How Can a Custom Iterator Simplify Iterating Through Nested Containers?

Flattening Nested Containers with a Custom Iterator

Introduction

Iterating through nested containers can often be a tedious and error-prone task. To simplify this process, we explore the concept of a "flattening" iterator, which allows us to treat a hierarchy of containers as a single, flattened sequence.

Problem

Suppose we have a set of vectors containing integers, and we want to iterate over them as a single, contiguous list. A simple std::vector> might not suffice, as it requires navigating through each nested vector separately.

Custom flattening_iterator

To address this issue, we can create a custom iterator, flattening_iterator, that iterates through the nested containers sequentially. This iterator:

• Takes as its input an iterator to the outermost container.
• Keeps track of the current position within both the outermost and innermost containers.
• Advances the iterator by incrementally moving through the nested containers until it reaches the end or encounters an empty innermost container.

Implementation

The flattening_iterator is implemented as follows:

template <typename OuterIterator>
class flattening_iterator
{
public:
    // ... iterator category, value type, etc.
    
    flattening_iterator(outer_iterator it) : outer_it_(it), outer_end_(it) { }
    flattening_iterator(outer_iterator it, outer_iterator end) : outer_it_(it), outer_end_(end) { advance_past_empty_inner_containers(); }
    
    // ... operators for comparison, dereferencing, and advancement
    
private:
    void advance_past_empty_inner_containers()
    {
        // Advance until we find a non-empty innermost container
        while (outer_it_ != outer_end_ && inner_it_ == outer_it_->end())
        {
            ++outer_it_;
            if (outer_it_ != outer_end_)
                inner_it_ = outer_it_->begin();
        }
    }
    
    outer_iterator outer_it_;
    outer_iterator outer_end_;
    inner_iterator inner_it_;
};

Usage

We can use the flattening_iterator to flatten nested containers as follows:

std::unordered_set<std::vector<int>> s;

s.insert({});
s.insert({1,2,3,4,5});
s.insert({6,7,8});
s.insert({9,10,11,12});

// Create a flattening iterator and iterate over the flattened set
for (auto it = flatten(s.begin()); it != s.end(); ++it)
{
    std::cout << *it << endl;
}

This will output the flattened list of all integers in the nested containers:

1
2
3
4
5
6
7
8
9
10
11
12

Conclusion

The flattening_iterator provides a simple and effective way to iterate over nested containers as a single, flattened sequence. This simplifies code and eliminates the need for complex nested loop structures or manual navigation through the nested hierarchy.

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