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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="SECTION0018110000000000000000">Pointers Are Variables</A></H2>
<P>
The C++ statement
<PRE>int* p;</PRE>
declares a <em>pointer</em><A NAME=56968> </A>.
A pointer is a variable the type of which has the form <IMG WIDTH=18 HEIGHT=11 ALIGN=BOTTOM ALT="tex2html_wrap_inline73359" SRC="img2594.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img2594.gif" >
where <I>T</I> is an arbitrary type.
In this case, <IMG WIDTH=57 HEIGHT=11 ALIGN=BOTTOM ALT="tex2html_wrap_inline73363" SRC="img2595.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img2595.gif" > and we say that <tt>p</tt> is a
<em>pointer</em> to <tt>int</tt>.
<P>
Since a pointer is a variable,
it has all the attributes of a variable.
In particular, it has both an <em>address</em> (<I>l</I>-value)
and a <em>value</em> (<I>r</I>-value).
<P>
Consider a C++ program that contains
the following global variable declarations:
<PRE>int i = 57;
int j = 31;
int* p = 0;
int* q = (int*) 1004;</PRE>
Suppose the four variables, <tt>i</tt>, <tt>j</tt>, <tt>p</tt> and <tt>q</tt>,
are stored in contiguous memory locations
starting at address 1000 as shown in Figure <A HREF="page588.html#figcpp1" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page588.html#figcpp1"><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>.
(We assume that <IMG WIDTH=233 HEIGHT=24 ALIGN=MIDDLE ALT="tex2html_wrap_inline73371" SRC="img2596.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img2596.gif" >).
Note that the variable named <tt>j</tt> has the value 31
and is found at address 1000.
Similarly, the variable named <tt>q</tt> has the value 1004
and is found at address 1012.
<P>
<P><A NAME="57084"> </A><A NAME="figcpp1"> </A> <IMG WIDTH=575 HEIGHT=203 ALIGN=BOTTOM ALT="figure56985" SRC="img2597.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img2597.gif" ><BR>
<STRONG>Figure:</STRONG> Memory Layout of C++ Variables and Pointers<BR>
<P>
<P>
An assignment statement such as
<PRE>i = j;</PRE>
takes the value (<I>r</I>-value) of <tt>j</tt>,
in this case 31,
and stores it in memory at the address (<I>l</I>-value) of <tt>i</tt>,
in this case 1000.
Similarly, the assignment
<PRE>p = q;</PRE>
takes the value of <tt>q</tt>,
in this case 1004,
and stores it in memory at the address of <tt>p</tt>,
in this case 1008.
<P>
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