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<html lang="en"><head><title>Using the GNU Compiler Collection (GCC)</title><meta http-equiv="Content-Type" content="text/html"><meta name="description" content="Using the GNU Compiler Collection (GCC)"><meta name="generator" content="makeinfo 4.6"><!--Copyright © 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc. <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below). A copy of the license isincluded in the section entitled "GNU Free Documentation License". <p>(a) The FSF's Front-Cover Text is: <p>A GNU Manual <p>(b) The FSF's Back-Cover Text is: <p>You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.--><meta http-equiv="Content-Style-Type" content="text/css"><style type="text/css"><!-- pre.display { font-family:inherit } pre.format { font-family:inherit } pre.smalldisplay { font-family:inherit; font-size:smaller } pre.smallformat { font-family:inherit; font-size:smaller } pre.smallexample { font-size:smaller } pre.smalllisp { font-size:smaller }--></style></head><body><div class="node"><p>Node: <a name="Temporaries">Temporaries</a>,Next: <a rel="next" accesskey="n" href="Copy-Assignment.html#Copy%20Assignment">Copy Assignment</a>,Previous: <a rel="previous" accesskey="p" href="Name-lookup.html#Name%20lookup">Name lookup</a>,Up: <a rel="up" accesskey="u" href="C---Misunderstandings.html#C++%20Misunderstandings">C++ Misunderstandings</a><hr><br></div><h3 class="subsection">Temporaries May Vanish Before You Expect</h4><p>It is dangerous to use pointers or references to <em>portions</em> of atemporary object. The compiler may very well delete the object beforeyou expect it to, leaving a pointer to garbage. The most common placewhere this problem crops up is in classes like string classes,especially ones that define a conversion function to type <code>char *</code>or <code>const char *</code>--which is one reason why the standard<code>string</code> class requires you to call the <code>c_str</code> memberfunction. However, any class that returns a pointer to some internalstructure is potentially subject to this problem. <p>For example, a program may use a function <code>strfunc</code> that returns<code>string</code> objects, and another function <code>charfunc</code> thatoperates on pointers to <code>char</code>:<pre class="smallexample"> string strfunc (); void charfunc (const char *); void f () { const char *p = strfunc().c_str(); ... charfunc (p); ... charfunc (p); } </pre><p>In this situation, it may seem reasonable to save a pointer to the Cstring returned by the <code>c_str</code> member function and use that ratherthan call <code>c_str</code> repeatedly. However, the temporary stringcreated by the call to <code>strfunc</code> is destroyed after <code>p</code> isinitialized, at which point <code>p</code> is left pointing to freed memory. <p>Code like this may run successfully under some other compilers,particularly obsolete cfront-based compilers that delete temporariesalong with normal local variables. However, the GNU C++ behavior isstandard-conforming, so if your program depends on late destruction oftemporaries it is not portable. <p>The safe way to write such code is to give the temporary a name, whichforces it to remain until the end of the scope of the name. Forexample:<pre class="smallexample"> string& tmp = strfunc (); charfunc (tmp.c_str ()); </pre> </body></html>⌨️ 快捷键说明
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