Convert string to int

In this program I have to enter 2 whole numbers a and b and the program should display on the screen the numbers harshad (numbers that are divisible by the sum of their numbers) between a and b. A and b must be positive, and b greater than a.

In the compilation it tells me that there are failures in the operator ++ of the for, and one of the whiles (I do not know which).

The program that I have used is:

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

int main(){
    int a,b,n,suma;
    string numero;
    bool harshad;

    harshad = (numero%suma=0);
    cout<<"introduce los numeros a y b:";
    cin>>a>>b;

    while((a<=0)||(b<a){
        cout<<"introduce los numeros a y b:";
        cin>>a>>b;
    }

    for(numero=a;numero<=b;numero++){
        n=0;
        while(numero.at(n)!=0){
            numero.at(n)=numero.at(n)+numero.at(n+1);
        }
    }
    suma=numero.at(n);

    while(harshad=true){
        cout<<numero<<endl;
        numero++;
    }
    return 0;
}