For many years, Boost has included a library to represent and operate on rational numbers. Its well crafted, has good documentation and is well maintained. Using the rational library is as easy as construction an instance with the expression rational r(n, d) where n and d are integers of the same type. From then on it can be used pretty much as any other number. Reading the documentation with safe integers in mind one finds

Which hints of trouble. Examination of the code reveals which suggest that care has been taken implement operations so as to avoid overflows, catch divide by zero, etc. But the code itself doesn't seem to consistently implement this idea. So we make a small demo program:

#include <iostream>
#include <limits>

#include <boost/rational.hpp>
#include <boost/safe_numerics/safe_integer.hpp>

int main(int, const char *[]){
    // simple demo of rational library
    const boost::rational<int> r {1, 2};
    std::cout << "r = " << r << std::endl;
    const boost::rational<int> q {-2, 4};
    std::cout << "q = " << q << std::endl;
    // display the product
    std::cout << "r * q = " << r * q << std::endl;

    // problem: rational doesn't handle integer overflow well
    const boost::rational<int> c {1, INT_MAX};
    std::cout << "c = " << c << std::endl;
    const boost::rational<int> d {1, 2};
    std::cout << "d = " << d << std::endl;
    // display the product - wrong answer
    std::cout << "c * d = " << c * d << std::endl;

    // 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, 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;
    }
    catch (std::exception const& e) {
        // catch exception due to multiplication overflow
        std::cout << e.what() << std::endl;
    }

    return 0;
}

which produces the following output

r = 1/2
q = -1/2
r * q = -1/4
c = 1/2147483647
d = 1/2
c * d = 1/-2
c = 1/2147483647
d = 1/2
c * d = multiplication overflow: positive overflow error

The rational library documentation is quite specific as to the type requirements placed on the underlying type. Turns out the our own definition of an integer type fulfills (actually surpasses) these requirements. So we have reason to hope that we can use safe<int> as the underlying type to create what we might call a "safe_rational". This we have done in the above example where we demonstrate how to compose the rational library with the safe numerics library in order to create what amounts to a safe_rational library - all without writing a line of code! Still, things are not perfect. Since the rational numbers library implements its own checking for divide by zero by throwing an exception, the safe numerics code for handling this - included exception policy will not be respected. To summarize: