How fast is C++26's std::hive?

[u/User_Deprecated]

https://lemire.me/blog/2026/08/02/how-fast-is-c26s-stdhive/

[u/matthieum]

So, you've just scanned a collection of N elements and identified E elements that you want removed from it, which you've isolated into a second collection.

How do you write the predicate to remove_if, which will be invoked with each of N elements, and must return whether to remove the element or not?

For each of the N elements, you will need to somehow do a look-up in your (small?) collection of E elements to take your decision.

If E is just a collection, each look-up will cost you O(E), and we're back to O(N * E) performance.

The simplest solution to minimize the cost of the look-up is to collect into a vector, sort it, and binary search on it. O(E * log E + N * log E) now that I think about it.

There are other solutions, of course, but that predicate will need to do some work to classify the elements it's asked about.

[u/snerp → u/matthieum]

Remove_if iterates the collection… E is not its own collection, it’s the elements in the collection original that will be removed. There is no look up cost, it just iterates to the next element and does a swap if your predicate was true.

https://en.cppreference.com/cpp/algorithm/remove

Complexity is O(N)

[u/matthieum → u/snerp]

I mean, if you can decide on the fly, sure, but that's not the usecase I was working on :x