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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!-- but may not be published or retransmitted without explicit permission.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/ternprec.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 3.17</title><link href="qcolonlhs.html" rev=precedes><link href="unswarn.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="qcolonlhs.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="unswarn.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 3.17</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I havesome codecontaining expressionslike<pre>a ? b = c : d</pre>and some compilers are accepting it but some are not.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>In the original definition ofthe language,<TT>=</TT> was of lower precedencethan <TT>?:</TT>,so early compilers tended to trip up on an expression likethe one above,attempting to parse it asif it had been written<pre> (a ? b) = (c : d)</pre>Since it has no other sensible meaning, however,later compilers have allowedthe expression,and interpret itas ifan inner set ofparentheses were implied:<pre> a ? (b = c) : d</pre>Here,the left-hand operand of the <TT>=</TT> is simply <TT>b</TT>,not the invalid <TT>a ? b</TT>.In fact,the grammar specified in the ANSI/ISO C Standardeffectively requires this interpretation.(The grammar in the Standardis not precedence-based,and says that any expression may appear betweenthe <TT>?</TT> and <TT>:</TT> symbols.)</p><p>An expression like the one in the question isperfectly acceptable to an ANSI compiler,but if you ever have to compile it under an older compiler,you can always add the explicit, inner parentheses.</p><p>References:K&R1 Sec. 2.12 p. 49<br>ISO Sec. 6.3.15<br>Rationale Sec. 3.3.15<br></p><!-- aend --><p><hr><a href="qcolonlhs.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="unswarn.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/ternprec.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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