HP Integrity NonStop J-Series - Manual (Page 167)

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©Copyright 1996 Rogue Wave Software
A Note on How Objects are Found
You may save yourself some difficulty by remembering the following point: the virtual
functions of the object within the collection, not those of the target, are called when
comparing or testing a target for equality.
The following code fragment illustrates the point:
SortedCollection sc;
RWCollectableString member;
sc.insert(&member);
RWCollectableString target;
RWCollectableString* p = (RWCollectableString*)sc.find(&target);
In this example, the virtual functions of member within the collection RWCollectableString are
called, not the virtual functions of target. In other words:
member.compareTo(&target); //This will get called.
target.compareTo(&member); //Not this.
Hashing
Hashing is an efficient method for finding an object within a collection. All the collection
classes that use hashing follow the same general strategy. First, member function hash() of the
target is called to find the proper bucket within the hash table. A buckets is implemented as a
singly-linked list. Because all the members of a bucket have the same hash value, the bucket is
linearly searched to find the exact match. This is done by calling member function isEqual() of
the candidate (see above) with each member of the bucket as the argument. The first argument
that returns TRUE is the chosen object. Be careful not to design your class so that two objects
that test true for isEqual() can have different hash values, since this algorithm will fail for such
objects.
In general, because of this combination of hashing and linear searching, as well as the
complexity of most hashing algorithms, the ordering of the objects within a hash collection will
not make a lot of sense. Hence, when the apply() function or an iterator scans through the
hashing table, the objects will be visited in what appears to be random order.