indexed(histogram) not usable with ranges

[mreininghaus]

While a boost::histogram can be used as a range in C++20, an indexed(histogram) cannot. At least for me (tested with clang++ 13 and g++ 11.3), the following code does not compile:

#include <random>
#include <ranges>
#include <iostream>

#include <boost/histogram.hpp>

int main() {
  using namespace boost::histogram;

  std::mt19937 rng;
  std::exponential_distribution dist;

  auto hist = make_histogram(axis::regular<>(20, 1, 10.0));

  for (int i = 0; i < 1'000; ++i) {
    hist(dist(rng));
  }
  
  // these work
  auto r1 = std::ranges::subrange(hist.cbegin(), hist.cend());
  auto v1 = hist | std::views::transform([] (auto&& x) {return 1;});
  
  // these don't:
  auto x = indexed(hist);
  auto r2 = std::ranges::subrange(x.cbegin(), x.cend());
  auto v2 = indexed(hist) | std::views::transform([] (auto&& x) {return 1;});

  return 0;
}

[HDembinski]

It seems that the iterators returned by indexed do not satisfy the std::input_or_output_iterator concept.
https://en.cppreference.com/w/cpp/iterator/input_or_output_iterator
It is not immediately obvious why, so this is needs more digging. Technically, they should satisfy that, these iterator requirements are very basic.

[HDembinski]

It is not the iterator concept.

#include <ranges>
#include <boost/histogram.hpp>

template<std::input_or_output_iterator I> 
void test_iter(I) {}

int main() {
    using namespace boost::histogram;

    auto hist = make_histogram(axis::integer(1, 2));

    auto ind = indexed(hist);

    // this works
    test_iter(ind.begin());

    // this fails
    auto r2 = std::ranges::subrange(ind.begin(), ind.end());

  return 0;
}

[HDembinski]

Using indexed in ranges is currently not supported, it is untested. I will try to support this, since the syntax is nice and it makes sense to use an indexed in a range expression to strip the flow content.