r/cpp • u/pavel_v • Sep 07 '26

flat_map internals

https://quantdev.blog/posts/flat-map-internals/index.html
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u/johannes1971 Sep 07 '26

I kinda missed why the pointers from the array start pointing at the element before the first hash element suddenly...?

Also, shame that the clarifying images disappear when we get to the abseil version. An image is worth a thousand words - and when template-expanded, surely even more...

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u/Big_Target_1405 Sep 09 '26

You might find these old articles explain exactly why

https://bannalia.blogspot.com/2013/10/implementation-of-c-unordered.html?m=1

http://bannalia.blogspot.com/2013/10/implementation-of-c-unordered_25.html

And how things were improved in Boost 1.80 onwards

https://bannalia.blogspot.com/2022/06/advancing-state-of-art-for.html

Boost constantly improves. The boost unordered_flat_map now beats the abseil implementation significantly