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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:45 2005 by faqproc version 2.7 --><!-- from source file aryptr.sgml dated Wed Dec 21 12:50:38 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/aryptr/aryptrequiv.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:49 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 6.3</title><link href="aryptr2.html" rev=precedes><link href="aryptrparam.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="aryptr2.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="aryptrparam.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>·</font><!-- qtag -->Question 6.3</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>So what is meant by the ``equivalence of pointers and arrays'' in C?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Much of the confusion surroundingarrays andpointers in C can be traced toa misunderstanding ofthis statement.Saying that arrays and pointers are ``equivalent''meansneitherthat they areidenticalnoreveninterchangeable.What it means is that array and pointer arithmetic is definedsuch that a pointer can be conveniently usedto access an arrayor to simulate an array.In other words, as Wayne Throop has put it,it's``pointer <em>arithmetic</em> and array <em>indexing</em>[that]are equivalent in C, pointers and arrays are <em>different</em>.'')</p><p>Specifically,the cornerstone of the equivalence is thiskey definition:<blockquote>A reference to an objectof type array-of-Twhich appears in anexpressiondecays(with three exceptions)into a pointer to its first element;the type of the resultant pointer is pointer-to-T.</blockquote></p><p>That is,whenever an array appears in an expression,the compiler implicitly generates a pointer to the array'sfirst element,just as if the programmer had written <TT>&a[0]</TT>.(The exceptions are when the array is the operand ofa <TT>sizeof</TT>or<TT>&</TT>operator,or is a string literal initializer for a characterarray.<a href="stringw.html" rel=subdocument>[footnote]</a>See questions<a href="arraynels.html">6.23</a>,<a href="aryvsadr.html">6.12</a>,and<a href="../decl/strlitinit.html">1.32</a>,respectively.)</p><p>As a consequence ofthis definition,and in spite of the fact thatthe underlying arrays and pointers are quite different,the compiler doesn't apply the array subscripting operator<TT>[]</TT>that differentlyto arrays and pointers,after all.<a href="fn28.html" rel=subdocument>[footnote]</a>Given an array <TT>a</TT> and pointer <TT>p</TT>,an expression of the form <TT>a[i]</TT>causes the array to decayinto a pointer,following the rule above,andthen to besubscripted just as would be a pointer variable inthe expression <TT>p[i]</TT>(although the eventual memory accesses will be different,as explained in question<a href="aryptr2.html">6.2</a>).If you were to assign the array's address to the pointer:<pre> p = a;</pre>then <TT>p[3]</TT> and <TT>a[3]</TT> would access the same element.</p><p>Thisharmonyof access explains how pointers can access arrays,serve in their stead as function parameters(see question <a href="aryptrparam.html">6.4</a>),and simulate dynamic arrays(see question <a href="dynarray.html">6.14</a>).</p><p>See also questions <a href="practdiff.html">6.8</a>, <a href="ptrkindofary.html">6.10</a>, and <a href="dynarray.html">6.14</a>.</p><p>References:K&R1 Sec. 5.3 pp. 93-6<br>K&R2 Sec. 5.3 p. 99<br>ISO Sec. 6.2.2.1, Sec. 6.3.2.1, Sec. 6.3.6<br>H&S Sec. 5.4.1 p. 124<br></p><!-- aend --><p><hr><a href="aryptr2.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="aryptrparam.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img 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