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| 22 | <title>Deleted Code (GNU C Language Manual)</title>
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Conditionals.html" rel="up" title="Conditionals">
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| 57 | <body lang="en">
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| 58 | <span id="Deleted-Code"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Conditional-Syntax.html" accesskey="p" rel="prev">Conditional Syntax</a>, Up: <a href="Conditionals.html" accesskey="u" rel="up">Conditionals</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Deleted-Code-1"></span><h4 class="subsection">26.6.3 Deleted Code</h4>
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| 64 | <span id="index-commenting-out-code"></span>
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| 65 |
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| 66 | <p>If you replace or delete a part of the program but want to keep the
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| 67 | old code in the file for future reference, commenting it out is not so
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| 68 | straightforward in C. Block comments do not nest, so the first
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| 69 | comment inside the old code will end the commenting-out. The probable
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| 70 | result is a flood of syntax errors.
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| 71 | </p>
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| 72 | <p>One way to avoid this problem is to use an always-false conditional
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| 73 | instead. For instance, put <code>#if 0</code> before the deleted code and
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| 74 | <code>#endif</code> after it. This works even if the code being turned
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| 75 | off contains conditionals, but they must be entire conditionals
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| 76 | (balanced <code>#if</code> and <code>#endif</code>).
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| 77 | </p>
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| 78 | <p>Some people use <code>#ifdef notdef</code> instead. This is risky, because
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| 79 | <code>notdef</code> might be accidentally defined as a macro, and then the
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| 80 | conditional would succeed. <code>#if 0</code> can be counted on to fail.
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| 81 | </p>
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| 82 | <p>Do not use <code>#if 0</code> around text that is not C code. Use a real
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| 83 | comment, instead. The interior of <code>#if 0</code> must consist of complete
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| 84 | tokens; in particular, single-quote characters must balance. Comments
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| 85 | often contain unbalanced single-quote characters (known in English as
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| 86 | apostrophes). These confuse <code>#if 0</code>. They don’t confuse
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| 87 | ‘<samp>/*</samp>’.
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| 88 | </p>
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| 89 |
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| 90 |
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| 91 |
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| 92 | </body>
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| 93 | </html>
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