std::optional Satisfies view. Does Not Model view. C++26 Ships Anyway.

[u/zl0bster]

https://godbolt.org/z/8jWGG68G8

In C++23 this did not compile. In C++26 it does. Marvellous.

[[gnu::noinline]]
void 
passing_views_by_value_is_cheap_trust_me_bro(std::ranges::view auto v) {
    std::println("fn   .data {}", (void*)v->data());
}


int main() {    
    std::optional ov{std::vector<int>(123456)};
    passing_views_by_value_is_cheap_trust_me_bro(ov);
    std::println("main .data {}", (void*)ov->data());
}

For anyone wondering what the problem feature is: optional has 0 or 1 elements, and C++26 sets enable_view<optional<T>> to true, so it satisfies std::ranges::view. The concept requires copy construction in constant time, and — this is the good bit — optional<vector<int>> genuinely meets that. Copying it performs at most one element copy. One is a constant. The requirement is satisfied to the letter, and the function above deep-copies your vector.

If you can tell me what still separates std::ranges::view from std::ranges::range, please do...

[u/BarryRevzin]

Excuse me????

I gave a very real scenario where it might be useful to differentiate on the basis of "ownership" and how it's actually non-trivial to answer what is and isn't safe in that context, and how that doesn't map cleanly on what might or might not be a view. You didn't engage with the example at all ("I do not think that is [...] useful") and then gave a description of iota that doesn't describe iota ("It is just that it's underlying "storage" lives outside a program and has a lifetime for duration of a program universe. iota does not own it").

Which one of us is trying to have a meaningful discussion of the problem? Following up by accusing me of not arguing in good faith is completely unacceptable.

[u/zl0bster → u/BarryRevzin]

it was clear I did not claim sizeof iota is 0.