Re: sticky question regarding boost rational

From: Marc Glisse (marc.glisse_at_[hidden])
Date: 2020-11-02 22:29:48

On Sat, 31 Oct 2020, Robert Ramey via Boost wrote:

> boost rational is tripping up one of my examples in safe numerics:
>
>    // solution: use safe integer in rational definition
>    using safe_rational = boost::rational<
>        boost::safe_numerics::safe<int>
>    >;
>
>    // use rationals created with safe_t
>    const safe_rational sc {1, std::numeric_limits<int>::max()};
>
>    std::cout << "c = " << sc << std::endl;
>    const safe_rational sd {1, 2};
>    std::cout << "d = " << sd << std::endl;
>    std::cout << "c * d = ";
>    try {
>        // multiply them. This will overflow
>        std::cout << sc * sd << std::endl;   // use of overload operator * is 
> ambiguous.

So you are multiplying 2 rationals, you could have made the example more 
minimal with sd * sd.

> rational.hpp contains - among other things, the following definitions for the 
> * operator:
>
> template <class IntType, class Arg>
> BOOST_CXX14_CONSTEXPR
> inline typename boost::enable_if_c <
>   rational_detail::is_compatible_integer<Arg, IntType>::value || 
> is_same<rational<IntType>, Arg>::value, rational<IntType> >::type
>   operator * (const rational<IntType>& a, const Arg& b)
> {
>      rational<IntType> t(a);
>      return t *= b;
> }
> template <class Arg, class IntType>
> BOOST_CXX14_CONSTEXPR
> inline typename boost::enable_if_c <
>   rational_detail::is_compatible_integer<Arg, IntType>::value, 
> rational<IntType> >::type
>   operator * (const Arg& b, const rational<IntType>& a)
> {
>      rational<IntType> t(a);
>      return t *= b;
> }
>
> Soooooo - it seems that sc * sd will match both of the above definitions. 
> Its unclear what the purpose of these two different overloads are.  They look 
> pretty similar to me.

They look very different. The first one is for rational * integer or 
rational * rational, and the second one for integer * rational.

> the definition for is_compatible_integer.
>
> namespace rational_detail{
>
>   template <class FromInt, class ToInt, typename Enable = void>
>   struct is_compatible_integer;
>
>   template <class FromInt, class ToInt>
>   struct is_compatible_integer<FromInt, ToInt, typename 
> enable_if_c<!is_array<FromInt>::value>::type>
>   {
>      BOOST_STATIC_CONSTANT(bool, value = 
> ((std::numeric_limits<FromInt>::is_specialized
> 	&& std::numeric_limits<FromInt>::is_integer
>         && (std::numeric_limits<FromInt>::digits 
> <=std::numeric_limits<ToInt>::digits)
>         && (std::numeric_limits<FromInt>::radix == 
> std::numeric_limits<ToInt>::radix)
>         && ((std::numeric_limits<FromInt>::is_signed == false) || 
> (std::numeric_limits<ToInt>::is_signed == true))
>         && is_convertible<FromInt, ToInt>::value)
>         || is_same<FromInt, ToInt>::value)
>         || (is_class<ToInt>::value && is_class<FromInt>::value && 
> is_convertible<FromInt, ToInt>::value));
>   };
> ...
>
> In fact, since it looks like if FromInt and ToInt are the same types (as they 
> are in my case, one will always got more than one match.

AFAICT, Arg is rational in your case, so it is **not** the same as 
IntType. And I hope that rational is not convertible to safe<int>, 
although a badly constrained constructor there could easily cause trouble.

-- 
Marc Glisse