#include <boost/multiprecision/cpp_int.hpp>

namespace boost{ namespace multiprecision{

typedef rational_adaptor<cpp_int_backend<> >    cpp_rational_backend;

typedef number<cpp_rational_backend>         cpp_rational;

}} // namespaces

The cpp_rational_backend type is used via the typedef boost::multiprecision::cpp_rational. It provides a rational number type that is a drop-in replacement for the native C++ number types, but with unlimited precision.

As well as the usual conversions from arithmetic and string types, instances of cpp_rational are copy constructible and assignable from type cpp_int.

There is also a two argument constructor that accepts a numerator and denominator: both of type cpp_int.

There are also non-member functions:

cpp_int numerator(const cpp_rational&);
cpp_int denominator(const cpp_rational&);

which return the numerator and denominator of the number.

Things you should know when using this type:

Example:

#include <boost/multiprecision/cpp_int.hpp>
#include <iostream>

int main()
{
   using namespace boost::multiprecision;

   cpp_rational v = 1;

   // Do some arithmetic:
   for(unsigned i = 1; i <= 1000; ++i)
      v *= i;
   v /= 10;

   std::cout << v << std::endl; // prints 1000! / 10
   std::cout << numerator(v) << std::endl;
   std::cout << denominator(v) << std::endl;

   cpp_rational w(2, 3);  // component wise constructor
   std::cout << w << std::endl; // prints 2/3
   return 0;
}