142 lines
6.3 KiB
HTML
142 lines
6.3 KiB
HTML
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<head>
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<title>Using the GNU Compiler Collection (GCC): Temporaries</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Temporaries">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C_002b_002b-Misunderstandings.html#C_002b_002b-Misunderstandings" rel="up" title="C++ Misunderstandings">
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<link href="Copy-Assignment.html#Copy-Assignment" rel="next" title="Copy Assignment">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Temporaries"></a>
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<div class="header">
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<p>
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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>
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</div>
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<hr>
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<a name="Temporaries-May-Vanish-Before-You-Expect"></a>
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<h4 class="subsection">12.7.3 Temporaries May Vanish Before You Expect</h4>
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<a name="index-temporaries_002c-lifetime-of"></a>
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<a name="index-portions-of-temporary-objects_002c-pointers-to"></a>
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<p>It is dangerous to use pointers or references to <em>portions</em> of a
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temporary object. The compiler may very well delete the object before
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you expect it to, leaving a pointer to garbage. The most common place
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where this problem crops up is in classes like string classes,
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especially ones that define a conversion function to type <code>char *</code>
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or <code>const char *</code>—which is one reason why the standard
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<code>string</code> class requires you to call the <code>c_str</code> member
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function. However, any class that returns a pointer to some internal
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structure is potentially subject to this problem.
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</p>
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<p>For example, a program may use a function <code>strfunc</code> that returns
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<code>string</code> objects, and another function <code>charfunc</code> that
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operates on pointers to <code>char</code>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">string strfunc ();
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void charfunc (const char *);
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void
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f ()
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{
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const char *p = strfunc().c_str();
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…
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charfunc (p);
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…
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charfunc (p);
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}
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</pre></div>
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<p>In this situation, it may seem reasonable to save a pointer to the C
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string returned by the <code>c_str</code> member function and use that rather
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than call <code>c_str</code> repeatedly. However, the temporary string
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created by the call to <code>strfunc</code> is destroyed after <code>p</code> is
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initialized, at which point <code>p</code> is left pointing to freed memory.
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</p>
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<p>Code like this may run successfully under some other compilers,
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particularly obsolete cfront-based compilers that delete temporaries
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along with normal local variables. However, the GNU C++ behavior is
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standard-conforming, so if your program depends on late destruction of
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temporaries it is not portable.
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</p>
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<p>The safe way to write such code is to give the temporary a name, which
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forces it to remain until the end of the scope of the name. For
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example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">const string& tmp = strfunc ();
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charfunc (tmp.c_str ());
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</pre></div>
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<hr>
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<div class="header">
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<p>
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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>
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</div>
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</body>
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