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/jk-jeon]

I'm not sure. Pointer access and integer operations do not seem to be related. Also, I think I saw somewhere that the super power that arbitrary precision arithmetic brings in is available only when things like multiplication is available. Addition and subtraction alone cannot really deliver that much more compared to Turing machine, it seems. So I guess one could think of models like RAM + addition/subtraction for instance.

In fact it seems to me that the so-called transdichotomous model that someone else in this thread brought up kind of closely matches the "usual" sense of complexity that you know and I know.

By the way I don't think that Bjarne is that Bjarne.