Should reversing a reverse_iterator give a forward iterator of the original type?

Question

I naively expected this program to compile and run with a success status:

#include <iterator>
#include <string>

int main()
{
    const std::string s = "foo";
    auto forward_iter = s.begin();
    auto reverse_iter = std::make_reverse_iterator(forward_iter);
    auto third_iter = std::make_reverse_iterator(reverse_iter);
    return forward_iter != third_iter;
}

It fails to compile, because the type of third_iter isn't the same as that of the forward_iter we started with; instead it's a reverse_iterator<reverse_iterator<normal_iterator>>:

0.cpp:10:25: error: no match for ‘operator!=’ (operand types are ‘__gnu_cxx::__normal_iterator<const char*, std::__cxx11::basic_string<char> >’ and ‘std::reverse_iterator<std::reverse_iterator<__gnu_cxx::__normal_iterator<const char*, std::__cxx11::basic_string<char> > > >’)
     return forward_iter != third_iter;
            ~~~~~~~~~~~~~^~~~~~~~~~~~~

Re-reading the documentation, it appears that std::make_reverse_iterator(it) is specified to always wrap it, even if it is already a reverse iterator (and to some extent that makes sense, because we'll be expecting to use reverse_iterator members (i.e. base()).

Is there a standard way to swap between normal (forward) and wrapped (reverse) iterators, without knowing which type I have? Or do I need to write a SFINAE pair of functions to return std::make_reverse_iterator(it) and it.base() appropriately?

Answer 1

I managed to make a pair of small functions to swap between forward and reverse iterators (using simple overloading rather than SFINAE):

template<typename Iter>
auto toggle_iterator_direction(Iter it) {
    return std::make_reverse_iterator(it);
}

template<typename Iter>
auto toggle_iterator_direction(std::reverse_iterator<Iter> it) {
    return it.base();
}

---

And an alternative version, using a helper type template, modelled on an answer to Determine if a (c++) iterator is reverse:

template<typename Iter>
struct is_reverse_iterator : std::false_type {};

template<typename Iter>
struct is_reverse_iterator<std::reverse_iterator<Iter>> : std::true_type {};


template<typename Iter>
auto toggle_iterator_direction(Iter it) {
    if constexpr (is_reverse_iterator<Iter>())
        return it.base();
    else
        return std::make_reverse_iterator(it);
}

---

Both of these fix the modified test program:

#include <iterator>
#include <string>

int main()
{
    const std::string s = "foo";
    auto forward_iter = s.begin();
    auto reverse_iter = toggle_iterator_direction(forward_iter);
    auto third_iter = toggle_iterator_direction(reverse_iter);
    return forward_iter != third_iter;
}

Answer 2

Should reversing a reverse_iterator give a forward iterator of the original type?

No. Or at least, that's not how make_reverse_iterator has been specified. The return type is specified to be reverse_iterator<Iterator> in the standard.

Is there a standard way to swap between normal (forward) and wrapped (reverse) iterators, without knowing which type I have?

No. Not as far as I know.

Or do I need to write a SFINAE pair of functions to return std::make_reverse_iterator(it) and it.base() appropriately?

You can write it. I can't say whether you need it.

Here's one implementation:

#include <iterator>
#include <type_traits>

// https://stackoverflow.com/a/35408829/2079303
template<typename I>
struct is_reverse_iterator : std::false_type {};

template<typename I>
struct is_reverse_iterator<std::reverse_iterator<I>>
: std::integral_constant<bool, !is_reverse_iterator<I>::value> {};

template<class It>
auto
reverse_or_base(It&& i)
{
    if constexpr (is_reverse_iterator<std::decay_t<It>>())
        return i.base();
    else
        return std::make_reverse_iterator(std::forward<It>(i));
}

And a test:

#include <vector>
#include <cassert>
int main() {
    std::vector<int> v;
    static_assert(
        std::is_same_v<
            decltype(reverse_or_base(v.rbegin())),
            std::vector<int>::iterator
        >
    );
    assert(v.end() == reverse_or_base(v.rbegin()));

    static_assert(
        std::is_same_v<
            decltype(reverse_or_base(v.begin())),
            std::vector<int>::reverse_iterator
        >
    );
    assert(v.rend() == reverse_or_base(v.begin()));
}