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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!-- but may not be published or retransmitted without explicit permission.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/evalorder2.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 3.2</title><link href="evalorder1.html" rev=precedes><link href="ieqiplusplus.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="evalorder1.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="ieqiplusplus.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 3.2</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Under my compiler, the code<pre>int i = 7;printf("%d\n", i++ * i++);</pre>prints 49.Regardless of the order of evaluation, shouldn't it print 56?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>It's true thatthe postincrement and postdecrement operators <TT>++</TT> and <TT>--</TT>performtheiroperations after yielding the former value.What's often misunderstood aretheimplicationsandprecise definitionofthe word``after.''It is<em>not</em>guaranteed thatan increment or decrementis performed immediately aftergiving up the previous value and before any other part of theexpression is evaluated.It is merely guaranteed that the update will be performedsometime before the expression is considered ``finished''(before the next``sequence point,'' in ANSI C's terminology;see question <a href="seqpoints.html">3.8</a>).In the example, the compiler chose to multiply the previous valueby itself and to perform both increments later.</p><p>The behavior of code which containsmultiple, ambiguous side effectshas always been undefined.(Loosely speaking, by``multiple, ambiguous side effects''we meanany combination ofincrement, decrement, and assignment operators(<TT>++</TT>, <TT>--</TT>, <TT>=</TT>, <TT>+=</TT>,<TT>-=</TT>, etc.)in a single expressionwhich causes the sameobjecteither to bemodified twiceormodified and then inspected.This is a rough definition;see question <a href="seqpoints.html">3.8</a> for a precise one,question <a href="confused.html">3.11</a> for a simpler one,and question<a href="../ansi/undef.html">11.33</a> for the meaning of ``undefined.'')Don't even try to find out how your compiler implements such things,let alone write code which depends on them(contrary to the ill-advised exercises in many C textbooks);asKernighan and Ritchiewisely point out,``if you don't know<em>how</em>they are done on various machines,that innocence may help to protect you.''</p><p>References:K&R1 Sec. 2.12 p. 50<br>K&R2 Sec. 2.12 p. 54<br>ISO Sec. 6.3<br>H&S Sec. 7.12 pp. 227-9<br>CT&P Sec. 3.7 p. 47<br>PCS Sec. 9.5 pp. 120-1<br></p><!-- aend --><p><hr><a href="evalorder1.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="ieqiplusplus.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/evalorder2.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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