Работа с неограничено большими числами

#include<iostream>
#include<string>
using namespace std;
 
int main(void)
{
    // the two "numbers" to be added. Make them as long as you like.
         string numStr1;
         string numStr2;
         cout << "Enter 1st number: "; cin >> numStr1;
         cout << "Enter 2nd number: "; cin >> numStr2;
 
    // keeping track of which string is longest using references
    string& rLongStr = numStr1.length() > numStr2.length() ? numStr1 : numStr2;
    string& rShortStr = numStr1.length() <= numStr2.length() ? numStr1 : numStr2;
 
    // initialize the sum with the long string but with space for a final carry at the beginning
    string numStrSum = '0' + rLongStr;// the '0' in case of a final carry
 
    // must go through the strings backwards since the least
    // significant digit is at the end, not the beginning.
    string::reverse_iterator r_itShort, r_itSum;
    r_itShort = rShortStr.rbegin();// point to last "digit" in the short string
    r_itSum = numStrSum.rbegin();// same for sum string
 
    // add the "digits" one by one from end to beginning of the short string
    while( r_itShort != rShortStr.rend() )
    {
        *r_itSum += *r_itShort - '0';// "add" the digits
        if( *r_itSum > '9' )// must carry a one to the next "digit"
        {
            *(r_itSum + 1) += 1;
            *r_itSum -= 10;
        }
        ++r_itShort;// move back 1 character
        ++r_itSum;// in each string
    }
    if( numStrSum.at(0) == '0' )// if 1st character is stiil '0'
        numStrSum.erase(0,1);// erase it
 
    // output result
    cout << numStrSum;
    cout << endl;
    return 0;
}

while( r_itShort != rShortStr.rend() )
{
    *r_itSum -= *r_itShort + '0';
    if( *r_itSum > '9' )
    {
        *(r_itSum + 1) -= 1;
        *r_itSum += 10;
    }
    --r_itShort;
    --r_itSum;
}
if( numStrSum.at(0) == '0' )
    numStrSum.erase(0,1);