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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="SECTION006240000000000000000">Object Wrappers for the Built-In Types</A></H2>
<A NAME="secadtswrappers"> </A>
<P>
One of the design goals of the C++ language is to
treat user-defined data types
and the built-in data types as equally as possible.
However, the built-in data types of C++ do
have one shortcoming with respect to user-defined ones--a built-in data type cannot be used as the base class
from which other classes are derived.
E.g., it is not possible to write:
<PRE>class D : public int // Wrong!</PRE>
Consequently, it is not possible to extend the functionality
of a built-in type using inheritance.
<P>
The usual design pattern for dealing with this deficiency
is to put instances of the built-in types
inside a <em>wrapper</em><A NAME=4700> </A> class
and to use the wrapped objects in place of the built-in types.
Program <A HREF="page115.html#progwrapper1h" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page115.html#progwrapper1h"><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> illustrates this idea.
The class <tt>Wrapper<T></tt> is a concrete class which is
derived from the abstract base class <tt>Object</tt>.
It is also a generic class,
the purpose of which is to encapsulate an object of type <tt>T</tt>.
Each instance of the <tt>Wrapper<T></tt> class contains a single member variable
of type <tt>T</tt> called <tt>datum</tt>.
<P>
<P><A NAME="5471"> </A><A NAME="progwrapper1h"> </A> <IMG WIDTH=575 HEIGHT=295 ALIGN=BOTTOM ALT="program4708" SRC="img687.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img687.gif" ><BR>
<STRONG>Program:</STRONG> <tt>Wrapper<T></tt> Class Definition<BR>
<P><BR> <HR>
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