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<title>The C Preprocessor: Alternatives to Wrapper #ifndef</title>

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<a name="Alternatives-to-Wrapper-_0023ifndef"></a>
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<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 &lsquo;<samp>#ifndef</samp>&rsquo;
and we recommend you do not use them in new programs, with the caveat
that &lsquo;<samp>#import</samp>&rsquo; is standard practice in Objective-C.
</p>
<a name="index-_0023import"></a>
<p>CPP supports a variant of &lsquo;<samp>#include</samp>&rsquo; called &lsquo;<samp>#import</samp>&rsquo; which
includes a file, but does so at most once.  If you use &lsquo;<samp>#import</samp>&rsquo;
instead of &lsquo;<samp>#include</samp>&rsquo;, then you don&rsquo;t need the conditionals
inside the header file to prevent multiple inclusion of the contents.
&lsquo;<samp>#import</samp>&rsquo; is standard in Objective-C, but is considered a
deprecated extension in C and C++.
</p>
<p>&lsquo;<samp>#import</samp>&rsquo; 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&rsquo;s implementor to write the file so that users
don&rsquo;t need to know this.  Using a wrapper &lsquo;<samp>#ifndef</samp>&rsquo; accomplishes
this goal.
</p>
<p>In the present implementation, a single use of &lsquo;<samp>#import</samp>&rsquo; will
prevent the file from ever being read again, by either &lsquo;<samp>#import</samp>&rsquo; or
&lsquo;<samp>#include</samp>&rsquo;.  You should not rely on this; do not use both
&lsquo;<samp>#import</samp>&rsquo; and &lsquo;<samp>#include</samp>&rsquo; 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 &lsquo;<samp>#pragma once</samp>&rsquo; directive.  If &lsquo;<samp>#pragma once</samp>&rsquo; is
seen when scanning a header file, that file will never be read again, no
matter what.
</p>
<p>&lsquo;<samp>#pragma once</samp>&rsquo; does not have the problems that &lsquo;<samp>#import</samp>&rsquo; does,
but it is not recognized by all preprocessors, so you cannot rely on it
in a portable program.
</p>
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