.. _More_on_HashCompare:
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));
Generated by dwww version 1.15 on Sun Jun 16 09:07:45 CEST 2024.