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<h4 class="subsection">12.7.3 Temporaries May Vanish Before You Expect</h4>
<p><a name="index-temporaries_002c-lifetime-of-4285"></a><a name="index-portions-of-temporary-objects_002c-pointers-to-4286"></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>
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