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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!-- but may not be published or retransmitted without explicit permission.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 Sun Jun 4 12:01:53 2006 by faqproc version 2.7 --><!-- from source file aryptr.sgml dated Sun Jun 4 12:01:44 2006 --><!-- corresponding to FAQ list version 4.1 --><html><!-- Mirrored from c-faq.com/aryptr/aryptr2.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 6.2</title><link href="aryptr1.html" rev=precedes><link href="aryptrequiv.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="aryptr1.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="aryptrequiv.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 6.2</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>But I heard that <TT>char a[]</TT> was identical to <TT>char *a</TT>.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Not at all.(What you heard has to do with formal parameters to functions;see question<a href="aryptrparam.html">6.4</a>.)Arrays are not pointers,though they are closely related(see question<a href="aryptrequiv.html">6.3</a>)and can be used similarly(see questions<a href="../ptrs/goodfor.html">4.1</a>,<a href="practdiff.html">6.8</a>,<a href="ptrkindofary.html">6.10</a>,and<a href="dynarray.html">6.14</a>).</p><p>The array declaration<TT>char a[6]</TT>requests that space for six characters be set aside,to be known by the name ``<TT>a</TT>''.That is,there is a location named ``<TT>a</TT>''at which six characters can sit.The pointer declaration<TT>char *p</TT>,on the other hand,requests a place which holds a pointer,to be known by the name ``<TT>p</TT>''.Thispointer can point almost anywhere:to any <TT>char</TT>,or to any contiguous array of <TT>char</TT>s,or nowhere<a href="anywhere.html" rel=subdocument>[footnote]</a>(see also questions <a href="../null/null1.html">5.1</a>and<a href="../decl/initval.html">1.30</a>).</p><p>As usual, a picture is worth a thousand words.The declarations<pre> char a[] = "hello"; char *p = "world";</pre>would initialize data structures which could be represented like this:<p><img src="aryptrstring.gif"><br></p><p>It is useful to realizethat areference like <I fallback=no>x</I><TT>[3]</TT>generates differentcode depending on whether <I fallback=no>x</I> is an array or a pointer.Given the declarations above,when the compiler sees the expression <TT>a[3]</TT>,it emits code to startat the location ``<TT>a</TT>'',move three past it,and fetch the character there.When itsees the expression <TT>p[3]</TT>,it emits code to start at the location ``<TT>p</TT>'',fetch the pointer value there,add three to the pointer,and finally fetch the character pointed to.In other words,<TT>a[3]</TT> is three places past(the start of)the object <em>named</em> <TT>a</TT>,while <TT>p[3]</TT> is three places past the object<em>pointed to</em> by<TT>p</TT>.In the example above,both <TT>a[3]</TT> and <TT>p[3]</TT>happen to be the character 'l',but the compiler gets there differently.(The essential difference is thatthe values ofan array like <TT>a</TT>and a pointer like <TT>p</TT>are computed differently <em>whenever</em> they appear in expressions,whether or not they are being subscripted,as explained further inquestion <a href="aryptrequiv.html">6.3</a>.)See also question<a href="../decl/strlitinit.html">1.32</a>.</p><p>References:K&R2 Sec. 5.5 p. 104<br>CT&P Sec. 4.5 pp. 64-5<br></p><!-- aend --><p><hr><a href="aryptr1.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="aryptrequiv.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 src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a> <a href="../eskimo.html">about eskimo</a> <a href="../search.html">search</a> <a href="../feedback.html">feedback</a> <a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/aryptr/aryptr2.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:49 GMT --></html>⌨️ 快捷键说明
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