#include <iostream>
#include <string>
using std::ostream;
using std::istream;
using std::string;
using <vector>
using namespace std;
 
class LargeDigit
{
  friend ostream &operator<<(ostream &input, const LargeDigit &);
  friend istream &operator>>(istream &output, LargeDigit &);
public:
  LargeDigit();
  LargeDigit(String );
  int operator+(const LargeDigit &);
private:
  vector<char> intz;
};
 
# include "largeDigit.h"
#include <iostream>
using std::cout;
using namespace std;
 
LargeDigit::LargeDigit()
{
  this.intz = 0;
}
 
LargeDigit::LargeDigit(String str)
{
  for (int i = 0; i < str.length(); i++)
  {
    if(!isdigit(str[i])
    {
      cout<<"non-digit number is entered, so 1 is used as default  
      value"<<endl;
      this.intz = 1;
    }
    else
    {
      intz.push_back(str[i]);
    }
  }
} 
 
istream &operator<<(istream &input, const LargeDigit &num)
{
  input >> num.intz;
  (how do I check if user entered valid number here?)
  return input;
}
 
ostream &operator<<(ostream &output, const LargeDigit &digit)
{
  output << digit.intz;
  return output;
}
 
int operator+::LargeDigit(const VeryLongInt &digit)
{
   return  digit.intz+=digit.intz;
}

int LargeDigit::operator +(const LargeDigit &digit)//SEE THE SYNTAX properly
{
     /*your code*/
}

int LargeDigit::operator +(const LargeDigit &digit)
{
     return (this->intz + digit.intz);
}