"Tobias Schwinger" <tschwinger@isonews2.com> wrote in message news:f9elpo$86r$1@sea.gmane.org...
Interesting. Note, that this optimization does not pay off in general: Worse distribution (no avalanche effect because bits are cut off) ==> more probing ==> more data cache misses.
If the hash function is poor, then modulo with a prime number will certainly help; however, I don't think containers should be responsible for improving the effectiveness of hash functions. If I already have a fine source of hash values, then applying further (expensive!) operations are of diminishing value.
Please, no more type template parameters! Add another type member to the traits of that container and keep the primary interface simple, instead.
(If "traits" here refers to value traits) I don't think bucket pointer, bucket size, and the modulo options belong in the value traits, since they pertain more to the container than to the templated types. I do support adding a "bucket trait" class, and perhaps folding the SizeType parameter into the bucket trait class as well. (I'm not sure whether converting the currently internal implementation types for the buckets into a trait-based system has been discussed already) The modulo option parameter can perhaps be folded into an expanded concept of a hash function class.
If 'dynamic_bucket_size_policy' is configured in the container's traits, the ctor and the 'set_bucket_size' member function accept an integer. If 'constant_bucket_size_policy' is used an attempt to use an integer will yield a compile error. EBCO can be exploited for a zero-overhead implementation.
That'll work.
BTW: Zero-sized objects do not exist in C++ (EBCO allows empty subobjects to be mapped to the same address and thus take up no extra space - a concrete object, however, takes up at least one byte, so pointer comparison can be used for checking object identity).
The goal is to use placement operator new to construct the container object directly on top of the bucket array, relying on the object having no members. Although then it seems silly to do such a convoluted thing just for the sake of using thiscalls and perhaps the library could just expose "hash table algorithms" as free functions.
Why are you assuming that a constant bucket size should always be a power of two?
They were just examples ;) Regards, RT