HP Integrity NonStop J-Series - Manual (Page 101)

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©Copyright 1996 Rogue Wave Software
Storage Methods of Collection Classes
The general objective of collection classes, called collections for short, is to store and retrieve
objects. In fact, you can classify collection classes according to how they store objects.
Value-based collections store the object itself; reference-based collections store a pointer or
reference to the object. The difference between the two will influence how you use some
features of collection classes in Tools.h++.
Value-based collection classes are simpler to understand and manipulate. You create a linked
list of integers or doubles, for example, or a hash table of shorts. Stored types can be more
complicated, like the
RWCString s, but the important point is that they act just like values, even
though they may contain pointers to other objects. When an object is inserted into a value-based
collection class, a copy is made. The procedure is similar to C's pass-by-value semantics in
function calls.
In a reference-based collection class, you store and retrieve pointers to other objects. For
example, you could create a linked list of pointers to integers or doubles, or a hash table of
pointers to
RWCString s.
Let's look at two code fragments that demonstrate the difference between the value-based and
the reference-based procedures:
Value-based Example Reference-based Example
/* A vector of RWCStrings: */
RWTValOrderedVector
v;
RWCString s("A string");
v.insert(s); /* A vector of pointers to
RWCStrings: */
RWTPtreOrderedVector
v;
RWCString* p = new RWCString("A
string");
v.insert(p);
Both code fragments insert an RWCString into vector v. In the first example, s is an RWCString
object containing "A string". The statement v.insert(s) copies the value of s into the vector. The
object that lies within the vector is distinct and separate from the original object s. In the second
example, p is a pointer to the RWCString object. When the procedure v.insert(p) is complete,
the new element in v will refer to the same RWCString object as p.