Re: [boost] a precision problem of lexical_cast

From: Matthias Troyer (troyer_at_[hidden])
Date: 2002-02-05 02:06:50

On Tuesday, February 5, 2002, at 05:10 AM, Jon Wang wrote:

>
> Maybe I've made a mistake. For double, the proper precision should be 
> 15 rather than 16.
>
> Mr Andy Koenig said "15 is not portable across all floating-point 
> implementations.  However, it is portable across all implementations 
> that support IEEE floating-point arithmetic, which is most computers 
> that are in common use today. If you want to do better than that, you 
> might consider using numeric_limits<double>::digits10, which is the 
> number of significant base-10 digits that can be accurately represented 
> in a double." I really appreciate his help.
>
> So maybe we can make such improvements:
>
>   #include <boost/limits.hpp>
>   //...
>   template<typename Target, typename Source>
>   Target lexical_cast(Source arg) {
>     	//...
>     	Target result;
>     	interpreter.precision(std::numeric_limits<Source>::digits10);
>     	if(	!(interpreter << arg) ||
> 			!(interpreter >> result) ||
> 			!(interpreter >> std::ws).eof())
> 		//...
>   }
>
> And we can get the right result.			
>

Actually I believe not even that is sufficient. Due to roundoff errors 
the
last bit can still be incorrect. To be really accurate we might need
a non-decimal format.

Matthias