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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="SECTION0018423000000000000000">Algorithmic Abstraction</A></H3>
<P>
Abstract classes can be used in many interesting ways.
One of the most useful paradigms is the use of an abstract class
for <em>algorithmic abstraction</em><A NAME=57676> </A>.
The <tt>Erase</tt> and <tt>MoveTo</tt> functions defined in Program <A HREF="page609.html#proggraphic1c" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page609.html#proggraphic1c"><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>
are examples of this.
<P>
The <tt>Erase</tt> and <tt>MoveTo</tt> functions are implemented in
the abstract class <tt>GraphicalObject</tt>.
The algorithms implemented are designed to work in any
concrete class derived from <tt>GraphicalObject</tt>,
be it <tt>Circle</tt>, <tt>Rectangle</tt> or <tt>Square</tt>.
In effect, we have written algorithms that work
regardless of the actual class of the object.
Therefore, such algorithms are called <em>abstract algorithms</em>.
<P>
Abstract algorithms typically invoke virtual member functions.
E.g., the <tt>MoveTo</tt> function invokes <tt>Erase</tt> and <tt>Draw</tt>
to do most of the actual work.
In this case,
the derived classes are expected to inherit the abstract algorithm <tt>MoveTo</tt>
and to override the pure virtual function <tt>Draw</tt>.
Thus, the derived class customizes the behavior of the abstract algorithm
by overriding the appropriate member functions.
The virtual function resolution mechanism ensures that
the ``correct'' member function is always called.
<P>
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