📄 safemacros.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!-- but may not be published or retransmitted without explicit permission.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 21:47:46 2005 by faqproc version 2.7 --><!-- from source file cpp.sgml dated Wed Dec 21 13:52:14 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/cpp/safemacros.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:53 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 10.1</title><link href="../bool/thirdparty.html" rev=precedes><link href="slm.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="../bool/thirdparty.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="slm.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 10.1</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I'm trying to definea few simple little function-like macrossuch as<pre> #define square(x) x * x</pre>but they're not always working.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>There are three important rules to remember when definingfunction-like macros:<OL><li>The macro expansion must always be parenthesizedto protectany lower-precedence operatorsfrom the surrounding expression.Given the(incorrect)<TT>square()</TT> macro above,the invocation<pre> 1 / square(n)</pre>would expand to<pre> 1 / n * n</pre>(which evaluates as <TT>(1 / n) * n</TT>),whilewhat you want is<pre> 1 / (n * n)</pre>(In this case, the problem is one of associativity rather than precedence, but the effect is the same.)<li>Within the macro definition,alloccurrencesof the parametersmust be parenthesizedto protectany low-precedence operatorsin the actual argumentsfrom therest of the macro expansion.Againgiven the <TT>square()</TT> macro above,the invocation<pre> square(n + 1)</pre>would expand to<pre> n + 1 * n + 1</pre>But what you want is<pre> (n + 1) * (n + 1)</pre><li>If a parameter appears several timesin the expansion,the macro may not work properlyif the actual argumentis an expression withside effects.Yet againgiven the <TT>square()</TT> macro above,the invocation<pre> square(i++)</pre>would expand to<pre> i++ * i++</pre>which is undefined(see question <a href="../expr/evalorder2.html">3.2</a>).</OL></p><p>The proper definition of a <TT>square</TT> macro,to comply with rules 1 and 2 above,is<pre> #define square(x) ((x) * (x))</pre>Complying with rule 3 is harder.Sometimes,careful exploitation of the short-circuiting behavior ofthe<TT>&&</TT>, <TT>||</TT>, or <TT>?:</TT>operators(see question <a href="../expr/shortcircuit.html">3.6</a>)can arrange that a parameter which appears several timesis guaranteed to be evaluated exactly once.Sometimes, the macro isjustdocumented as being unsafe,and callers must remembernot to use it on arguments with side effects.Other times,it may be advisable <em>not</em> tocompose a function-like macroif it can't be made safe.</p><p>(As a stylistic convention,macros are often defined with capitalized or all-upper-case names,to make it obvious that they are macros.It may be acceptable to define a function-like macrowith an all-lower-case name,if it truly simulates a function,but only if it complies with all three rules above.Since the squaring macro we've been discussing does not,it should be defined as something like<pre> #define Square(x) ((x) * (x)) /* UNSAFE */</pre>if it is to be used at all.)</p><p>References:K&R1 Sec. 4.11 p. 87<br>K&R2 Sec. 4.11.2 p. 90<br>H&S Secs. 3.3.6,3.3.7 pp. 49-50<br>CT&P Sec. 6.2 pp. 78-80<br></p><!-- aend --><p><hr><a href="../bool/thirdparty.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="slm.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/cpp/safemacros.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:53 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 + -