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

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<a name="Temporaries"></a>
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<hr>
<a name="Temporaries-May-Vanish-Before-You-Expect"></a>
<h4 class="subsection">13.7.3 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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