More on HashCompare
There are several ways to make the
HashCompare
argument for
concurrent_hash_map
work for your own types.- Specify theHashCompareargument explicitly
- Let theHashComparedefault totbb_hash_compare<Key>and do one of the following:
- Define a specialization of templatetbb_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 methodhashthat maps aKeyto asize_t
- A methodequalthat 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));