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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!-- but may not be published or retransmitted without explicit permission.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 null.sgml dated Wed Dec 21 13:03:25 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/null/ptrtest.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:48 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 5.3</title><link href="null2.html" rev=precedes><link href="macro.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="null2.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="macro.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 5.3</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Is the abbreviated pointer comparison ``<TT>if(p)</TT>'' to test fornon-null pointers valid?What if the internal representation for null pointers is nonzero?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>It is always valid.</p><p>When C requires the Booleanvalue of an expression,a false value isinferredwhen the expression comparesequal to zero, and a true value otherwise.That is, wheneverone writes<pre> if(expr)</pre>where``expr''is any expression at all, the compileressentially acts as if it had been written as<pre> if((expr) != 0)</pre>Substituting the trivial pointer expression``p''for``expr'',we have<pre> if(p) is equivalent to if(p != 0)</pre>and this is a comparison context, so the compiler can tellthat the (implicit) <TT>0</TT> is actually a null pointer constant,anduse the correct null pointer value.There is no trickery involved here;compilers do work this way,and generate identical code for both constructs.The internal representation of a null pointer does<em>not</em>matter.</p><p>The boolean negation operator, <TT>!</TT>, can be describedas follows:<pre> !expr is essentially equivalent to (expr)?0:1 or to ((expr) == 0)</pre>which leads to the conclusion that<pre> if(!p) is equivalent to if(p == 0)</pre></p><p>``Abbreviations'' such as <TT>if(p)</TT>,though perfectly legal<a href="assertptr.html" rel=subdocument>[footnote]</a>,are considered by some to be bad style(and by others to be good style;see question <a href="../style/stylewars.html">17.10</a>).</p><p>See also question<a href="../bool/bool2.html">9.2</a>.</p><p>References:K&R2 Sec. A7.4.7 p. 204<br>ISO Sec. 6.3.3.3, Sec. 6.3.9, Sec. 6.3.13, Sec. 6.3.14, Sec. 6.3.15, Sec. 6.6.4.1, Sec. 6.6.5<br>H&S Sec. 5.3.2 p. 122<br></p><!-- aend --><p><hr><a href="null2.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="macro.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/null/ptrtest.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:48 GMT --></html>⌨️ 快捷键说明
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