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<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><span class="samp">#ifndef</span></samp>&rsquo;
and we recommend you do not use them in new programs, with the caveat
that &lsquo;<samp><span class="samp">#import</span></samp>&rsquo; is standard practice in Objective-C.

   <p><a name="index-g_t_0023import-33"></a>CPP supports a variant of &lsquo;<samp><span class="samp">#include</span></samp>&rsquo; called &lsquo;<samp><span class="samp">#import</span></samp>&rsquo; which
includes a file, but does so at most once.  If you use &lsquo;<samp><span class="samp">#import</span></samp>&rsquo;
instead of &lsquo;<samp><span class="samp">#include</span></samp>&rsquo;, then you don't need the conditionals
inside the header file to prevent multiple inclusion of the contents. 
&lsquo;<samp><span class="samp">#import</span></samp>&rsquo; is standard in Objective-C, but is considered a
deprecated extension in C and C++.

   <p>&lsquo;<samp><span class="samp">#import</span></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's implementor to write the file so that users
don't need to know this.  Using a wrapper &lsquo;<samp><span class="samp">#ifndef</span></samp>&rsquo; accomplishes
this goal.

   <p>In the present implementation, a single use of &lsquo;<samp><span class="samp">#import</span></samp>&rsquo; will
prevent the file from ever being read again, by either &lsquo;<samp><span class="samp">#import</span></samp>&rsquo; or
&lsquo;<samp><span class="samp">#include</span></samp>&rsquo;.  You should not rely on this; do not use both
&lsquo;<samp><span class="samp">#import</span></samp>&rsquo; and &lsquo;<samp><span class="samp">#include</span></samp>&rsquo; to refer to the same header file.

   <p>Another way to prevent a header file from being included more than once
is with the &lsquo;<samp><span class="samp">#pragma once</span></samp>&rsquo; directive.  If &lsquo;<samp><span class="samp">#pragma once</span></samp>&rsquo; is
seen when scanning a header file, that file will never be read again, no
matter what.

   <p>&lsquo;<samp><span class="samp">#pragma once</span></samp>&rsquo; does not have the problems that &lsquo;<samp><span class="samp">#import</span></samp>&rsquo; does,
but it is not recognized by all preprocessors, so you cannot rely on it
in a portable program.

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