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 <a name="Temporaries"></a>
 <div class="header">
 <p>
 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>
 </div>
 <hr>
 <a name="Temporaries-May-Vanish-Before-You-Expect"></a>
 <h4 class="subsection">11.7.3 Temporaries May Vanish Before You Expect</h4>

 <a name="index-temporaries_002c-lifetime-of"></a>
 <a name="index-portions-of-temporary-objects_002c-pointers-to"></a>
 <p>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>&mdash;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>
 <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>:
 </p>
 <div class="smallexample">
 <pre class="smallexample">string strfunc ();
 void charfunc (const char *);

 void
 f ()
 {
   const char *p = strfunc().c_str();
   &hellip;
   charfunc (p);
   &hellip;
   charfunc (p);
 }
 </pre></div>

 <p>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>
 <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>
 <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:
 </p>
 <div class="smallexample">
 <pre class="smallexample">const string&amp; tmp = strfunc ();
 charfunc (tmp.c_str ());
 </pre></div>

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	<a name="Temporaries"></a>
	<div class="header">
	<p>
	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>
	</div>
	<hr>
	<a name="Temporaries-May-Vanish-Before-You-Expect"></a>
	<h4 class="subsection">11.7.3 Temporaries May Vanish Before You Expect</h4>

	<a name="index-temporaries_002c-lifetime-of"></a>
	<a name="index-portions-of-temporary-objects_002c-pointers-to"></a>
	<p>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>
	<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>:
	</p>
	<div class="smallexample">
	<pre class="smallexample">string strfunc ();
	void charfunc (const char *);

	void
	f ()
	{
	const char *p = strfunc().c_str();
	…
	charfunc (p);
	…
	charfunc (p);
	}
	</pre></div>

	<p>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>
	<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>
	<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:
	</p>
	<div class="smallexample">
	<pre class="smallexample">const string& tmp = strfunc ();
	charfunc (tmp.c_str ());
	</pre></div>

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