Re: [Boost-users] boost::rational does not maintain its invariant in case of overflow

Subject: Re: [Boost-users] boost::rational does not maintain its invariant in case of overflow
From: Robert Ramey (ramey_at_[hidden])
Date: 2018-04-11 14:28:42

On 4/11/18 6:11 AM, Frédéric via Boost-users wrote:
> With the program shown below, I obtained the following output:
> std::numeric_limits<long>::max() == 9223372036854775807
> r == boost::rational<long>(4294967296, 1853020188851841) ==
> 4294967296/1853020188851841
> r * r == 0/8733086111712066817
> 
> Clearly there are 2 overflows but this leads to the invariant of the
> class not being maintained, i.e. the faction is not simplified to 0/1
> (gcd(num, den) != 1).
> 
> F
> 
> #include <boost/rational.hpp>
> 
> #include <iostream>
> 
> int main() {
>    std::cout << "std::numeric_limits<long>::max() == "
>              << std::numeric_limits<long>::max() << '\n';
>    boost::rational<long> r(4294967296, 1853020188851841);
>    std::cout << "r == boost::rational<long>(4294967296, 1853020188851841) == "
>              << r << '\n';
>    std::cout << "r * r == " << (r * r) << '\n';
>    return 0;
> }
> 

safe_numerics can address this.

Robert Ramey