How to deduce the most nested iterator type?

Question

I would like to write a type trait that, given a ContainerType, is capable to deduce the most nested IteratorType, meaning that given for example a std::vector<int> or a std::vector<std::vector<int>> or a std::vector<std::vector<std::vector<int>>> always the same IteratorType will be deduced, as if it is an std::vector<int>.

Answer 1

Here you go, I have written a trait and a tiny demonstration how it works:

#include <type_traits>
#include <iterator>
#include <vector>

template<class ...>
using void_t = void;    

template<class T, class = void>
struct is_container : std::false_type{};

template<class T>
struct is_container<T, void_t<decltype(std::begin(std::declval<T>())), decltype(std::end(std::declval<T>()))>> : std::true_type{};

template<class T, class = void>
struct drill_iterator {
    using type =  typename T::iterator;
};

template<class T>
struct drill_iterator<T, typename std::enable_if<is_container<typename T::value_type>::value>::type > {
    using type = typename drill_iterator<typename T::value_type>::type;
};


int main(){
    drill_iterator<std::vector<std::vector<int>>>::type a;
    drill_iterator<std::vector<int>>::type b;

    if(std::is_same<decltype(b), std::vector<int>::iterator>::value
    && std::is_same<decltype(a), std::vector<int>::iterator>::value)
        return 0;

    return 1;
}

Online demo