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Why are RB-trees used to implement std::set in C++?

+2 votes
130 views

I'm curious as to why libstdc++ is using a RB-tree to implement std::set (details here
https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/std/set and here https://github.com/gcc-mirror/gcc/blob/master/libstdc++-v3/include/bits/stl_tree.h ),
when there are faster alternatives?

I'm mainly curious, because from all the people I've asked there hasn't been a single answer in favor of RB-trees, other than "they're already popular" or "easy to implement".

If you'd like more details on that, here's a link to my question on stackexchange http://cs.stackexchange.com/questions/41969/why-are-red-black-trees-so-popular,
where nobody has yet answered as well.

Using a variant of B-tree (or (a,b)-tree) should be faster, and everyone seems to suggest so, which makes me wonder what is the reason for picking RB-trees? This is not a question about their theoretical speed, but about real world behavior on real hardware with respect to CPU caches, etc.

posted May 11, 2015 by anonymous

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2 Answers

+1 vote

Because the original HP STL that most implementations (including libstdc++) are derived from was written that way. Changing the underlying data structure would likely break binary compatibility and so the benefits of such a change would have to significantly outweigh its costs.

answer May 11, 2015 by Kiran Kumar
+1 vote

Imagine you write a library and you compile and distribute the binaries. Someone else builds against those. If std::set is used in your API, and they have a different version of libstdc++, do you still want the library and the binary to be compatible, even across different versions of libstdc++?

answer May 11, 2015 by Jagan Mishra
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