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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="SECTION0018120000000000000000">References are Not Variables</A></H2>
<A NAME="seccppreference">&#160;</A>
<P>
In C++, a <em>reference</em><A NAME=57128>&#160;</A>
is an alternative name for a variable.
The declaration of a reference looks
very much like the declaration of a variable.
However, a reference is not a variable.
The notation  <IMG WIDTH=19 HEIGHT=11 ALIGN=BOTTOM ALT="tex2html_wrap_inline73429" SRC="img2598.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img2598.gif"  > denotes a reference to a variable of type <I>T</I>.
For example the code sequence
<PRE>int i = 57;
int&amp; r = i;</PRE>
defines one variable, <tt>i</tt>,
and one reference, <tt>r</tt>.
<P>
In C++ all references must be initialized.
In this case the reference <tt>r</tt> <em>refers</em> to the variable <tt>i</tt>.
A reference can be used in a program everywhere that a variable can be used.
In particular, if the reference <tt>r</tt> is used
where a <I>r</I>-value is required,
it is the <I>r</I>-value of <tt>i</tt> that is obtained.
Similarly, if the reference <tt>r</tt> is used where an <I>l</I>-value is required,
it is the <I>l</I>-value of <tt>i</tt> that is obtained.
<P>
Is it important to note that in C++,
despite appearances,
there are no operators that operate on references.
In particular,
it is not possible to change the variable to which a reference refers.
And since every reference must be initialized by associating it with a variable,
there is no such thing as a <em>null</em> reference.
References are mainly used in C++ programs to specify the arguments
and return values of functions.
<P>
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