What is MSVC's unordered_map::lower_bound?

Question

I found out that, contrary to my expectation and the documentation, the MSVC's std::unordered_map and std::unordered_multimap (and the unordered sets, too) provide lower_bound and upper_bound.

What do these member functions do? Is there any documentation?

I tried with MSVC 2019 and 2022 in C++20 mode; the following seems to compile:

#include <iostream>
#include <unordered_map>
int main() {
    std::unordered_map<int, int> m;
    m.emplace(1, 1);
    std::cout << m.lower_bound(1)->first;
}

Answer 1

It's a non-standard extension for compatibility purposes only. It is deprecated and has the following warning:

The hash_meow and unordered_meow containers' non-Standard lower_bound() member was provided for interface compatibility with the ordered associative containers, and doesn't match the semantics of the hash_meow or unordered_meow containers. Please use the find() member instead. You can define _SILENCE_STDEXT_HASH_LOWER_BOUND_DEPRECATION_WARNING to suppress this warning.

From yvals_core.h (upper_bound has a similar warning).

Your code actually doesn't compile for me, on VS 17.5 using /std:c++20, as I get the above "warning" (which is actually an error for me).

As to what it actually does, lower_bound is the same as calling find; upper_bound is a bit more complex, but still tries to find the next element after the key, using _Find_last (internal MS function for hash-type objects)