Re: [boost] [serialization] Serialisation/deserialisation offloating-point values

From: Caleb Epstein (caleb.epstein_at_[hidden])
Date: 2006-03-15 18:19:45

On 3/15/06, Janek Kozicki <janek_listy_at_[hidden]> wrote:
>
>
> try following modifications in above code. If you still get a mistake by
> one bit, then .... well, I'd be very surprised.

Prepare to be suprised.

Here's the exact code I compiled:

#include <iostream>
#include <sstream>
#include <cassert>
#include <boost/lexical_cast.hpp>

int main ()
{
    std::stringstream stream;
    double orig_value = 0.0019075645054089487;
    stream << boost::lexical_cast<std::string> (orig_value);
    double num = boost::lexical_cast<double> (stream.str());
    assert (num == orig_value);
}

On gcc + Linux this fails:

lc: lc.cpp:12: int main(): Assertion `num == orig_value' failed.

Breakpoint 1, main () at lc.cpp:12
12          assert (num == orig_value);
(gdb) print num
$3 = 0.0019075645054089489
(gdb) print orig_value
$4 = 0.0019075645054089487

On MSVC 8, the program also asserts and the values are similarly mismatched:

        orig_value    0.0019075645054089487    double
        num    0.0019075645054089489    double

Note that boost::lexical_cast uses a precision of
std::numeric_limits<T>::digits10
+ 1 in its T-to-string conversions.  For double, this is 16 which would
probably explain the mismatch on the 17th significant digit on two separate
platforms.

--
Caleb Epstein
caleb dot epstein at gmail dot com