[stevemk14ebr]

Neat/Funny i guess but not really technically impressive. They took normal C++ code and hid it behind a macro with some names. The actual implementation isn't even there, Hashtable::hash and the 'table' member for example.

I was totally hoping for some fancy templates and macro tricks to compile time gen all the code for a working hashtable implementation.

[Xophmeister]

For some _real_ CPP abuse, check out klib’s hash table implementation: https://github.com/attractivechaos/klib/blob/master/khash.h

[stochastic_monk]

I've had real world applications where the full program's performance increased by a factor of 10 when switching to khash over std::unordered_map. I've seen papers which have them performing rather comparably, but whenever I've compared the two, khash has always soared over std::unordered_map. I use std::unordered_map for typical, average use cases (where speed doesn't matter as much and I'm feeling lazy), but khash for inner loops/core functionality.

I wonder if part of the reason is that, by nature of the standard, std::unordered_map has to resolve its collisions by linear chaining, while khash is able to use quadratic probing.

[CyberDildonics]

The standard dictates performance characteristics that imply the use of buckets, so there is more pointer hopping. A hash map that uses flat memory will end up being better. I would guess robin hood hashing + flat memory would be one of the most competitive techniques.

[maxlybbert]

On top of the performance guarantees, the API explicitly mentions buckets (e.g., http://en.cppreference.com/w/cpp/container/unordered_map/max... , http://en.cppreference.com/w/cpp/container/unordered_map/buc... ).

[stochastic_monk]

Well, even khash uses buckets. It’s just that you skip over your “correct” bucket for insertion if it’s full when using open addressing.

[ComputerGuru]

Fwiw quadratic hashing was only added to khash in 2013. Depending on when you first tested it, that may not be the reason it was faster.

[stochastic_monk]

Thanks for the tip. This testing was 2015, so it would have been after that point. You're right, though -- even linear probing would probably have better locality than chaining.