C++20 std::ranges: Range adapter to skip every nth element

Question

I'm trying to get more acquainted with C++20's std::ranges implementation, and I've come across a seemingly simple problem of which I cannot find a standard solution without rolling my own implementation.

The problem is simple: I would like to only access and process every Nth element in a range using a C++20 range-adapter. For example, I'm looking for a utility where the following:

for (auto x : std::ranges::iota_view{0, 10} | std::ranges::<some api>(3)) {
    std::cout << x << " ";
}

will produce an output like:

0 3 6 9

This can of-course be accomplished with something like std::ranges::filter -- however filter actually accesses and processes the iterator at each value, e.g. it evaluates the expression "predicate(*it)". For small simple ranges this is fine, but for more complex/expensive generator iterators, then evaluating *it can be costly and undesirable since these values will be otherwise unused.

I'm looking for something more equivalent to the behavior of std::ranges::take or std::ranges::drop, which simply bypass the value by iterating over it, rather than accessing it.

Is there a simple, C++20 solution to accomplish this? Or will I have to roll my own with a wrapper iterator/sentinel where operator++ is done N times? This sounds like a utility that should already be part of the standard, but I can't seem to find anything fitting this description.

Answer 1

I don't think there is a range adaptor for this in the standard library.

Ranges-v3 library does however have it:

ranges::iota_view{0, 10} | ranges::stride_view(3)

There is a proposal to add such adaptor into the standard: P1899.

Github Issue says:

Discussed by LEWG in Prague. http://wiki.edg.com/bin/view/Wg21prague/P1899

Ensure that stride is positive. Unanimous consent

Revise (many suggestions in the notes) and return to LEWG. Unanimous consent

I didn't find public copy of the notes.

---

There is also another proposal to add step parameter to iota_view that would achieve the same for the example case: P2016.