make_iterator_range: The Range Counterpart to make_iterator

[chayden83]

I wanted to inquire as to whether there would be interest in adding a make_iterator_range utility function similar to make_iterator except that make_iterator_range would operate on ranges instead of iterators. The idea is to be able to return ranges local to a function w/out requiring a caster definition or a copy into a Python sequence. make_iterator_range enables code like this (pardon the pseudo-code).

static std::vector<int> const foo = { 1, 2, 3, 4, 5 };

pybind11::cpp_function([]() -> pybind11::object {
  return pybind11::make_iterator_range(foo | std::views::transform([](int i) { return i +1; }));
});

Accomplishing the same thing w/make_iterator is more difficult b/c the iterator for a transform_view is invalid after the lifetime of the transform_view ends. For example, this will not work.

pybind11::cpp_function([]() -> pybind11::object {
  auto tv = foo | std::views::transform([](int i) { return i +1; });
  return pybind11::make_iterator(std::ranges::begin(tv), std::ranges::end(tv)); // iterators dangle!
});

The make_iterator_range utility I'm proposing would not suffer this issue because it would store the range internally in its private state, and the __iter__ function would be implemented with a keep_alive<0,1>() in order to keep the underlying range alive as long as necessary. In addition, the binding returned by make_iterator_range would provide different methods depending on whether the underlying range is an input range, a forward range, etc.

I also wanted to know if pybind11 code is allowed to make use of feature macros in order to make some functionality conditionally available. For the current proposal, make_iterator_range would require a C++20 compiler w/__cpp_lib_ranges defined.

If this is something the project would be interested in, please let me know and I will prepare a PR.

[chayden83]

Just following up to see if there is any interest in this idea.

[chayden83]

It seems like most of these threads don't get a lot of discussion. Would it be better if I just prepared a PR so everyone could take a look?