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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!-- but may not be published or retransmitted without explicit permission.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 pointers.sgml dated Wed Dec 21 13:07:10 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/ptrs/genericpp.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 4.9</title><link href="passptrinit.html" rev=precedes><link href="refconst.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="passptrinit.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="refconst.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 4.9</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Suppose I want to write a function that takes a generic pointeras an argument and I want to simulate passing it by reference.Can I give the formal parameter type <TT>void **</TT>,and do something like this?<pre> void f(void **); double *dp; f((void **)&dp);</pre></p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Not portably.Code like this may work and is sometimes recommended,but itrelies onall pointer types having the same internal representation(which is common, but not universal;see question <a href="../null/machexamp.html">5.17</a>).</p><p>There is no generic pointer-to-pointer type in C.<TT>void *</TT> acts as a generic pointeronly becauseconversions(if necessary)are applied automatically when other pointer typesare assigned to and from <TT>void *</TT>'s;these conversions cannot be performedif an attempt is madeto indirect upon a <TT>void **</TT> valuewhich points ata pointer typeother than <TT>void *</TT>.When you make use of a <TT>void **</TT> pointer value(for instance, when you use the <TT>*</TT> operatorto access the <TT>void *</TT> valueto which the <TT>void **</TT> points),the compiler has no way of knowingwhether that <TT>void *</TT> valuewas once converted from some other pointer type.It must assume that it is nothing more than a <TT>void *</TT>;it cannot perform any implicit conversions.</p><p>In other words,any <TT>void **</TT> value you play withmust be the address of an actual <TT>void *</TT> value somewhere;casts like <TT>(void **)&dp</TT>,though they may shut the compiler up,arenonportable(and may not even do what you want;see also question <a href="../lib/qsort2.html">13.9</a>).If the pointer that the <TT>void **</TT> points tois not a <TT>void *</TT>,and ifithas a different size or representation than a <TT>void *</TT>,thenthe compilerisn't going to be able to access it correctly.</p><p>To make the code fragment above work,you'd have to use an intermediate <TT>void *</TT> variable:<pre> double *dp; void *vp = dp; f(&vp); dp = vp;</pre>The assignments to and from <TT>vp</TT>give the compiler the opportunity to perform any conversions,if necessary.</p><p>Again,the discussion so far assumes that different pointer typesmight have different sizes or representations,which is rare today,but not unheard of.Toappreciate theproblem with <TT>void **</TT> more clearly,compare the situation to an analogous one involving, say,types <TT>int</TT> and <TT>double</TT>,which probably have different sizesand certainly have different representations.If we have a function<pre> void incme(double *p) { *p += 1; }</pre>then we can do something like<pre> int i = 1; double d = i; incme(&d); i = d;</pre>and <TT>i</TT> will be incremented by 1.(This is analogous to the correct <TT>void **</TT> code involving the auxiliary <TT>vp</TT>.)If, on the other hand,we were to attempt something like<pre> int i = 1; incme((double *)&i); /* WRONG */</pre>(this code is analogous to the fragment in the question),it would be highly unlikely to work.</p><!-- aend --><p><hr><a href="passptrinit.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="refconst.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/ptrs/genericpp.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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