SG::ptrhash Struct Reference

Improved hash function for pointers. More...

#include <ptrhash.h>

Collaboration diagram for SG::ptrhash:

Public Member Functions
std::size_t	operator() (const void *p) const

Detailed Description

Improved hash function for pointers.

The default hash function for pointers used by unordered_map and friends is just the result of casting the pointer to an int. The problem is that most pointers one deals with are going to be aligned; if the pointers are to objects obtained through malloc, then on all systems we deal with, at least the lower three bits of the pointer will always be clear. Since unordered_map uses a bucket-hash scheme, this means that only 1/8 of the buckets will be used in such a case, resulting in lower efficiency.

Here, we try to avoid this problem.

Definition at line 40 of file ptrhash.h.

Member Function Documentation

operator()()

std::size_t SG::ptrhash::operator() ( const void * p ) const

inline

Definition at line 42 of file ptrhash.h.

  {
    std::size_t x = reinterpret_cast<std::size_t> (p);
    return (x>>3) ^ (x&7);
  }

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The documentation for this struct was generated from the following file:

• ptrhash.h