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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!-- but may not be published or retransmitted without explicit permission.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 21:47:46 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/charize.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:54 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 10.21</title><link href="oldpaste.html" rev=precedes><link href="macstrexp2.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="oldpaste.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="macstrexp2.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 10.21</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I have an old macro<pre>#define CTRL(c) ('c' & 037)</pre>that doesn'tseem towork any more.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>The intended use of this macrois in code like<pre> tchars.t_eofc = CTRL(D);</pre>which is expected to expand to<pre> tchars.t_eofc = ('D' & 037);</pre>based on the assumptionthatthe actual value ofthe parameter <TT>c</TT> will be substitutedeven inside the single quotes of a character constant.Preprocessing was never supposed to work this way;it was somewhat of an accidentthat a <TT>CTRL()</TT> macro like this ever worked.</p><p>ANSI C defines a new ``stringizing'' operator(see question <a href="../ansi/stringize.html">11.17</a>),but there is no corresponding ``charizing'' operator.</p><p>The best solution to the problem is probably to move the single quotes from the definition to the invocation,by rewriting the macro as<pre> #define CTRL(c) ((c) & 037)</pre>and invoking it as<pre> CTRL('D')</pre>(Doing so also makes the macro ``syntactic'';see question <a href="slm.html">10.2</a>.)</p><p>It may also be possible to use the stringizing operatorand some indirection:<pre> #define CTRL(c) (*#c & 037)</pre>or<pre> #define CTRL(c) (#c[0] & 037)</pre>but neither of these would work as well as the original,since they wouldn't be valid in <TT>case</TT> labelsor as global variable initializers.(Global variable initializers and <TT>case</TT> labels require various flavors of constant expressions,and string literals and indirection aren't allowed.)</p><p>See also question <a href="../ansi/macstrexp.html">11.18</a>.</p><p>References:ISO Sec. 6.8.3<br>H&S Secs. 7.11.2,7.11.3 pp. 226-7<br></p><!-- aend --><p><hr><a href="oldpaste.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="macstrexp2.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/charize.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:54 GMT --></html>⌨️ 快捷键说明
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