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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="SECTION006220000000000000000">Objects</A></H2>
<A NAME="secadtsobjects"> </A>
<P>
The abstract class at the top of the class hierarchy
is called <tt>Object</tt>.
With the exception of the <tt>Ownership</tt> class,
all the other classes in the hierarchy are derived from this class.
Program <A HREF="page111.html#progobject1h" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page111.html#progobject1h"><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 the declaration
of the <tt>Object</tt> class.
Altogether only six member functions are declared:
the destructor, <tt>IsNull</tt>, <tt>Hash</tt>, <tt>Put</tt>,
<tt>Compare</tt> and <tt>CompareTo</tt>.
<P>
<P><A NAME="5435"> </A><A NAME="progobject1h"> </A> <IMG WIDTH=575 HEIGHT=219 ALIGN=BOTTOM ALT="program4502" SRC="img679.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img679.gif" ><BR>
<STRONG>Program:</STRONG> <tt>Object</tt> Class Definition<BR>
<P>
<P>
The <tt>Object</tt> class destructor is declared <tt>virtual</tt>.
It is essential to declare it so,
because the <tt>Object</tt> is an abstract class,
and instances of derived class objects are very likely to be
accessed through the base class interface.
In particular, it is necessary for the destructor of the derived class
to be called through the base class interface--this is precisely what the <tt>virtual</tt> keyword achieves.
<P>
The <tt>IsNull</tt> function is a pure virtual member function
which returns a Boolean value.
This function is used in conjunction with the <tt>NullObject</tt>
concrete classes described below in Section <A HREF="page113.html#secadtsnullobject" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page113.html#secadtsnullobject"><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>.
The <tt>IsNull</tt> function shall return <tt>false</tt> for
all object instances derived from <tt>Object</tt>
except if the object instance is the <tt>NullObject</tt>.
We will see later that certain functions return object references.
For example, a function which searches through a data structure
for a particular object returns a reference to that object if it is found.
If the object is not found,
the search returns a reference to the <tt>NullObject</tt> instance.
By using the <tt>IsNull</tt> function,
the programmer can test whether the search was successful.
This is analogous to the <tt>NULL</tt> pointer in the C
programming language and to the pointer value <tt>0</tt> in C++.
<P>
The <tt>Hash</tt> function is a pure virtual member function
which returns a <tt>HashValue</tt>.
The <tt>Hash</tt> function is used in the implementation of hash tables
which are discussed in Chapter <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>.
We have put the <tt>Hash</tt> function in the <tt>Object</tt> class interface,
so that it is possible to store any object derived from <tt>Object</tt>
in a hash table.
What the <tt>Hash</tt> function computes is unspecified.
The only requirement is that it is <em>idempotent</em><A NAME=4545> </A>.
I.e., given an object instance <tt>obj</tt>,
<tt>obj.Hash()</tt> does not modify the object in any way,
and as long as <tt>obj</tt> is not modified in the interim,
repeated calls always give exactly the same result.
<P>
Two functions for comparing objects are declared--<tt>Compare</tt>
and <tt>CompareTo</tt>.
<tt>Compare</tt> is a public member function.
that takes a <tt>const</tt> reference to <tt>Object</tt> and returns an <tt>int</tt>.
Given two objects <tt>obj1</tt> and <tt>obj2</tt>,
calling <tt>obj1.Compare (obj2)</tt> <em>compares</em>
the value of <tt>obj1</tt> with the value of <tt>obj2</tt>.
The result is equal to zero if <IMG WIDTH=87 HEIGHT=21 ALIGN=MIDDLE ALT="tex2html_wrap_inline61275" SRC="img680.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img680.gif" >;
less than zero if <IMG WIDTH=88 HEIGHT=21 ALIGN=MIDDLE ALT="tex2html_wrap_inline61277" SRC="img681.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img681.gif" >; and,
greater than zero if <IMG WIDTH=87 HEIGHT=21 ALIGN=MIDDLE ALT="tex2html_wrap_inline61279" SRC="img682.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img682.gif" >.
<P>
It is not necessary that the two objects compared using <tt>Compare</tt>
have the same type.
As long as they are instances of classes derived from <tt>Object</tt>,
they can be compared.
The <tt>Compare</tt> function is used
in the overloading of the comparison operators
<tt>operator==</tt>, <tt>operator!=</tt>,
<tt>operator<</tt>, <tt>operator<=</tt>,
<tt>operator></tt>, and <tt>operator>=</tt>,
as shown in Program <A HREF="page111.html#progobject2h" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page111.html#progobject2h"><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>
The second comparison function, <tt>CompareTo</tt>,
is a pure virtual function.
An implementation for this function must be given in every
concrete class derived from <tt>Object</tt>.
The purpose of this function is to compare two objects
that are both instances of the same derived class.
<P>
The purpose of the <tt>Put</tt> member function of the <tt>Object</tt>
class is to output a human-readable representation of the object
on the specified output stream.
The <tt>Put</tt> function is a pure virtual function.
However, since it has the side-effect of output,
it is not strictly idempotent.
Nevertheless, it is a <tt>const</tt> member function
which means that calling the <tt>Put</tt> member function does
not modify the object in any way.
The use of the <tt>Put</tt> in the overloading of the <tt>ostream</tt>
insertion operator, <tt>operator<<</tt>,
is shown in Program <A HREF="page111.html#progobject2h" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page111.html#progobject2h"><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="5443"> </A><A NAME="progobject2h"> </A> <IMG WIDTH=575 HEIGHT=393 ALIGN=BOTTOM ALT="program4588" SRC="img683.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img683.gif" ><BR>
<STRONG>Program:</STRONG> <tt>Object</tt> Operator Definitions<BR>
<P>
<P>
The use of polymorphism in the way
shown gives the programmer enormous leverage.
The fact almost all objects will be derived from
the <tt>Object</tt> base class,
together with the fact that the <tt>Put</tt> and <tt>CompareTo</tt>
member functions are virtual functions,
ensures that the overloaded operators work as we expect
for all derived class instances.
<P>
<BR> <HR>
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