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

Flattening Iterators in C

Consider a scenario where you have a container of containers, such as a vector>, and you want to iterate over the elements in a flattened manner, as if it were a single sequence. This is where the concept of flattening iterators comes into play.

In the absence of existing flattening iterator implementations in major libraries, we present a basic implementation below:

#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
}

This implementation enables you to iterate over nested containers in a flattened manner, as demonstrated in the following example:

// 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 << ", ";
}

This implementation has not been thoroughly tested, and further testing is recommended before putting it into production. Should you encounter any bugs, please feel free to contact the author for corrections.

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