Re: [Boost-users] [math] log_max_value<long double>

Subject: Re: [Boost-users] [math] log_max_value<long double>
From: John Maddock (john_at_[hidden])
Date: 2009-03-20 05:40:59

> Was this discussed before?
>
> <cmath>
>
> using namespace math::tools;
>
> typedef double RealType;
> log_max_value<RealType>()     709
> log(max_value<RealType>())    709.783
> !math::isinf(exp(log_max_value<RealType>())) //true
>
>
> typedef long double RealType;
> log_max_value<RealType>()     11356
> log(max_value<RealType>())    inf
> !math::isinf(exp(log_max_value<RealType>())) //false
>
> i.e. exp in cmath seems to "assume" RealType = double.
>
> i686-apple-darwin9-gcc-4.0.1

Nod, sadly there are some platforms where the long double overloads just 
forward to the double versions :-(

It can lead to absurdities like the sqrt of a finite long double value 
coming back as infinity !

Boost.Math should define BOOST_MATH_NO_LONG_DOUBLE_MATH_FUNCTIONS when this 
is the case: as it's basically impossible to reason sensibly about how long 
doubles will behave in such an environment.  I'm surprised to see this occur 
on a 686 Mac though as I didn't think their std lib has this issue: could be 
a bug in std::log I guess that it can't handle std::numeric_limits<>::max() 
as an argument?

HTH, John.