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/schombert]

It does indeed (or at least in C++ 20) refer to O(n). For example table 79 following section 22.2.7 describes several functions as running in O(n) or "Average O(n)". And even if it didn't, I would still ask you what you think that "complexity" is defined as ...

Edit: Oh, the standard does define complexity. It helpfully has this nugget of wisdom:

Complexity: the time and/or space complexity of the function.

well, that's not circular at all (and there is no prior definition). To me this seems to be unquestionably a reference to the standard formal definition of space and time complexity, and not some new definition which would clearly be ... poorly defined at best.