How Can I Get the Index of the Current Object in a C Range-Based For Loop?

Finding the Current Object's Index in a Range-Based For Loop

The range-based for loop is a convenient syntax for iterating over a range of values. However, it does not provide a way to access the index of the current object within the loop. This can be problematic if you need to perform operations on the object based on its position in the container.

Fortunately, there is a way to find the index of the current object without maintaining a separate iterator. The trick is to use a composition technique. Instead of iterating over the container directly, we can "zip" it with an index along the way.

Here's how it works:

The Zipper Code

The zipper code is a class that creates a new iterator type that wraps the original iterator and adds an index field. The iterator_extractor struct is used to extract the underlying iterator type from the container.

template <typename T>
class Indexer {
public:
    class iterator {
        typedef typename iterator_extractor<T>::type inner_iterator;
        typedef typename std::iterator_traits<inner_iterator>::reference inner_reference;
    public:
        typedef std::pair<size_t, inner_reference> reference;

        iterator(inner_iterator it): _pos(0), _it(it) {}

        reference operator*() const { return reference(_pos, *_it); }

        iterator& operator++() { ++_pos; ++_it; return *this; }
        iterator operator++(int) { iterator tmp(*this); ++*this; return tmp; }

        bool operator==(iterator const& it) const { return _it == it._it; }
        bool operator!=(iterator const& it) const { return !(*this == it); }

    private:
        size_t _pos;
        inner_iterator _it;
    };

    Indexer(T& t): _container(t) {}

    iterator begin() const { return iterator(_container.begin()); }
    iterator end() const { return iterator(_container.end()); }

private:
    T& _container;
}; // class Indexer

template <typename T>
Indexer<T> index(T& t) { return Indexer<T>(t); }

Using the Zipper Code

To use the zipper code, simply wrap the container in the indexer function and iterate over the resulting iterator range. The iterator will provide both the index and the value of the current object.

std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9};

for (auto p: index(v)) {
    std::cout << p.first << ": " << p.second << "\n";
}

This will output:

0: 1
1: 2
2: 3
3: 4
4: 5
5: 6
6: 7
7: 8
8: 9

Alternative Approaches

While the zipper code is a powerful tool for finding the index of the current object in a range-based for loop, there are also alternative approaches that may be more suitable in certain situations.

Separate Iterator: Maintaining a separate iterator allows for more direct control over the iteration process. You can use the iterator to explicitly find the index of the current object or to perform other operations on the container.

Boost.Range: The Boost.Range library provides a number of tools for manipulating ranges, including the indexed adaptor. The indexed adaptor can be used to create an iterator range that pairs each element in the original range with its index.

Custom Range Class: You can create your own custom range class that provides an iterator that includes the index of the current object. This approach gives you the most flexibility in controlling the iteration process.

Conclusion

There are several options available for finding the index of the current object in a range-based for loop. The best choice for your application will depend on the specific requirements and trade-offs involved.

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