c++ - How to find the index of current object in range-based for loop?

Question

Assume I have the following code:

vector<int> list;
for(auto& elem:list) {
    int i = elem;
}

Can I find the position of elem in the vector without maintaining a separate iterator?

Answer 1

Yes you can, it just take some massaging ;)

The trick is to use composition: instead of iterating over the container directly, you "zip" it with an index along the way.

Specialized zipper code:

template <typename T>
struct iterator_extractor { typedef typename T::iterator type; };

template <typename T>
struct iterator_extractor<T const> { typedef typename T::const_iterator type; };


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); }

And using it:

#include <iostream>
#include <iterator>
#include <limits>
#include <vector>

// Zipper code here

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

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

You can see it at ideone, though it lacks the for-range loop support so it's less pretty.

EDIT:

Just remembered that I should check Boost.Range more often. Unfortunately no zip range, but I did found a pearl: boost::adaptors::indexed. However it requires access to the iterator to pull of the index. Shame :x

Otherwise with the counting_range and a generic zip I am sure it could be possible to do something interesting...

In the ideal world I would imagine:

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

    for (auto tuple: zip(iota(0), v)) {
        std::cout << tuple.at<0>() << ": " << tuple.at<1>() << "
";
    }
}

With zip automatically creating a view as a range of tuples of references and iota(0) simply creating a "false" range that starts from 0 and just counts toward infinity (or well, the maximum of its type...).