Nesting with Combinations Compilation Issue in MSVC

[ryandougherty]

Hi, I have the following code snippet that does not compile in MSVC (if it helps, I'm using Visual Studio 2022):

auto cols = combinations(range(k), t);
auto d_sets = combinations(cols, d);
for (const auto& d_set : d_sets) { ... }

Attached is a screenshot of MSVC's output. All variables k, t, d are unsigned integers. It appears that the errors have nothing to do with this project's actual code (or mine), but within the xmemory and xutility headers. I have no idea why these are happening. I have an equivalent Python version which works.

The error goes away if I remove the for loop. The inside of the loop has nothing to do with the error; even with nothing in it I still get compilation errors.

[ryanhaining]

I'm afraid the answer here is going to be pretty unsatisfying, but you will need to throw in some extra steps to make this work.

Combinations is one of a few itertools that requires that the thing you pass to it have a ForwardIterator, but, combinations itself only provides an InputIterator.

The general advice if you need an itertool to behave like a forward iterable (or anything more functional than an input iterable) is to expand it into a vector or other container, the simplest (not pretty) way to do it here imo is:

auto cols_comb = iter::combinations(iter::range(3), 2);
auto m = iter::imap([] (auto&& c) {
      // expand the inner iterables as well.
      return std::vector<int>(c.begin(), c.end());
    }, cols_comb);
auto cols = std::vector<std::vector<int>>(m.begin(), m.end());

The rest of the code should work given that

#include "cppitertools/imap.hpp"
#include "cppitertools/combinations.hpp"
#include "cppitertools/range.hpp"

#include <iostream>
#include <vector>

int main() {
  auto cols_comb = iter::combinations(iter::range(3), 2);
  auto m = iter::imap([] (auto&& c) {
        return std::vector<int>(c.begin(), c.end());
      }, cols_comb);
  auto cols = std::vector<std::vector<int>>(m.begin(), m.end());

  auto d_sets = iter::combinations(cols, 2);

  for (const auto& d_set : d_sets) { 
    std::cout << "{ ";
    for (const auto& d : d_set) {
      std::cout << "{ ";
      for (const auto & i : d) {
        std::cout << i << " ";
      }
      std::cout << "} ";
    }
    std::cout << "}\n";
  }
}