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141 lines
6.2 KiB
HTML
141 lines
6.2 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Temporaries (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Temporaries (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Temporaries (Using the GNU Compiler Collection (GCC))">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings" rel="up" title="C++ Misunderstandings">
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<link href="Copy-Assignment.html#Copy-Assignment" rel="next" title="Copy Assignment">
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<link href="Name-lookup.html#Name-lookup" rel="prev" title="Name lookup">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Temporaries"></a>
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<div class="header">
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<p>
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Next: <a href="Copy-Assignment.html#Copy-Assignment" accesskey="n" rel="next">Copy Assignment</a>, Previous: <a href="Name-lookup.html#Name-lookup" accesskey="p" rel="prev">Name lookup</a>, Up: <a href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings" accesskey="u" rel="up">C++ Misunderstandings</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Temporaries-May-Vanish-Before-You-Expect"></a>
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<h4 class="subsection">13.7.3 Temporaries May Vanish Before You Expect</h4>
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<a name="index-temporaries_002c-lifetime-of"></a>
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<a name="index-portions-of-temporary-objects_002c-pointers-to"></a>
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<p>It is dangerous to use pointers or references to <em>portions</em> of a
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temporary object. The compiler may very well delete the object before
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you expect it to, leaving a pointer to garbage. The most common place
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where this problem crops up is in classes like string classes,
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especially ones that define a conversion function to type <code>char *</code>
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or <code>const char *</code>—which is one reason why the standard
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<code>string</code> class requires you to call the <code>c_str</code> member
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function. However, any class that returns a pointer to some internal
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structure is potentially subject to this problem.
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</p>
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<p>For example, a program may use a function <code>strfunc</code> that returns
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<code>string</code> objects, and another function <code>charfunc</code> that
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operates on pointers to <code>char</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">string strfunc ();
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void charfunc (const char *);
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void
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f ()
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{
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const char *p = strfunc().c_str();
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…
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charfunc (p);
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…
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charfunc (p);
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}
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</pre></div>
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<p>In this situation, it may seem reasonable to save a pointer to the C
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string returned by the <code>c_str</code> member function and use that rather
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than call <code>c_str</code> repeatedly. However, the temporary string
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created by the call to <code>strfunc</code> is destroyed after <code>p</code> is
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initialized, at which point <code>p</code> is left pointing to freed memory.
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</p>
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<p>Code like this may run successfully under some other compilers,
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particularly obsolete cfront-based compilers that delete temporaries
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along with normal local variables. However, the GNU C++ behavior is
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standard-conforming, so if your program depends on late destruction of
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temporaries it is not portable.
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</p>
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<p>The safe way to write such code is to give the temporary a name, which
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forces it to remain until the end of the scope of the name. For
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example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">const string& tmp = strfunc ();
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charfunc (tmp.c_str ());
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Copy-Assignment.html#Copy-Assignment" accesskey="n" rel="next">Copy Assignment</a>, Previous: <a href="Name-lookup.html#Name-lookup" accesskey="p" rel="prev">Name lookup</a>, Up: <a href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings" accesskey="u" rel="up">C++ Misunderstandings</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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