(split_)interval_map always preserve the values of the intersecting ranges during erase

[justinasvd]

Consider a test program

#include <boost/icl/split_interval_map.hpp>

#include <iostream>

struct foo {
  int v;
};

inline bool operator==(const foo &lhs, const foo &rhs) noexcept {
  return lhs.v == rhs.v;
}

inline std::ostream &operator<<(std::ostream &os, const foo &u) {
  return os << u.v;
}

template <class T> struct dummy_ {
  using first_argument_type = T;
};

int main() {
  using interval_map =
      boost::icl::split_interval_map<int, foo, boost::icl::partial_enricher,
                                     std::less, dummy_, dummy_,
                                     boost::icl::right_open_interval<int>>;

  interval_map tst;

  for (int i = 0; i < 10; i += 2) {
    tst.insert(std::make_pair(boost::icl::right_open_interval<int>(i, i + 2),
                              foo{10 * i}));
  }

  std::cout << "map: " << tst << std::endl;

  const boost::icl::right_open_interval<int> test_range(3, 7);

  std::cout << "after remove: ";
  tst -= test_range;
  std::cout << tst << std::endl;
}

The output of this program is going to be

map: {([0,2)->0)([2,4)->20)([4,6)->40)([6,8)->60)([8,10)->80)}
after remove: {([0,2)->0)([2,3)->20)([7,8)->60)([8,10)->80)}

This behavior is very surprising, because I would have expected that an inter_sect (in our case dummy_) f-tor would be used for the intersecting ranges [2,4) and [6,8). In my own use case, the correct output after removal should be

after remove: {([0,2)->0)([2,3)->0)([7,8)->0)([8,10)->80)}

But I see no way to achieve this with the current implementation of erase(interval).

[justinasvd]

N.B. the behavior of

tst.erase(std::make_pair(test_range, foo{}));

is even more baffling, as it is a no-op!

[jofaber]

Hi Justinas,
subtracting an interval from an Interval_map using the -= operator is effectively erasing the all content form the interval_map within the interval test_range.
If you'd like intersection to be propagated to values of the map, you need to combine two interval_maps that must have the same codomain type being a model of concept Set.
You may take a look at example user groups for this kind of operations.
Best regards,
Joachim