| <html lang="en"> |
| <head> |
| <title>Temporaries - 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 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings" title="C++ Misunderstandings"> |
| <link rel="prev" href="Name-lookup.html#Name-lookup" title="Name lookup"> |
| <link rel="next" href="Copy-Assignment.html#Copy-Assignment" title="Copy Assignment"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, |
| 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, |
| 2008 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.2 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Funding Free Software'', the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| ``GNU Free Documentation License''. |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| 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 } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Temporaries"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Copy-Assignment.html#Copy-Assignment">Copy Assignment</a>, |
| Previous: <a rel="previous" accesskey="p" href="Name-lookup.html#Name-lookup">Name lookup</a>, |
| Up: <a rel="up" accesskey="u" href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings">C++ Misunderstandings</a> |
| <hr> |
| </div> |
| |
| <h4 class="subsection">11.8.3 Temporaries May Vanish Before You Expect</h4> |
| |
| <p><a name="index-temporaries_002c-lifetime-of-3240"></a><a name="index-portions-of-temporary-objects_002c-pointers-to-3241"></a>It is dangerous to use pointers or references to <em>portions</em> of a |
| temporary object. The compiler may very well delete the object before |
| you expect it to, leaving a pointer to garbage. The most common place |
| where 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> member |
| function. However, any class that returns a pointer to some internal |
| structure 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> that |
| operates 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 class="noindent">In this situation, it may seem reasonable to save a pointer to the C |
| string returned by the <code>c_str</code> member function and use that rather |
| than call <code>c_str</code> repeatedly. However, the temporary string |
| created by the call to <code>strfunc</code> is destroyed after <code>p</code> is |
| initialized, 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 temporaries |
| along with normal local variables. However, the GNU C++ behavior is |
| standard-conforming, so if your program depends on late destruction of |
| temporaries it is not portable. |
| |
| <p>The safe way to write such code is to give the temporary a name, which |
| forces it to remain until the end of the scope of the name. For |
| example: |
| |
| <pre class="smallexample"> const string& tmp = strfunc (); |
| charfunc (tmp.c_str ()); |
| </pre> |
| </body></html> |
| |
