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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!-- but may not be published or retransmitted without explicit permission.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 expr.sgml dated Sat Jul 3 17:10:33 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/expr/evalorder4.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:47 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 3.9</title><link href="seqpoints.html" rev=precedes><link href="experiment.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="seqpoints.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="experiment.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 3.9</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Soif I write<pre>a[i] = i++;</pre>and I don't carewhich cell of <TT>a[]</TT> gets written to,the code is fine,and <TT>i</TT> getsincremented by one,right?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Not necessarily!For one thing,if you don't care which cell of <TT>a[]</TT> gets written to,why write code which seems to write to <TT>a[]</TT> at all?More significantly,oncean expression or program becomes undefined,<em>all</em> aspects of it become undefined.When an undefined expression has(apparently)two plausible interpretations,do not mislead yourself by imaginingthat the compiler will choose one or the other.The Standard does not require thata compiler make an obvious choice,and some compilers don't.In this case,not only do we not knowwhether <TT>a[i]</TT> or <TT>a[i+1]</TT> is written to,it ispossiblethat a completely unrelated cell of the array(or any random part of memory)is written to,and it is also not possible to predictwhat final value <TT>i</TT> will receive.See questions <a href="evalorder2.html">3.2</a>,<a href="ieqiplusplus.html">3.3</a>,<a href="../ansi/undef.html">11.33</a>,and<a href="../ansi/experiment.html">11.35</a>.</p><!-- aend --><p><hr><a href="seqpoints.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="experiment.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/expr/evalorder4.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:47 GMT --></html>⌨️ 快捷键说明
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