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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.

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