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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="SECTION0018421000000000000000">Virtual Member Functions</A></H3>
<P>
In Program&nbsp;<A HREF="page609.html#proggraphic1h" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page609.html#proggraphic1h"><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>
<tt>Draw</tt>, <tt>Erase</tt> and <tt>MoveTo</tt>
are all declared as <tt>virtual</tt> member functions of the 
<tt>GraphicalObject</tt> class.
Declaring a member function <tt>virtual</tt> changes the way
in which the compiler determines the member function to call.
<P>
Consider the following sequence of instructions:
<PRE>GraphicalObject&amp; g1 = *new Circle (Point (0,0), 5);
GraphicalObject&amp; g2 = *new Square (Point (0,0), 5);
g1.Draw ();
g2.Draw ();</PRE>
If the <tt>Draw</tt> function was not declared virtual,
then both <tt>g1.Draw()</tt> and
<tt>g2.Draw()</tt>
would invoke <tt>GraphicalObject::Draw</tt>.
However, because <tt>Draw</tt> is a <tt>virtual</tt> function,
<tt>g1.Draw()</tt> calls <tt>Circle::Draw</tt>
and <tt>g2.Draw()</tt> calls <tt>Rectangle::Draw</tt>.
<P>
It is as if every object of a class contains a pointer to the actual routine
to be invoked when a virtual function is called on that object.
E.g, <tt>Circle</tt> objects carry pointers to
<tt>Circle::Draw</tt>,
<tt>GraphicalObject::Erase</tt> and <tt>GraphicalObject::MoveTo</tt>,
whereas <tt>Square</tt> objects carry pointers to
<tt>Rectangle::Draw</tt>,
<tt>GraphicalObject::Erase</tt> and <tt>GraphicalObject::MoveTo</tt>.
<P>
Virtual functions ensure that the ``correct''
member function is actually called,
regardless of how the object is accessed.
Consider the following sequence:
<PRE>Square s (Point (0,0), 5);
Rectangle&amp; r = s;
GraphicalObject&amp; g = r;</PRE>
Here <tt>s</tt>, <tt>r</tt> and <tt>g</tt> all refer to the same object,
even though they are all of different types.
However, because <tt>Draw</tt> is <tt>virtual</tt>,
<tt>s.Draw()</tt>, <tt>r.Draw()</tt> and <tt>g.Draw()</tt>
all invoke <tt>Rectangle::Draw</tt>.
<P>
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