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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="SECTION0018422000000000000000">Abstract Classes and Concrete Classes</A></H3>
<P>
In C++ an <em>abstract class</em><A NAME=57647> </A>
is one which defines an interface,
but does not necessarily provide implementations for all its member functions.
An abstract class is meant to be used as the base class from
which other classes are derived.
The derived class is expected to provide implementations for the member
functions that are not implemented in the base class.
A derived class that implements all the missing functionality is called
a <em>concrete class</em><A NAME=57649> </A>.
<P>
E.g., the <tt>GraphicalObject</tt> class
defined in Programs <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> and <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> is an abstract class.
In particular,
no implementation is given for the virtual member function <tt>Draw</tt>.
The fact that no implementation is given is indicated by the <tt>=0</tt>
attached to the <tt>Draw</tt> function prototype
in the class definition (Program <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>, line 19).
Recall that an object carries a pointer to the appropriate routine
for every virtual member function it supports.
The <tt>=0</tt> says that the pointer for the <tt>Draw</tt> function is
not defined in the <tt>GraphicalObject</tt> class and,
therefore, it must be defined in a derived class.
<P>
A virtual member function for which no implementation is given is called
a <em>pure virtual function</em><A NAME=57661> </A>.
If a C++ class contains a pure virtual function,
it is an <em>abstract class</em>.
In C++ it is not possible to instantiate an abstract class.
E.g., the following declaration is illegal:
<PRE>GraphicalObject g (Point (0,0)); // Wrong.</PRE>
If we were allowed to declare <tt>g</tt> in this way,
then we could attempt to invoke the non-existent
member function <tt>g.Draw()</tt>.
<P>
In fact, there is a second reason why the declaration
of <tt>g</tt> is an error:
The <tt>GraphicalObject</tt> constructor is not <tt>public</tt>
(see Program <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>, lines 15-16).
So, even if the <tt>GraphicalObject</tt> class was a concrete class,
we are still prevented from instantiating it.
Since the constructor is <tt>protected</tt>,
it can be called by a derived class.
Therefore, it is possible to instantiate <tt>Circle</tt>,
<tt>Rectangle</tt> and <tt>Square</tt>.
<P>
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