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83 lines
3.5 KiB
HTML
83 lines
3.5 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Deleted Code - The C Preprocessor</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="The C Preprocessor">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Conditionals.html#Conditionals" title="Conditionals">
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<link rel="prev" href="Conditional-Syntax.html#Conditional-Syntax" title="Conditional Syntax">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1987-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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the license is included in the
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section entitled ``GNU Free Documentation License''.
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This manual contains no Invariant Sections. The Front-Cover Texts are
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(a) (see below), and the Back-Cover Texts are (b) (see below).
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development.
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<style type="text/css"><!--
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pre.display { font-family:inherit }
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pre.format { font-family:inherit }
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span.roman { font-family:serif; font-weight:normal; }
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span.sansserif { font-family:sans-serif; font-weight:normal; }
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Deleted-Code"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Conditional-Syntax.html#Conditional-Syntax">Conditional Syntax</a>,
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Up: <a rel="up" accesskey="u" href="Conditionals.html#Conditionals">Conditionals</a>
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<hr>
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</div>
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<h3 class="section">4.3 Deleted Code</h3>
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<p><a name="index-commenting-out-code-92"></a>
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If you replace or delete a part of the program but want to keep the old
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code around for future reference, you often cannot simply comment it
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out. Block comments do not nest, so the first comment inside the old
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code will end the commenting-out. The probable result is a flood of
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syntax errors.
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<p>One way to avoid this problem is to use an always-false conditional
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instead. For instance, put <code>#if 0</code> before the deleted code and
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<code>#endif</code> after it. This works even if the code being turned
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off contains conditionals, but they must be entire conditionals
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(balanced ‘<samp><span class="samp">#if</span></samp>’ and ‘<samp><span class="samp">#endif</span></samp>’).
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<p>Some people use <code>#ifdef notdef</code> instead. This is risky, because
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<code>notdef</code> might be accidentally defined as a macro, and then the
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conditional would succeed. <code>#if 0</code> can be counted on to fail.
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<p>Do not use <code>#if 0</code> for comments which are not C code. Use a real
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comment, instead. The interior of <code>#if 0</code> must consist of complete
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tokens; in particular, single-quote characters must balance. Comments
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often contain unbalanced single-quote characters (known in English as
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apostrophes). These confuse <code>#if 0</code>. They don't confuse
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‘<samp><span class="samp">/*</span></samp>’.
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</body></html>
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