From: Boost-users [mailto:boost-users-bounces@lists.boost.org] On Behalf Of dariomt@gmail.com
Sent: 08 September 2014 14:54
To: boost-users@lists.boost.org
Subject: [Boost-users] Numeric Cast from integer to floating point
Hi,
Quoting the docs for boost::numeric_cast: "There are several situations where conversions are unsafe: [...] Conversions from floating point types to integral types."
What about conversions from integral types to floating point types? E.g. from 64bit int to double.
The following example shows what I mean:
#include <iostream>
#include <cmath>
#include
int main()
{
const uint64_t i = 123445678911188878;
std::cout << "i=" << i << std::endl;
const double d = i;
std::cout << "d=" << std::fixed << d << std::endl;
std::cout << "next=" << std::fixed << std::nextafter(d, std::numeric_limits<double>::max() ) << std::endl;
std::cout << "prev=" << std::fixed << std::nextafter(d, -std::numeric_limits<double>::max()) << std::endl;
// I'd expect the following cast to fail
const double dd = boost::numeric_cast<double>(i);
std::cout <<"dd=" << std::fixed << dd << std::endl;
return 0;
}
prints
i=123445678911188878
d=123445678911188880.000000
next=123445678911188896.000000
prev=123445678911188864.000000
dd=123445678911188880.000000
because that integer cannot be represented in double precision
Is there something in Boost to help here?
Would using 128-bit floating point help?
http://www.boost.org/doc/libs/1_54_0/libs/multiprecision/doc/html/boost_mult...
(though of course, the biggest 128-bit int cannot be represented in 128-bit FP.)
Paul
---
Paul A. Bristow
Prizet Farmhouse
Kendal UK LA8 8AB
+44 (0) 1539 561830
