C++ source code to calculate factorials with GMP.

factorial.cpp

/***************************************************************
  Factorial of N
  by GMP(The GNU Multi Presicion Arithmetic Library).

  $ g++ -std=c++17 -Wall -O2 --pedantic-errors -lgmp -lgmpxx -o factorial factorial.cpp

    DATE          AUTHOR          VERSION
    2020.08.17    mk-mode.com     1.00 新規作成

  Copyright(C) 2020 mk-mode.com All Rights Reserved.
***************************************************************/
#include <iostream>  // for cout
#include <gmp.h>
#include <gmpxx.h>

class Factorial {

public:
    mpz_class fact(mpz_class);
};

/*
 * Caculation of fatorial
 */
mpz_class Factorial::fact(mpz_class n) {
  try {
    if (n == 0)
      return 1;
    return n * fact(n-1);
  } catch (...) {
    throw;
  }
}

int main(int argc, char* argv[]) {
  Factorial f;
  mpz_class n;
  std::string buf;

  try {
    while (true) {
      std::cout << "n? ";
      getline(std::cin, buf);
      if (buf.empty())
        break;
      n.set_str(buf, 10);
      std::cout << n << "! = "
                << f.fact(n).get_str() << std::endl;
      std::cout << "---" << std::endl;
    }
  } catch (...) {
    std::cerr << "EXCEPTION!" << std::endl;
    return EXIT_FAILURE;
  }

  return EXIT_SUCCESS;
}