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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="SECTION0062110000000000000000">Associations</A></H2>
<A NAME="secadtsassociations">&#160;</A>
<P>
An association is an ordered pair of objects.
The first element of the pair is called the <em>key</em><A NAME=5211>&#160;</A>;
the second element is the <em>value</em><A NAME=5213>&#160;</A>
associated with the given key.
<P>
Associations are useful for storing
information in a database<A NAME=5214>&#160;</A> for later retrieval.
E.g., a database can be viewed as a collection of key-and-value pairs.
The information associated with a given key
is retrieved from the database by searching the database
for an the ordered pair in which the key matches the given key.
<P>
An association is like a container in that it contains two objects--the key and the value.
On the other hand,
an association is fundamentally different from a container
since the contained objects are treated very differently.
Normally, the objects in a container are not distinguished
one from another.
In an association, the two contained objects are distinct--one of them is <em>the key</em>
and the other is a value <em>associated with</em> the key.
<P>
Program&nbsp;<A HREF="page125.html#progassoch" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page125.html#progassoch"><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 concrete object class <tt>Association</tt>.
The declaration of the <tt>Association</tt> class
is like that of <tt>Container</tt> class
in that both of them are derived from the same set of base classes--<tt>Object</tt> and <tt>Ownership</tt>.
<P>
<P><A NAME="5546">&#160;</A><A NAME="progassoch">&#160;</A> <IMG WIDTH=575 HEIGHT=353 ALIGN=BOTTOM ALT="program5223" SRC="img699.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img699.gif"  ><BR>
<STRONG>Program:</STRONG> <tt>Association</tt> Class Definition<BR>
<P>
<P>
An association has two member variables,
<tt>key</tt> and <tt>value</tt>,
both of which are of type <em>pointer</em> to <tt>Object</tt>.
Since pointers are used,
the <tt>Association</tt> class implements <em>indirect containment</em>.
<P>
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