https://qiita.com/S_H_/items/bfd224e96732163add75

check_digit.cpp

#include <iostream>
#include <cstdint>

class digit_t {
  friend std::istream& operator>>(std::istream&, digit_t&);
  friend std::ostream& operator<<(std::ostream&, digit_t&);
private:
  using value_type = uint_least64_t;
  value_type data;
  unsigned int check_digit;
  unsigned int make_check_digit(value_type digit);
public:
  digit_t(value_type data);
  digit_t(void);
  digit_t& operator=(value_type);
};

digit_t::digit_t(value_type data)
  : data(data), check_digit(make_check_digit(data)) {}

digit_t::digit_t(void)
  : data(0), check_digit(0) {}

digit_t& digit_t::operator=(value_type data) {
  this->data = data;
  this->check_digit = make_check_digit(data);
  return *this;
}

template<class T>
auto is_even(T x) -> typename std::enable_if<std::is_integral<T>::value, bool>::type {
  return x%2==0;
}

unsigned int digit_t::make_check_digit(value_type digit) {
  unsigned int d=0;

  for(int i=0; i<12; i++, digit/=10) {
    int c = digit%10;
    if(is_even(i)) c*=2;
    d += c/10+c%10;
  }

  return d ? 10-d%10 : 0;
}

std::istream& operator>>(std::istream& is, digit_t& d) {
  digit_t::value_type v=0;

  for(int i=0; i<12; i++) {
    int ch=is.peek();
    if(isdigit(ch)) {
      is.ignore();
      v*=10;
      v+=ch-'0';
    } else {
      is.setstate(is.failbit);
      return is;
    }
  }

  int ch=is.peek();
  if(ch=='\n') {
    is.ignore();
  } else {
    is.setstate(is.failbit);
    return is;
  }

  d = v;

  return is;
}

std::ostream& operator<<(std::ostream& os, digit_t& d) {
  os.width(12);
  os.fill('0');
  os << d.data;
  os << d.check_digit;
  return os;
}

int main(void) {
  digit_t d;
  while(std::cin >> d) {
    std::cout << d << std::endl;
  }
  
  return 0;
}