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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!-- but may not be published or retransmitted without explicit permission.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 decl.sgml dated Wed Dec 21 12:56:18 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/decl/constparm.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:44 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 1.20b</title><link href="constptrconst.html" rev=precedes><link href="cdecl1.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="constptrconst.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="cdecl1.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 1.20b</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>What does it mean for a function parameter to be <TT>const</TT>?What do the two <TT>const</TT>'s in<pre> int f(const * const p)</pre>mean?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>In<pre> int f(const * const p)</pre>the first of the two <TT>const</TT>'sis perfectly appropriate and quite useful; manyfunctions declare parameters which are pointers to const data,and doing so documents (and tends to enforce) the function'spromise that it won't modify the pointed-to data in the caller.The second <TT>const</TT>, on the other hand, is almost useless; all itsays is that the function won't alter its own copy of thepointer, even though it wouldn't cause the caller <em>or</em> thefunction any problems if it did, nor is this anything the callershould care about in any case.The situation is the same as if afunction declared an ordinary (non-pointer) parameter as const:<pre> int f2(const int x)</pre>This says that nowhere in the body of <TT>f2()</TT> will the functionassign a different value to <TT>x</TT>.(Compilers should try to enforce this promise, too.)But assigning adifferent value to <TT>x</TT> wouldn't affect the value that the callerhad passed (because C always uses call-by-value), so it's anunimportant guarantee, and in fact a pretty useless one, becausewhat does the function gain by promising (to itself, since it'sthe only one that could care) whether it will or won't bemodifying in the passed-in copy of the value?</p><!-- aend --><p><hr><a href="constptrconst.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="cdecl1.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/decl/constparm.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:44 GMT --></html>⌨️ 快捷键说明
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