Practicing recursion

Hello, need to do some exercices that involves recursion, the
substitution model in particular (I believe that what it's called). I
decided to translate the following lisp program that sums the elements
in a list that are numbers:

(defun is-integer (x) ; Helper function to sum-only-numbers
   (typep x 'integer)
)

(defun sum-only-numbers (list)
    (cond
     ((endp list) 0)
     ((is-integer (first list)) (+ (first list) (sum-only-numbers(rest
list))))
     (t (sum-only-numbers (rest list)))
     )
)

(sum-only-numbers '(1 2 a b 33)) ; which evals to 36

I don't really know lisp and I'm interested in C++ right now, just
showing it for references.

Here's my attempt:
#include <iostream>
#include <list>
#include <sstream>
#include <string>

using namespace std;

bool is_num(const string& s)
{
    istringstream iss(s);

    int n;

    return iss >> n;
}

int to_num(const string& s)
{
    istringstream iss(s);

    int n;

    iss >> n;

    return n;
}

int sum_only_numbers(list<string>::const_iterator itr,
list<string>::const_iterator end)
{
    if (itr != end)
    {
       string s = *itr;

       if (is_num(s))
       {
          return to_num(s) + sum_only_numbers(++itr, end);
       }
       else
       {
          return sum_only_numbers(++itr, end);
       }
    }
    else
    {
       return 0;
    }
}

int main(void)
{
    string ss[] = {"1", "2", "a", "b", "33"};
    list<string> l(ss, ss + sizeof(ss) / sizeof(ss[0]));

    cout << sum_only_numbers(l.begin(), l.end()) << endl;
}

It prints "36" when run so it seems to work! Any helpful suggestions
before I hand it in?

- Fencer