gmp_rational

#include <boost/multiprecision/gmp.hpp>

namespace boost{ namespace multiprecision{

class gmp_rational;

typedef number<gmp_rational >         mpq_rational;

}} // namespaces

The gmp_rational back-end is used via the typedef boost::multiprecision::mpq_rational. It acts as a thin wrapper around the GMP mpq_t to provide 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 number<gmp_rational> are copy constructible and assignable from:

The GMP native types: mpz_t, mpq_t.
number<gmp_int>.

There is also a two-argument constructor that accepts a numerator and denominator (both of type number<gmp_int>).

There are also non-member functions:

mpz_int numerator(const mpq_rational&);
mpz_int denominator(const mpq_rational&);

which return the numerator and denominator of the number.

It's also possible to access the underlying mpq_t via the data() member function of mpq_rational.

Things you should know when using this type:

Default constructed mpq_rationals have the value zero (this is the GMP default behavior).
Division by zero results in a std::overflow_error being thrown.
Conversion from a string results in a std::runtime_error being thrown if the string can not be interpreted as a valid rational number.
No changes are made to the GMP library's global settings, so this type can coexist with existing GMP code.
The code can equally be used with MPIR as the underlying library - indeed that is the preferred option on Win32.

Example:

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

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

   mpq_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;

   mpq_rational w(2, 3);  // component wise constructor
   std::cout << w << std::endl; // prints 2/3

   // Access the underlying data:
   mpq_t q;
   mpq_init(q);
   mpq_set(q, v.backend().data());
   mpq_clear(q);
   return 0;
}