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94 lines
4.9 KiB
HTML
94 lines
4.9 KiB
HTML
4 years ago
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<title>Alternatives to Wrapper #ifndef - The C Preprocessor</title>
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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="Header-Files.html#Header-Files" title="Header Files">
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<link rel="prev" href="Once_002dOnly-Headers.html#Once_002dOnly-Headers" title="Once-Only Headers">
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<link rel="next" href="Computed-Includes.html#Computed-Includes" title="Computed Includes">
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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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</head>
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<body>
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<div class="node">
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<a name="Alternatives-to-Wrapper-%23ifndef"></a>
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<a name="Alternatives-to-Wrapper-_0023ifndef"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Computed-Includes.html#Computed-Includes">Computed Includes</a>,
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Previous: <a rel="previous" accesskey="p" href="Once_002dOnly-Headers.html#Once_002dOnly-Headers">Once-Only Headers</a>,
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Up: <a rel="up" accesskey="u" href="Header-Files.html#Header-Files">Header Files</a>
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<hr>
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</div>
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<h3 class="section">2.5 Alternatives to Wrapper #ifndef</h3>
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<p>CPP supports two more ways of indicating that a header file should be
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read only once. Neither one is as portable as a wrapper ‘<samp><span class="samp">#ifndef</span></samp>’
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and we recommend you do not use them in new programs, with the caveat
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that ‘<samp><span class="samp">#import</span></samp>’ is standard practice in Objective-C.
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<p><a name="index-g_t_0023import-33"></a>CPP supports a variant of ‘<samp><span class="samp">#include</span></samp>’ called ‘<samp><span class="samp">#import</span></samp>’ which
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includes a file, but does so at most once. If you use ‘<samp><span class="samp">#import</span></samp>’
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instead of ‘<samp><span class="samp">#include</span></samp>’, then you don't need the conditionals
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inside the header file to prevent multiple inclusion of the contents.
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‘<samp><span class="samp">#import</span></samp>’ is standard in Objective-C, but is considered a
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deprecated extension in C and C++.
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<p>‘<samp><span class="samp">#import</span></samp>’ is not a well designed feature. It requires the users of
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a header file to know that it should only be included once. It is much
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better for the header file's implementor to write the file so that users
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don't need to know this. Using a wrapper ‘<samp><span class="samp">#ifndef</span></samp>’ accomplishes
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this goal.
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<p>In the present implementation, a single use of ‘<samp><span class="samp">#import</span></samp>’ will
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prevent the file from ever being read again, by either ‘<samp><span class="samp">#import</span></samp>’ or
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‘<samp><span class="samp">#include</span></samp>’. You should not rely on this; do not use both
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‘<samp><span class="samp">#import</span></samp>’ and ‘<samp><span class="samp">#include</span></samp>’ to refer to the same header file.
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<p>Another way to prevent a header file from being included more than once
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is with the ‘<samp><span class="samp">#pragma once</span></samp>’ directive. If ‘<samp><span class="samp">#pragma once</span></samp>’ is
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seen when scanning a header file, that file will never be read again, no
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matter what.
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<p>‘<samp><span class="samp">#pragma once</span></samp>’ does not have the problems that ‘<samp><span class="samp">#import</span></samp>’ does,
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but it is not recognized by all preprocessors, so you cannot rely on it
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in a portable program.
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</body></html>
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