WTF my while loop is on permenent vacation

[MK27]

In further pursing my silly solution to lilostitch382's "assign string to int array" I have been hamstrung:

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

class nlist : public string {
	public:
		nlist(string in) {
			assign(in);
		}
		int extract(size_t pos) {
			int retv = 0, m = 1;
			size_t end;
			while (at(pos) < '0'|| at(pos) > '9') {
				printf("%d %d\n",pos,at(pos)); fflush(stdout);
				pos++;
			}
			cout << "hey";
			end = pos;
			while (at(end) > '0'&& at(end) < '9') end++;
			end--;
			while (end>=pos) {
				retv += (at(pos)-48)*m;
				m*=10;
			}
			return retv;
		}
};

int main() {
	string src="1,44,56,33";
	nlist nums(src);

	cout << nums << "\n" << nums.extract(1);

	return 0;
}

Execution pauses indefinitely at pos++.

WHY????