📄 mutrefstructs.html
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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!-- but may not be published or retransmitted without explicit permission.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/mutrefstructs.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 1.15</title><link href="selfrefstruct.html" rev=precedes><link href="structtypdf.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="selfrefstruct.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="structtypdf.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 1.15</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How can I define a pair of mutually referential structures?I tried<pre> typedef struct { int afield; BPTR bpointer; } *APTR; typedef struct { int bfield; APTR apointer; } *BPTR;</pre>but the compiler doesn't know about<TT>BPTR</TT>when it is used inthe first structure declaration.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>As in question <a href="selfrefstruct.html">1.14</a>,the problem lies not inthe structures orthe pointers but the typedefs.First,give the two structures tags,anddefinethe link pointerswithout using typedefs:<pre> struct a { int afield; struct b *bpointer; }; struct b { int bfield; struct a *apointer; };</pre>The compiler can accept the field declaration <TT>struct b *bpointer</TT>within <TT>struct a</TT>,even though it has not yet heard of<TT>struct b</TT>.(<TT>struct b</TT> is``incomplete''at that point.)Occasionally it is necessary to precede this couplet with theempty declaration<pre> struct b;</pre>to mask the declarations (if in an inner scope) from a different<TT>struct b</TT> in an outer scope.</p><p>After declaring the two structures using struct tags,you can then declare the typedefs separately:<pre> typedef struct a *APTR; typedef struct b *BPTR;</pre></p><p>Alternatively,you can define the typedefsbefore the struct definitions<a href="jst2.html" rel=subdocument>[footnote]</a>,in which case you can use them when declaring the link pointer fields:<pre> typedef struct a *APTR; typedef struct b *BPTR; struct a { int afield; BPTR bpointer; }; struct b { int bfield; APTR apointer; };</pre></p><p>See also question <a href="selfrefstruct.html">1.14</a>.</p><p>References:K&R2 Sec. 6.5 p. 140<br>ISO Sec. 6.5.2.3<br>H&S Sec. 5.6.1 p. 132<br></p><!-- aend --><p><hr><a href="selfrefstruct.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="structtypdf.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/mutrefstructs.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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