Re: [Boost-users] [Conversion] imprecision with lexical_cast from double to std::string

From: Michael Linck (mgl_at_[hidden])
Date: 2008-07-30 18:08:04

Andreas Ries wrote:
> Hi,
> I'm using the borland toolset 5.8.2 under Win XP and have a problem 
> doing the following:
>
>   double val = 795.6352;
>   string result = boost::lexical_cast <string> (val);
>
> the resulting string is:  "795.63520000000005"    with boost 1_35_0
> the resulting string is: "795.6352000000001" with boost_1_34_0
> Can anybody help?

This describes a bug I just found as well.  I didn't  see any follow up 
to this message when I looked through my archived boost mail.  Looks 
like boost::lexical_cast for double and float got broken between boost 
1_33 and boost 1_34 and now changes the precision of the cast and 
introduces error, and this was never fixed.  This could cause havoc on 
the application I'm maintaining, which uses lexical _cast extensively 
and expects a precision of 2 so our users don't ever have to look at a 
number like the ones above.  Unfortunately, I can't revert to boost 1_33 
because we switched to Visual Studio 9 for our windows port.  We build 
on gcc 4.1.2 on Unix platforms and, as I already mentioned VS9 on 
Windows, and we get this problem everywhere.  Is there a workaround for 
this or a fix in the pipeline?  If not, I guess it's time to write our 
own string wrappers again, so we know what it's going to do >hurghle<.

mike