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<b>Data Structures and Algorithms 
with Object-Oriented Design Patterns in C++</b><br>
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<H2><A NAME="SECTION006250000000000000000">Containers</A></H2>
<A NAME="secadtscontainers">&#160;</A>
<P>
The <tt>Container</tt> class abstracts the notion of a container--an object that holds within it other objects.
A container is itself an object.
Therefore, the <tt>Container</tt> class is derived
from the <tt>Object</tt> class.
A consequence of this is that containers
can be held in other containers!
<P>
The declaration of the <tt>Container</tt> class is given
in Program&nbsp;<A HREF="page117.html#progcontainer1h" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page117.html#progcontainer1h"><IMG  ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A>.
Notice that the <tt>Container</tt> declaration
uses <em>multiple inheritance</em><A NAME=4823>&#160;</A>.
It is derived from two base classes, <tt>Object</tt> and <tt>Ownership</tt>.
We have already seen the definition of the <tt>Object</tt> class.
The <tt>Ownership</tt> class is defined
below in Section&nbsp;<A HREF="page124.html#secadtsowner" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page124.html#secadtsowner"><IMG  ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A>.
<P>
<P><A NAME="5491">&#160;</A><A NAME="progcontainer1h">&#160;</A> <IMG WIDTH=575 HEIGHT=333 ALIGN=BOTTOM ALT="program4829" SRC="img690.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img690.gif"  ><BR>
<STRONG>Program:</STRONG> <tt>Container</tt> Class Definition<BR>
<P>
<P>
The <tt>Container</tt> class is an <em>abstract base class</em>.
It is intended to be used as the base class from which
concrete container realizations are derived.
The <tt>Container</tt> class public interface comprises eight
virtual member functions--<tt>Count</tt>, <tt>IsEmpty</tt>, <tt>IsFull</tt>, <tt>Hash</tt>,
<tt>Put</tt>, <tt>NewIterator</tt>, <tt>Purge</tt> and <tt>Accept</tt>.
Default implementations are provided for the first six;
the last two are pure virtual member functions.
<P>
A single member variable, <tt>count</tt>, is declared.
This variable is used to keep track of the number of objects
held in the container.
The <tt>count</tt> field is set initially to zero.
It is the responsibility of the derived class
to update this field as required.
<P>
The <tt>Count</tt> member function is an accessor that
returns the number of items contained in the container.
The <tt>IsEmpty</tt> and <tt>IsFull</tt> functions are
Boolean-valued accessors which indicate whether a given
container is empty or full, respectively.
The <tt>Hash</tt> function is used in conjunction with hash tables
as discussed in Chapter&nbsp;<A HREF="page203.html#chaphashing" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page203.html#chaphashing"><IMG  ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A> and the <tt>Put</tt> function
inserts a human-readable representation of the container and its
contents in a specified output stream.
The purpose of the <tt>Purge</tt> function
is to remove all the objects from a container,
making it empty.
<P>
In order to describe the remaining two member functions,
<tt>Accept</tt> and <tt>NewIterator</tt>,
we need to become familiar first with the concepts of a visitor
and an iterator,
as well as with the <tt>Visitor</tt> and <tt>Iterator</tt> classes
which embody these concepts.
Visitors are discussed below in Section&nbsp;<A HREF="page118.html#secadtsvisitors" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page118.html#secadtsvisitors"><IMG  ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A>
and iterators are discussed in Section&nbsp;<A HREF="page121.html#secadtsiterators" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page121.html#secadtsiterators"><IMG  ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A>.
<P>
Program&nbsp;<A HREF="page117.html#progcontainer1c" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page117.html#progcontainer1c"><IMG  ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A> gives code that defines the
<tt>Container</tt> class constructor
as well as the default behaviors for several of the member functions.
The three functions <tt>Count</tt>, <tt>IsEmpty</tt> and <tt>IsFull</tt>
are member variable accessors--they are idempotent <tt>const</tt> member functions which
provide information about the status of the container.
<P>
<P><A NAME="5501">&#160;</A><A NAME="progcontainer1c">&#160;</A> <IMG WIDTH=575 HEIGHT=239 ALIGN=BOTTOM ALT="program4884" SRC="img691.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img691.gif"  ><BR>
<STRONG>Program:</STRONG> <tt>Container</tt> Class Member Function Definitions<BR>
<P>
<P>
The <tt>Count</tt> function simply returns
the value of the <tt>count</tt> member variable.
The <tt>IsEmpty</tt> function is a Boolean-valued accessor
which returns the value <tt>true</tt> if the container is empty,
i.e., it contains no objects.
Notice that the <tt>IsEmpty</tt> function
does not directly access the <tt>count</tt> field.
Instead it calls <tt>Count</tt> which is virtual function.
<P>
In some cases, a container is implemented in a way
which makes its capacity finite.
When this is the case,
it is necessary to be able to determine when the container is full.
The <tt>IsFull</tt> function is a Boolean-valued accessor
which returns the value <tt>true</tt> if the container is full.
However, the default version always returns <tt>false</tt>.
<P>
Default behaviors are also provided by the <tt>Container</tt> class
for the <tt>Hash</tt> and <tt>Put</tt> operations.
The implementation of the <tt>Put</tt> function is discussed
in the following section.
The <tt>Hash</tt> function is given in Chapter&nbsp;<A HREF="page203.html#chaphashing" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page203.html#chaphashing"><IMG  ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A>.
<P>
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