Bigint class operator

#include <iostream>
#include <string>
#include <vector>
using namespace std;

class Mint {
public:

    Mint();
    Mint(int);
    Mint (const char  *s);
    string afficher();
    Mint operator+=(const Mint &rhs); //returns mint + rhs
    Mint operator+(const Mint &rhs);
private:
    vector<char> num;
};
#include "Mint.h"
#include <string.h>

Mint::Mint()
{
    num.push_back(0);
}
Mint::Mint(int n)
{
    while(n!=0)
    {
        num.push_back(n%10);
        n = n/10;
    }
}
Mint::Mint(const char* s)
{
        int i = 0 ;
        for(i=strlen(s)-1;i>=0;i--)
        num.push_back(s[i] - '0');
}
string Mint::afficher(){
    string s="";
    int i;
    for(i=num.size()-1;i>=0;i--)
        s += char('0'+num[i]);
    return s;
}
Mint Mint::operator+=(const Mint &rhs) {
    unsigned int len = num.size();
    char carry = 0;

    if (len > rhs.num.size())
        len = rhs.num.size();

    unsigned int i;
    for (i = 0; i <= len - 1; i++)
    {
        char result = num[i] + rhs.num[i] + carry;
        num[i] = result % 10;
        carry = result / 10;
    }

    while (carry)
    {
        if (i < num.size())
        {
            num[i] += carry;
            if (num[i] >= 10)
            {
                num[i] -= 10;
                i++;
            }
            else
                carry = 0;
        }
        else {
            num.push_back(carry);
            carry = 0;
        }
    }
    return *this;
}
Mint Mint::operator+(const Mint &rhs){
    Mint result;
    result = *this;
    return result+=rhs;
}
#include "Mint.h"
#include <iostream>
#include <string>
using namespace std;



int main()
{
    Mint x="123456",z="22222222222222222222",f;
    Mint a = "655478461469974272005572";
    Mint b = 8;
    x += z;
    f = x + z;
    cout << x.afficher()<<endl;

}

hello everyone please check out this code and tell me why is the operator += only works if the numbers have the same size when i tried x += z; i got 345678 i think the problem is int for (i = 0; i <= len - 1; i++) loop i tried to fix it but i wasn't successful any solutions ?