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<b>Data Structures and Algorithms
with Object-Oriented Design Patterns in C++</b><br>
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<H3><A NAME="SECTION006261000000000000000">The <tt>IsDone</tt> Member Function</A></H3>
<P>
As shown in Program <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>,
the <tt>Visitor</tt> class interface
also includes the member function <tt>IsDone</tt>.
The <tt>IsDone</tt> member function is an accessor
which is used to determine whether a visitor has finished its work.
I.e., the <tt>IsDone</tt> member function returns the boolean value
<tt>true</tt> if the visitor ``is done.''
<P>
The idea is this:
Sometimes a visitor does not need to visit all the objects in a container.
I.e., in some cases, the visitor may be finished its task
after having visited only a some of the objects.
The <tt>IsDone</tt> member function can be used by the
container to terminate the <tt>Accept</tt> function like this:
<P>
<PRE>void SomeContainer::Accept (Visitor& visitor) const
<P>
<em>for each</em> Object i <em>in this container</em>
<P>
if (visitor.IsDone ())
return;
visitor.Visit (i);
<P>
</PRE>
<P>
To illustrate the usefulness of <tt>IsDone</tt>,
consider a visitor which visits the objects in a container
with the purpose of finding the first object that matches a given object.
Having found the first matching object in the container,
the visitor is done and does not need to visit any more contained objects.
<P>
The following code fragment defines a visitor
which finds the first object in the container
that matches a given object.
<PRE>class MatchingVisitor : public Visitor
{
Object const& target;
Object* found;
public:
MatchingVisitor (Object const& object) :
target (object), found (0)
{}
void Visit (Object& object)
{
if (found == 0 && object == target)
found = &object;
}
bool IsDone ()
{ return found != 0; }
};</PRE>
<P>
The constructor of the <tt>MatchingVisitor</tt> visitor takes a reference
to an <tt>Object</tt> instance that is the target of the search.
I.e., we wish to find an object in a container that matches the target.
For each object the <tt>MatchingVisitor</tt> visitor visits,
it compares that object with the target
and makes <tt>found</tt> point at that object if it matches.
Clearly, the <tt>MatchingVisitor</tt> visitor is done
when the <tt>found</tt> pointer is non-zero.
<P>
Suppose we have 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>;
and an object <tt>x</tt>
that is an instance of a concrete object class, <tt>SomeObject</tt>,
which is derived from the abstract base class <tt>Object</tt>.
Then, we can call use the <tt>MatchingVisitor</tt> visitor as follows:
<PRE>SomeContainer c;
SomeObject x;
MatchingVisitor v (x);
c.Accept (v);</PRE>
<P>
The observant reader will have noticed in Program <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>
that the <tt>Visit</tt> member function
of the abstract <tt>Iterator</tt> class is <em>pure</em> virtual function
whereas the <tt>IsDone</tt> function is not.
It turns out that it is convenient to define a default
implementation for the <tt>IsDone</tt> function
that always returns <tt>false</tt>.
<P>
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