iterator.cpp

#include <iostream>

using namespace std;

auto upto(int& beg, int& end) -> decltype(std::function<int()>()) {
    return [&]() {
        if (beg <= end) {
            return beg++;
        }
        return 0;
    };
}

int main(void) {
    int i = 0;
    int j = 3;

    auto it = upto(i, j);
    cout << it() << endl; // 0
    cout << it() << endl; // 1
    cout << it() << endl; // 2
    cout << it() << endl; // 3
    cout << it() << endl; // 0
}