#include <iostream>
#include <memory>
#include <vector>

using namespace std;

class Iterator {
public:
    virtual bool has_next() = 0;
    virtual int next() = 0;
    virtual ~Iterator() = default;
};

class IntContainer {
private:
    vector<int> v;
public:
    void add_i(int i) {
        v.push_back(i);
    }

    shared_ptr<Iterator> begin();
};

class IntIterator : public Iterator {
    vector<int>& v;
    int index;
public:
    IntIterator(vector<int>& v) : v(v), index(0) {}
    virtual bool has_next() {
        return index != v.size();
    }

    virtual int next() {
        index += 1;
        return v[index - 1];
    }
};

shared_ptr<Iterator> IntContainer::begin() {
    return make_shared<IntIterator>(this->v);
}

int main() {
    IntContainer c;
    c.add_i(1);
    c.add_i(3);
    c.add_i(5);
    c.add_i(6);

    // When you want to design a new container, you just
    // implement a iterator, so user don't have to care the
    // detail how the container work.
    auto v = c.begin();
    while (v->has_next()) {
        cout << v->next() << endl;
    }
}