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Could you explain why the _Hash class manages buckets using both lower and upper bounds?

형진 김 20 Reputation points
2025-05-07T06:23:50.5233333+00:00

Hello,

First of all, I’d like to kindly ask for your understanding as I’m using a translation tool to write this message.

While looking into the msvc C++ _Hash class, I came across something I didn’t fully understand, so I’m reaching out with a question. From what I can tell, the _Hash class seems to manage buckets in the form of [2n, 2n + 1] I’m curious about the reasoning behind this design. Is there a specific optimization technique involved in this approach?

In particular, when calling find(), it appears that it accesses 2n + 1 and then searches backwards through the list. I’d like to understand whether this behavior is part of some optimization strategy.

Thank you very much to anyone who takes the time to read my question.

Developer technologies | C++
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  1. Omkara Varshitha Kunapalli (INFOSYS LIMITED) 80 Reputation points Microsoft External Staff
    2025-07-08T08:31:36.91+00:00

    Regarding your observation about the bucket structure [2n, 2n + 1] and the behavior of find():

    The _Hash class in MSVC C++ uses a bucket pairing strategy—specifically [2n, 2n + 1]—as part of an internal optimization technique. This design helps improve memory alignment and lookup efficiency.

     Why [2n, 2n + 1]

    • Memory locality: Grouping buckets in adjacent pairs improves cache performance during lookups.
    • Efficient probing: The find() function typically starts at 2n + 1 and searches backward. This reverse traversal prioritizes recently inserted elements, which are more likely to be accessed again.
    • Collision resolution: This structure supports efficient chaining and collision handling by organizing entries predictably

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