >
>
> > Construction:
> >
>
> > template <typename T, typename T2>constexpr decimal32(T coeff, T2 exp,
> >
>
> > bool sign = false) noexcept;
> >
>
> > Is the sign flag helpful here? It seems simpler to just handle a negative
> > coefficient -- like it already does. When I look at it there's already
> > cognitive dissidence in my brain about if the sign flag overrides or
> > inverts the coefficient sign. The N2849 api doesn't have this flag.
> >
>
>
> I think a couple people have mentioned confusion over this so it's likely
> in everyone's best interest to remove.
>

Excellent.

> > Conversions:
> >
>
> > I'd like to see the library pull back on convertibility.
> >
>
> > long double ld{1.23456789234567};
> > decimal32 d(ld); // what happens -- round, truncate, doesn't compile?
> >
>
> > The best answer is doesn't compile. There's been significant work in the
> > standard now to define floating point rank and limit conversions with the
> > extended floating point types. Specifically:
> >
>
> >
> https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1467r9.html#rank
> >
>
>
> That's not a terrible idea. We made them explicit and added notes in the
> docs that it may not do what you expect it to do. It's probably best to
> just remove that particular foot gun.
>

Yep, there's too many of them for sure.

> > Literals
> >
>
> > constexpr auto operator "" _DF(const char* str) -> decimal32constexpr
> >
>
> > auto operator "" _df(const char* str) -> decimal32
> >
>
> >
>
> > Why not _d32, _d64, _d128? _df makes me think fast, not 32.
> >
>
>
> The current ones match the TR and what GCC has been doing with the
> _DecimalXX types.
>

hmm, ugh right.

> We will be switching a bunch of things to tables after Peter Turcan and
> other brought it up.
>
> > In the using the library section I'd like to see some:
> > - examples of using it it math
> > - examples of the behavior of NAN/inf values
> > - conversions from other types like float expanded
> >
>
>
> On the math one there are a couple examples with math (including binding
> with Boost.Math). The others we can add.
>

Example .cpp files are good -- but in a *tutorial like form* in the docs is
better.

Jeff