Re: [boost] [rational] Inadvertent construction from double

Subject: Re: [boost] [rational] Inadvertent construction from double
From: John Maddock (boost.regex_at_[hidden])
Date: 2014-07-25 11:13:38

> I have encountered something I consider a bug in boost::rational's
> interface. Its documentation discusses in detail why conversion from double
> is not supported, but the following code works fine:
>
> #include <iostream>
> #include <boost/rational.hpp>
>
> int main()
> {
>    double d = 31.82;
>    boost::rational<int> r = d;
>    std::cout << r << std::endl;
> }
>
> With the following result:
> 31/1
> It just discards the fractional part. Such conversion is really confusing
> an I claim should be explicitly deleted.

It should indeed be prohibited and not proceed via an implicit 
conversion.  It's probably an easy fix via enable/disable_if as well.

Note that the Boost.Multiprecision rational classes *do* allow implicit 
construction from floating point values since these are actually exact 
conversions when correctly written (this is a new change in response to 
a bug report - will be in the next release).  So there's a discrepancy 
there.  However, rational<int> doesn't have enough bits to do such a 
conversion correctly, so I guess it's maybe safer to forbid it in that case?

John.