C++ and GMP help

#include <stdlib.h>
#include <stdio.h>
#include <gmp.h>


mpz_t fact (int num)
{

 int i;
 mpz_t result;

 mpz_init (result);
 mpz_set_ui (result, 1);

 for (i = 1 ; i <= num ; i++) 
  mpz_mul_ui (result, result, i);

 return (result);

}
 


int main (int argc, char **argv)
{

 int num;
 mpz_t ans;

 mpz_init (ans);

 if (argc < 2) {
  fprintf (stdout, "Usage: %s <Integer>\n", argv[0]);
  exit (1);
 }

 num = atoi (argv[1]);
 ans = fact (num);

 gmp_printf ("%Zd\n", ans);

 return (0);
}

The string I use to compile this program and the error message:

[vince@vincent: ~/prog/cpp]% g++ -O6 -fomit-frame-pointer -lgmp fact.cpp -o fact fact.cpp:7: `fact' declared as function returning an array fact.cpp: In function `int fact(int)':
fact.cpp:18: return to `int' from `__mpz_struct *' lacks a cast
fact.cpp: In function `int main(int, char **)':
fact.cpp:38: incompatible types in assignment of `int' to `__mpz_struct[1]'
fact.cpp:40: implicit declaration of function `int gmp_printf(...)'
[vince@vincent: ~/prog/cpp]%

Does anyone know what the problem is and would care to explain it to me?

Could anyone help?

[andrzej]

From http://www.gnu.org/manual/gmp-3.1.1/...de/gmp_12.html

mpz_t is actually implemented as a one-element array of a certain structure type. This is why using it to declare a variable gives an object with the fields GMP needs, but then using it as a parameter passes a pointer to the object.

In C++ a function cannot return an array "by value".

#include <stdlib.h>
#include <stdio.h>
#include <gmp.h>


void fact (mpz_t *result, int num)
{

 int i;

 mpz_set_ui (*result, 1);

 for (i = 1 ; i <= num ; i++)
  mpz_mul_ui (*result, *result, i);

}



int main (int argc, char **argv)
{

 int num;
 mpz_t ans;
 int gmp_fprintf;

 mpz_init (ans);

 if (argc < 2) {
  fprintf (stdout, "Usage: %s <Integer>\n", argv[0]);
  exit (1);
 }

 num = atoi (argv[1]);
 fact (&ans, num);

 gmp_printf ("%Zd\n", ans);

 return (0);
}

gcc -lgmp -O6 -fomit-frame-pointer fact.c -o fact

./fact 65536 22.79s user 0.10s system 99% cpu 22.941 total