📄 confused.html
字号:
<!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!-- but may not be published or retransmitted without explicit permission.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/confused.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 3.11</title><link href="expec0.html" rev=precedes><link href="prevspost.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="expec0.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="prevspost.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 3.11</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How can I avoid these undefined evaluation order difficultiesif I don't feel like learning the complicated rules?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>The easiest answeris that if you steer clear of expressionswhich don't havereasonablyobviousinterpretations,for the most part you'll steer clear ofthe undefined ones,too.(Of course,``reasonably obvious''means different things to different people.This answer works as long as you agreethat<TT>a[i] = i++</TT> and <TT>i = i++</TT>are not ``reasonably obvious.'')</p><p>To be a bit more precise,here are somesimpler rules which,though slightly more conservative thanthe ones in the Standard,willhelp tomake sure that your code is``reasonably obvious''andequally understandable to both the compiler <em>and</em> your fellow programmers:<OL><li>Make sure that eachexpressionmodifies at most one object.By ``object'' we mean eithera simple variable,ora cell of an array,orthelocationpointed to by a pointer (e.g. <TT>*p</TT>).A ``modification'' is eithersimple assignment with the <TT>=</TT> operator,ora compound assignmentwith an operator like <TT>+=</TT>, <TT>-=</TT>, or <TT>*=</TT>,or an increment or decrement with <TT>++</TT> or <TT>--</TT>(in either pre or post forms).<li>If an object(as defined above)appears more than once in an expression,and is the object modified in the expression,make sure that<em>all</em> appearances of the objectwhich fetch its valueparticipate in the computation of the new value which is stored.This rule allows the expression<pre> i = i + 1</pre>because although the object <TT>i</TT> appears twice and is modified,the appearance (on the right-hand side)which fetches <TT>i</TT>'s old valueis used to compute <TT>i</TT>'s new value.<li>If you want to break rule 1,make sure that the several objects being modified are distinctly different,and try to limit yourself to two or at most three modifications,and of a style matching those of the following examples.(Also,make sure that you continue to follow rule 2for each object modified.)The expression<pre> c = *p++</pre>is allowed under this rule,because the two objects modified (<TT>c</TT> and <TT>p</TT>)are distinct.The expression<pre> *p++ = c</pre>is also allowed,because <TT>p</TT> and <TT>*p</TT>(i.e. <TT>p</TT> itself and what it points to)are both modified but are almost certainly distinct.Similarly, both<pre> c = a[i++]and a[i++] = c</pre>are allowed,because <TT>c</TT>, <TT>i</TT>, and <TT>a[i]</TT> arepresumablyall distinct.Finally, expressions like<pre> *p++ = *q++</pre>and<pre> a[i++] = b[j++]</pre>in which <em>three</em> things are modified(<TT>p</TT>, <TT>q</TT>, and <TT>*p</TT> in the first expression,and <TT>i</TT>, <TT>j</TT>, and <TT>a[i]</TT> in the second),are allowed <em>if</em> all three objects are distinct,i.e. only if two <em>different</em> pointers <TT>p</TT> and <TT>q</TT>or two <em>different</em> array indices <TT>i</TT> and <TT>j</TT>are used.<li>You may also break rule 1 or 2as long as you interposea defined sequence point operatorbetween the two modifications,or between the modification and the access.The expression<pre> (c = getchar()) != EOF && c != '\n'</pre>(commonly seen in a <TT>while</TT> loop whilereading a line)is legal becausethe second access of the variable <TT>c</TT>occurs after the sequence point implied by <TT>&&</TT>.(Without the sequence point,the expression would be illegalbecause the access of <TT>c</TT> while comparing it to <TT>'\n'</TT>on the rightdoesnot ``determine the value to be stored''on the left.)</OL></p><!-- aend --><p><hr><a href="expec0.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="prevspost.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/confused.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:47 GMT --></html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -