124 lines
6.2 KiB
HTML
124 lines
6.2 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1987-2016 Free Software Foundation, Inc.
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
|
any later version published by the Free Software Foundation. A copy of
|
|
the license is included in the
|
|
section entitled "GNU Free Documentation License".
|
|
|
|
This manual contains no Invariant Sections. The Front-Cover Texts are
|
|
(a) (see below), and the Back-Cover Texts are (b) (see below).
|
|
|
|
(a) The FSF's Front-Cover Text is:
|
|
|
|
A GNU Manual
|
|
|
|
(b) The FSF's Back-Cover Text is:
|
|
|
|
You have freedom to copy and modify this GNU Manual, like GNU
|
|
software. Copies published by the Free Software Foundation raise
|
|
funds for GNU development. -->
|
|
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>The C Preprocessor: Alternatives to Wrapper #ifndef</title>
|
|
|
|
<meta name="description" content="The C Preprocessor: Alternatives to Wrapper #ifndef">
|
|
<meta name="keywords" content="The C Preprocessor: Alternatives to Wrapper #ifndef">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content="makeinfo">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<link href="index.html#Top" rel="start" title="Top">
|
|
<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="Header-Files.html#Header-Files" rel="up" title="Header Files">
|
|
<link href="Computed-Includes.html#Computed-Includes" rel="next" title="Computed Includes">
|
|
<link href="Once_002dOnly-Headers.html#Once_002dOnly-Headers" rel="prev" title="Once-Only Headers">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.indentedblock {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
|
|
div.smalllisp {margin-left: 3.2em}
|
|
kbd {font-style:oblique}
|
|
pre.display {font-family: inherit}
|
|
pre.format {font-family: inherit}
|
|
pre.menu-comment {font-family: serif}
|
|
pre.menu-preformatted {font-family: serif}
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
|
pre.smallexample {font-size: smaller}
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
|
pre.smalllisp {font-size: smaller}
|
|
span.nocodebreak {white-space:nowrap}
|
|
span.nolinebreak {white-space:nowrap}
|
|
span.roman {font-family:serif; font-weight:normal}
|
|
span.sansserif {font-family:sans-serif; font-weight:normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
|
|
<a name="Alternatives-to-Wrapper-_0023ifndef"></a>
|
|
<div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
<hr>
|
|
<a name="Alternatives-to-Wrapper-_0023ifndef-1"></a>
|
|
<h3 class="section">2.5 Alternatives to Wrapper #ifndef</h3>
|
|
|
|
<p>CPP supports two more ways of indicating that a header file should be
|
|
read only once. Neither one is as portable as a wrapper ‘<samp>#ifndef</samp>’
|
|
and we recommend you do not use them in new programs, with the caveat
|
|
that ‘<samp>#import</samp>’ is standard practice in Objective-C.
|
|
</p>
|
|
<a name="index-_0023import"></a>
|
|
<p>CPP supports a variant of ‘<samp>#include</samp>’ called ‘<samp>#import</samp>’ which
|
|
includes a file, but does so at most once. If you use ‘<samp>#import</samp>’
|
|
instead of ‘<samp>#include</samp>’, then you don’t need the conditionals
|
|
inside the header file to prevent multiple inclusion of the contents.
|
|
‘<samp>#import</samp>’ is standard in Objective-C, but is considered a
|
|
deprecated extension in C and C++.
|
|
</p>
|
|
<p>‘<samp>#import</samp>’ is not a well designed feature. It requires the users of
|
|
a header file to know that it should only be included once. It is much
|
|
better for the header file’s implementor to write the file so that users
|
|
don’t need to know this. Using a wrapper ‘<samp>#ifndef</samp>’ accomplishes
|
|
this goal.
|
|
</p>
|
|
<p>In the present implementation, a single use of ‘<samp>#import</samp>’ will
|
|
prevent the file from ever being read again, by either ‘<samp>#import</samp>’ or
|
|
‘<samp>#include</samp>’. You should not rely on this; do not use both
|
|
‘<samp>#import</samp>’ and ‘<samp>#include</samp>’ to refer to the same header file.
|
|
</p>
|
|
<p>Another way to prevent a header file from being included more than once
|
|
is with the ‘<samp>#pragma once</samp>’ directive. If ‘<samp>#pragma once</samp>’ is
|
|
seen when scanning a header file, that file will never be read again, no
|
|
matter what.
|
|
</p>
|
|
<p>‘<samp>#pragma once</samp>’ does not have the problems that ‘<samp>#import</samp>’ does,
|
|
but it is not recognized by all preprocessors, so you cannot rely on it
|
|
in a portable program.
|
|
</p>
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
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>
|
|
</div>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|