[PATCH] D48896: [libcxx][c++17] P0083R5: Splicing Maps and Sets Part 2: merge

[Richard Smith - zygoloid via Phabricator]

rsmith added a comment.

In https://reviews.llvm.org/D48896#1274779, @ldionne wrote:

> When merging  a multi-container into a unique-container (e.g. `map.merge(multimap)` or `set.merge(multiset)`), I think we could do the following optimization: Once we've inserted an element from the `multimap` into the `map`, we could skip all the following equivalent elements in the `multimap` without having to perform a search in the `map` each time (since we know we won't insert the element anyway).


We can only do that if the comparators divide elements into the same equivalence classes, which we cannot know in general. (We can do this optimization if we can prove the comparator is a pure stateless function, but we probably can't detect that -- even an empty class type could store its state outside the class.) However, if the source and destination have the same comparator, and that comparator is `std::less<T>`, then I think it's correct to do this optimization.


https://reviews.llvm.org/D48896