Making interchangable forward/backwards for-range iterator

Question

I wanted to be able to iterate collections (I will only we doing this on vectors, if that's important) in reverse order with simple for (const auto& item : collection) style, and I found this answer which worked perfectly: https://stackoverflow.com/a/28139075/2195721. But then I wanted to be able to iterate either forward or backward, so I modified the code as shown here:

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

template <typename T>
struct reversable_wrapper {
    T& iterable;
    bool reverse;

    reversable_wrapper(T&& iterable) : reversable_wrapper(iterable, true) {};
    reversable_wrapper(T&& iterable, bool reverse) : iterable(iterable), reverse(reverse) {};
};

template <typename T>
auto std::begin (reversable_wrapper<T> w)
{
    if (w.reverse) return std::rbegin(w.iterable);
    else return std::begin(w.iterable);
}

template <typename T>
auto std::end (reversable_wrapper<T> w)
{
    if (w.reverse) return std::rend(w.iterable);
    else return std::end(w.iterable);
}

template <typename T>
reversable_wrapper<T> reverse (T&& iterable) { return reversable_wrapper<T>(iterable); }

template <typename T>
reversable_wrapper<T> forward (T&& iterable) { return reversable_wrapper<T>(iterable, false); }

int main()
{
    std::vector<int> vec = {1,2,3};
    for (const auto& i : reverse(vec)) std::cout<<i<<std::endl;
    return 0;
}

Basically, what I wanted to achieve was for (const auto& item : cond ? forward(coll) : reverse(coll)). However, this code gives me following errors:

> $ g++ -std=c++14 iterators.cpp -o iterators
iterators.cpp: In instantiation of ‘auto std::begin(reversable_wrapper<T>) [with T = std::vector<int>&]’:
iterators.cpp:37:37:   required from here
iterators.cpp:18:38: error: inconsistent deduction for ‘auto’: ‘std::reverse_iterator<__gnu_cxx::__normal_iterator<int*, std::vector<int> > >’ and then ‘__gnu_cxx::__normal_iterator<int*, std::vector<int> >’
     else return std::begin(w.iterable);
                                      ^
iterators.cpp: In instantiation of ‘auto std::end(reversable_wrapper<T>) [with T = std::vector<int>&]’:
iterators.cpp:37:37:   required from here
iterators.cpp:25:36: error: inconsistent deduction for ‘auto’: ‘std::reverse_iterator<__gnu_cxx::__normal_iterator<int*, std::vector<int> > >’ and then ‘__gnu_cxx::__normal_iterator<int*, std::vector<int> >’
     else return std::end(w.iterable);

I can't figure which signature can I give to std::begin and std::end to overcome this and if this is possible at all.

I'm using g++ 5.4.0.

score 0 · Accepted Answer · answered Nov 11 '16 at 09:01

Your problem is that you are trying to return one of std::vector::iterator or std::vector::reverse_iterator from your functions. Sadly, these are unrelated types.

I think your only chance is to write a reversible_iterator_wrapper which holds both types, and forwards its arguments to the right one. This is going to be tedious. The simplest (but not most efficient) option probably involves virtual functions!

Making interchangable forward/backwards for-range iterator

1 Answers1