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123 lines
6.1 KiB
HTML
123 lines
6.1 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1987-2018 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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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Alternatives to Wrapper #ifndef (The C Preprocessor)</title>
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<meta name="description" content="Alternatives to Wrapper #ifndef (The C Preprocessor)">
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<meta name="keywords" content="Alternatives to Wrapper #ifndef (The C Preprocessor)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Header-Files.html#Header-Files" rel="up" title="Header Files">
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<link href="Computed-Includes.html#Computed-Includes" rel="next" title="Computed Includes">
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<link href="Once_002dOnly-Headers.html#Once_002dOnly-Headers" rel="prev" title="Once-Only Headers">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Alternatives-to-Wrapper-_0023ifndef"></a>
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<div class="header">
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<p>
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Next: <a href="Computed-Includes.html#Computed-Includes" accesskey="n" rel="next">Computed Includes</a>, Previous: <a href="Once_002dOnly-Headers.html#Once_002dOnly-Headers" accesskey="p" rel="prev">Once-Only Headers</a>, Up: <a href="Header-Files.html#Header-Files" accesskey="u" rel="up">Header Files</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Alternatives-to-Wrapper-_0023ifndef-1"></a>
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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>#ifndef</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>#import</samp>’ is standard practice in Objective-C.
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</p>
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<a name="index-_0023import"></a>
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<p>CPP supports a variant of ‘<samp>#include</samp>’ called ‘<samp>#import</samp>’ which
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includes a file, but does so at most once. If you use ‘<samp>#import</samp>’
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instead of ‘<samp>#include</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>#import</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>
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<p>‘<samp>#import</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>#ifndef</samp>’ accomplishes
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this goal.
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</p>
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<p>In the present implementation, a single use of ‘<samp>#import</samp>’ will
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prevent the file from ever being read again, by either ‘<samp>#import</samp>’ or
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‘<samp>#include</samp>’. You should not rely on this; do not use both
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‘<samp>#import</samp>’ and ‘<samp>#include</samp>’ to refer to the same header file.
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</p>
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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>#pragma once</samp>’ directive. If ‘<samp>#pragma once</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>
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<p>‘<samp>#pragma once</samp>’ does not have the problems that ‘<samp>#import</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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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Computed-Includes.html#Computed-Includes" accesskey="n" rel="next">Computed Includes</a>, Previous: <a href="Once_002dOnly-Headers.html#Once_002dOnly-Headers" accesskey="p" rel="prev">Once-Only Headers</a>, Up: <a href="Header-Files.html#Header-Files" accesskey="u" rel="up">Header Files</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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