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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

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  25. <title>Temporaries (Using the GNU Compiler Collection (GCC))</title>

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  35. <link href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings" rel="up" title="C++ Misunderstandings">

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  70. <a name="Temporaries"></a>

  71. <div class="header">

  72. <p>

  73. 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> &nbsp; [<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>

  74. </div>

  75. <hr>

  76. <a name="Temporaries-May-Vanish-Before-You-Expect"></a>

  77. <h4 class="subsection">14.7.3 Temporaries May Vanish Before You Expect</h4>

  78. 

  79. <a name="index-temporaries_002c-lifetime-of"></a>

  80. <a name="index-portions-of-temporary-objects_002c-pointers-to"></a>

  81. <p>It is dangerous to use pointers or references to <em>portions</em> of a

  82. temporary object.  The compiler may very well delete the object before

  83. you expect it to, leaving a pointer to garbage.  The most common place

  84. where this problem crops up is in classes like string classes,

  85. especially ones that define a conversion function to type <code>char *</code>

  86. or <code>const char *</code>&mdash;which is one reason why the standard

  87. <code>string</code> class requires you to call the <code>c_str</code> member

  88. function.  However, any class that returns a pointer to some internal

  89. structure is potentially subject to this problem.

  90. </p>

  91. <p>For example, a program may use a function <code>strfunc</code> that returns

  92. <code>string</code> objects, and another function <code>charfunc</code> that

  93. operates on pointers to <code>char</code>:

  94. </p>

  95. <div class="smallexample">

  96. <pre class="smallexample">string strfunc ();

  97. void charfunc (const char *);

  98. 

  99. void

  100. f ()

  101. {

  102.   const char *p = strfunc().c_str();

  103.   &hellip;

  104.   charfunc (p);

  105.   &hellip;

  106.   charfunc (p);

  107. }

  108. </pre></div>

  109. 

  110. <p>In this situation, it may seem reasonable to save a pointer to the C

  111. string returned by the <code>c_str</code> member function and use that rather

  112. than call <code>c_str</code> repeatedly.  However, the temporary string

  113. created by the call to <code>strfunc</code> is destroyed after <code>p</code> is

  114. initialized, at which point <code>p</code> is left pointing to freed memory.

  115. </p>

  116. <p>Code like this may run successfully under some other compilers,

  117. particularly obsolete cfront-based compilers that delete temporaries

  118. along with normal local variables.  However, the GNU C++ behavior is

  119. standard-conforming, so if your program depends on late destruction of

  120. temporaries it is not portable.

  121. </p>

  122. <p>The safe way to write such code is to give the temporary a name, which

  123. forces it to remain until the end of the scope of the name.  For

  124. example:

  125. </p>

  126. <div class="smallexample">

  127. <pre class="smallexample">const string&amp; tmp = strfunc ();

  128. charfunc (tmp.c_str ());

  129. </pre></div>

  130. 

  131. <hr>

  132. <div class="header">

  133. <p>

  134. 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> &nbsp; [<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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  136. 

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