Get index in C++11 foreach loop

Question

Is there a convenient way to get the index of the current container entry in a C++11 foreach loop, like enumerate in python:

for idx, obj in enumerate(container):     pass 

I could imagine an iterator that can also return the index or similar.

Of course I could have a counter, but often iterators don't give guarantees of the order they iterate over a container.

Answer 1

A good implementation of the feature you are requested can be found here:

https://github.com/ignatz/pythonic

The idea behind is, that you build a wrapper struct with a custom iterator that does the counting. Below is a very minimal exemplary implementation to illustrate the idea:

// Distributed under the terms of the GPLv2 or newer  #include <iostream> #include <vector> #include <tuple>  // Wrapper class template <typename T> class enumerate_impl { public:     // The return value of the operator* of the iterator, this     // is what you will get inside of the for loop     struct item     {         size_t index;         typename T::value_type & item;     };     typedef item value_type;      // Custom iterator with minimal interface     struct iterator     {         iterator(typename T::iterator _it, size_t counter=0) :             it(_it), counter(counter)         {}          iterator operator++()         {             return iterator(++it, ++counter);         }          bool operator!=(iterator other)         {             return it != other.it;         }          typename T::iterator::value_type item()         {             return *it;         }          value_type operator*()         {             return value_type{counter, *it};         }          size_t index()         {             return counter;         }      private:         typename T::iterator it;         size_t counter;     };      enumerate_impl(T & t) : container(t) {}      iterator begin()     {         return iterator(container.begin());     }      iterator end()     {         return iterator(container.end());     }  private:     T & container; };  // A templated free function allows you to create the wrapper class // conveniently  template <typename T> enumerate_impl<T> enumerate(T & t) {     return enumerate_impl<T>(t); }    int main() {     std::vector<int> data = {523, 1, 3};     for (auto x : enumerate(data))     {         std::cout << x.index << ": " << x.item << std::endl;     } }