Print to file from MPFR

Question

I want to print the result of a calculation using MPFR to file but I don't know how. MPFR is used to do floating point operations with high accuracy. To print an mpfr_t number you use the function:

size_t mpfr_out_str (FILE *stream, int base, size t n, mpfr t op, mp rnd t rnd)

I guess my problem is that I don't understand FILE* objects and how they are related to fstream objects.

If I change my_file in the mpfr_out_str line to stdout then the number will print to the screen as I'd hoped but I don't know how to get it into the file.

#include <mpfr.h>
#include <iostream>
#include <fstream>
using namespace std;
int main() {
   mpfr_t x;
   mpfr_init(x);
   mpfr_set_d(x, 1, MPFR_RNDN);

   ofstream my_file;
   my_file.open("output.txt");
   mpfr_out_str(my_file, 2, 0, x, MPFR_RNDN);
   my_file.close();
}

Answer 1

It is possible to use the std::ostream methods with mpfr functions like mpfr_as_printf or mpfr_get_str. It however requires an additional string allocation.

  #include <mpfr.h>
  #include <iostream>
  #include <fstream>
  using namespace std;
  int main() {
     mpfr_t x;
     mpfr_init(x);
     mpfr_set_d(x, 1, MPFR_RNDN);

     ofstream my_file;
     my_file.open("output.txt");

     char* outString = NULL;
     mpfr_asprintf(&outString, "%RNb", x);
     my_file << outString;
     mpfr_free_str(outString);
     my_file.close();

     mpfr_clear(x);
  }