Re: [Boost-users] generalize boost macros

Subject: Re: [Boost-users] generalize boost macros
From: John Maddock (jz.maddock_at_[hidden])
Date: 2019-02-03 09:10:07

> That's exactly my problem.
>
> namespace std
> {
> // declare missing log2, cannot say constexpr: compilererror
> inline float128_t log2(const float128_t& arg)	noexcept
> {
>   return log2q(arg);
> }
> }

There's no conceivable way we can implement that as a constexpr algorithm.

>
> // also not constexpr
> inline float128_t myfunc(const float128_t& arg)	noexcept
> {
>   // log2_e not available
>   // constexpr float128_t factor =
> boost::math::constants::log2_e<float128_t>();
>
>   // compiler generates a function call to log2
>   const float128_t factor =
> std::log2(boost::math::constants::e<float128_t>());
>
>   return arg*factor;
> }

log2(e) == 1/log(2)

So:

constexpr float128_t log2_e = 1 / 
boost::math::constants::ln_two<float128_t>();

does actually work just fine.

Note that we cannot provide every single conceivable constant, so you 
can also define your own:

namespace boost {
 Â Â  namespace math {
 Â Â Â Â Â  namespace constants {

 Â Â Â Â Â Â Â Â  BOOST_DEFINE_MATH_CONSTANT(log2_e, 
1.4426950408889634073599246810018921374266459541529859341, 
"1.4426950408889634073599246810018921374266459541529859341354494069311092191811850798855266228935063444969975183096");

 Â Â Â Â Â  }
 Â Â  }
}

Which is also constepxr for float128_t.

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