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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="SECTION006260000000000000000">Visitors</A></H2>
<A NAME="secadtsvisitors">&#160;</A>
<P>
The <tt>Container</tt> class described in the preceding section
interacts closely with the <tt>Visitor</tt> class
shown in Program&nbsp;<A HREF="page118.html#progvisitor1h" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page118.html#progvisitor1h"><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>.
In particular, the <tt>Accept</tt> member function of the <tt>Container</tt>
class takes as its lone argument a reference to a <tt>Visitor</tt>.
<P>
<P><A NAME="5506">&#160;</A><A NAME="progvisitor1h">&#160;</A> <IMG WIDTH=575 HEIGHT=143 ALIGN=BOTTOM ALT="program4922" SRC="img692.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img692.gif"  ><BR>
<STRONG>Program:</STRONG> <tt>Visitor</tt> Class Definition<BR>
<P>
<P>
But what is a visitor?
A shown in Program&nbsp;<A HREF="page118.html#progvisitor1h" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page118.html#progvisitor1h"><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>,
a visitor is an object that has the two member functions
<tt>Visit</tt> and <tt>IsDone</tt>.
Of these, the <tt>Visit</tt> function is the most interesting.
The <tt>Visit</tt> function takes as its lone argument a reference
to an <tt>Object</tt> instance.
<P>
The interaction between a container and a visitor goes like this.
The container is passed a reference to a visitor by calling
the container's <tt>Accept</tt> member function.
I.e., the container ``accepts'' the visitor.
What does a container do with a visitor?
It calls the <tt>Visit</tt> member function of that visitor
one-by-one for each object contained in the container.
<P>
The interaction between a <tt>Container</tt> and its <tt>Visitor</tt>
are best understood by considering an example.
The following code fragment gives the design framework
for the implementation of the <tt>Accept</tt> function
in some concrete class, say <tt>SomeContainer</tt>,
which is derived from the abstract base class <tt>Container</tt>:
<P>
<PRE>void SomeContainer::Accept (Visitor& visitor) const
<P>
    <em>for each</em> Object i <em>in this container</em>
<P>
        visitor.Visit (i);
<P>
</PRE>
<P>
The <tt>Accept</tt> function calls <tt>Visit</tt>
for each object <tt>i</tt> in the container.
Since the class <tt>Visitor</tt> is an abstract base class
which does not provide an implementation for the <tt>Visit</tt> operation,
what the visitor actually does with an object
depends on the type of visitor used.
<P>
Suppose that we want to print all of the objects in the container.
One way to do this is to create a <tt>PrintingVisitor</tt>
which prints every object it visits,
and then to pass the visitor to the container
by calling the <tt>Accept</tt> member function.
The following code shows how we can declare the <tt>PrintingVisitor</tt> class
which prints an object on the standard output stream, <tt>cout</tt>.
<PRE>class PrintingVisitor : public Visitor
{
public:
    void Visit (object&amp; object)
        { cout &lt;&lt; object; }
};</PRE>
<P>
Finally, given a container <tt>c</tt> that is an instance of
a concrete container class <tt>SomeContainer</tt>
which is derived from the abstract base class <tt>Container</tt>,
we can call the <tt>Accept</tt> function as follows:
<PRE>SomeContainer c;
PrintingVisitor v;
c.Accept (v);</PRE>
The effect of this call is to call the <tt>Visit</tt> member function
of the visitor for each object in the container.
<P>
<BR> <HR>
<UL> 
<LI> <A NAME="tex2html3373" HREF="page119.html#SECTION006261000000000000000" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page119.html#SECTION006261000000000000000">The <tt>IsDone</tt> Member Function</A>
<LI> <A NAME="tex2html3374" HREF="page120.html#SECTION006262000000000000000" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page120.html#SECTION006262000000000000000"><tt>Container</tt> Class Default <tt>Put</tt> Member Function</A>
</UL>
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