How Can I Create a Flattening Iterator in C to Iterate Over Nested Containers?

#include <algorithm>
#include <iostream>
#include <set>
#include <vector>

template <typename OuterIterator>
class flattening_iterator
{
public:
    // ... Iterator definitions and implementation

private:
    void advance_past_empty_inner_containers()
    {
        // ... Advance logic
    }

    // ... Private members
};

template <typename Iterator>
flattening_iterator<Iterator> flatten(Iterator it)
{
    // ... Return a flattening iterator for a single range
}

template <typename Iterator>
flattening_iterator<Iterator> flatten(Iterator first, Iterator last)
{
    // ... Return a flattening iterator for a range
}

// Generate some test data
std::vector<std::vector<int>> v(3);
int i(0);
for (auto it(v.begin()); it != v.end(); ++it)
{
    it->push_back(i++);
    it->push_back(i++);
    it->push_back(i++);
    it->push_back(i++);
}

// Flatten the data and print all the elements
for (auto it(flatten(v.begin(), v.end())); it != flatten(v.end()); ++it)
{
    std::cout << *it << ", ";
}