subtracting two strings as numbers

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

string subtracting(string, string);
string str_reverse(string a)
{
    string b;
    for(int i = a.length()-1; i >= 0; i--)
        b += a[i];
    return  b;
}

int main()
{
    cout << subtracting("67437341", "67437299") << endl;
    return 0;
}

string subtracting(string a, string b)
{
    int length = max(a.length(), b.length());
    a = str_reverse(a);
    b = str_reverse(b);
    if(a.length() != b.length())
        ((a.length() > b.length()) ? (b) : (a)).resize(length, '0');
    string c;
    c.resize(length);

    for(int i = 0; i < length; i++)
        c[i] = a[i] + '0' - b[i];

    for(int i = 0; i < length; i++)
        if(c[i] < '0')
        {
            c[i] += 10;
            c[i+1]--;
        }
    return str_reverse(c);
}