• 2021.4
  • 09/27/2021
  • Public Content
Contents

More on HashCompare

There are several ways to make the
HashCompare
argument for
concurrent_hash_map
work for your own types.
  • Specify the
    HashCompare
    argument explicitly
  • Let the
    HashCompare
    default to
    tbb_hash_compare<Key>
    and do one of the following:
    • Define a specialization of template
      tbb_hash_compare<Key>
      .
For example, if you have keys of type
Foo
, and
operator==
is defined for
Foo
, you just have to provide a definition of
tbb_hasher
as shown below:
size_t tbb_hasher(const Foo& f) { size_t h = ...compute hash code for f... return h; };
In general, the definition of
tbb_hash_compare<Key>
or
HashCompare
must provide two signatures:
  • A method
    hash
    that maps a
    Key
    to a
    size_t
  • A method
    equal
    that determines if two keys are equal
The signatures go together in a single class because
if two keys are equal, then they must hash to the same value
, otherwise the hash table might not work. You could trivially meet this requirement by always hashing to
0
, but that would cause tremendous inefficiency. Ideally, each key should hash to a different value, or at least the probability of two distinct keys hashing to the same value should be kept low.
The methods of
HashCompare
should be
static
unless you need to have them behave differently for different instances. If so, then you should construct the
concurrent_hash_map
using the constructor that takes a
HashCompare
as a parameter. The following example is a variation on an earlier example with instance-dependent methods. The instance performs both case-sensitive or case-insensitive hashing, and comparison, depending upon an internal flag
ignore_case
.
// Structure that defines hashing and comparison operations class VariantHashCompare { // If true, then case of letters is ignored. bool ignore_case; public: size_t hash(const string& x) const { size_t h = 0; for(const char* s = x.c_str(); *s; s++) h = (h*16777179)^*(ignore_case?tolower(*s):*s); return h; } // True if strings are equal bool equal(const string& x, const string& y) const { if( ignore_case ) strcasecmp(x.c_str(), y.c_str())==0; else return x==y; } VariantHashCompare(bool ignore_case_) : ignore_case(ignore_case_) {} }; typedef concurrent_hash_map<string,int, VariantHashCompare> VariantStringTable; VariantStringTable CaseSensitiveTable(VariantHashCompare(false)); VariantStringTable CaseInsensitiveTable(VariantHashCompare(true));

Product and Performance Information

1

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