Implementing an iterator over combinations of many vectors

Question

I am working on a problem that requires iterating over all combinations of elements of K vectors taken one at a time. So for example for K=2 vector

Answer 1

Why would you like to use custom iterators? One could instead implement a very simple class that will iterate through all combinations:

class Combinator
{
public:
    Combinator(std::vector<std::vector<int> >& vectors)
        : m_vectors(vectors)
    {
        m_combination.reserve(m_vectors.size());
        for(auto& v : m_vectors)
            m_combination.push_back(v.begin());
    }

    bool next()
    {
        // iterate through vectors in reverse order
        for(long long i = m_vectors.size() - 1; i >= 0; --i)
        {
            std::vector<int>& v = m_vectors[i];
            std::vector<int>::iterator& it = m_combination[i];

            if(++it != v.end())
                return true;
            it = v.begin();
        }
        return false;
    }

    std::vector<std::vector<int>::iterator> combination() const
    {
        return m_combination;
    }

private:
    std::vector<std::vector<int> >& m_vectors; // reference to data
    std::vector<std::vector<int>::iterator> m_combination;
};

Live Demo

UPDATE: If you would still like to use iterators, I suggest iterating over combinations. One can put all the combinations from Combinator into a container and then work with container's own iterators. In my opinion it's a cleaner solution. The only drawback is the extra-memory needed to store all combinations explicitly.