Iterating and inverting a const views::filter

Via Alexej Harm on Slack, two little hacks for dealing with std::ranges::filter_view.

First, recall that filter_view isn’t const-iterable — because its iterators keep pointers into the view itself, and modify it as you iterate. So if you are “constexpr’ing all the things” (not that I recommend that), how do you iterate a const filter_view?

constexpr auto isPrint = [](auto c) { return std::isprint(c); };
constexpr auto isXDigit = [](auto c) { return std::isxdigit(c); };
constexpr auto hexdigital = std::views::filter(isXDigit);
constexpr auto printable = std::views::filter(isPrint);

constexpr auto digits = std::views::iota('\0')
                      | std::views::take(256)
                      | hexdigital;

for (char c : digits) ~~~             // Error
for (char c : digits | printable) ~~~ // Error

C++23 to the rescue!

for (char c : auto(digits)) ~~~             // OK
for (char c : auto(digits) | printable) ~~~ // OK

Here we aren’t trying to iterate over the const-qualified digits: we’re asking the compiler to make us a copy (just like int(i) makes a prvalue copy of i), and then iterating over that copy.

Second, given an existing filter_view, can we “invert its sense”? For example, given digits as above, can we easily iterate over the non-digits? It turns out that C++20 filter_view exposes enough public getters to make this possible!

constexpr auto rest = [](auto fv) {
    return fv.base() | std::views::filter(std::not_fn(fv.pred()));
};

for (char c : rest(digits)) ~~~             // OK
for (char c : rest(digits) | printable) ~~~ // OK

Notice that we’re using fv.base() on the filter_view itself, to extract the base view. We might have written ranges::subrange(fv.begin().base(), fv.end().base()) instead, but that would cause dangling-pointer bugs if those iterators referred back into something captured inside fv. We should never do that.

Never prematurely “debone” a range type by splitting it into begin and end. Do that only when you’re about to traverse it, if ever at all.

For greater applicability to move-only views such as owning_view, we might consider calling std::move(fv).base() instead of fv.base(), or taking auto&& fv and calling decltype(fv)(fv).base(). As always, there’s a tradeoff between generality and maintainability.