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94 lines
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94 lines
4.9 KiB
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<title>Alternatives to Wrapper #ifndef - The C Preprocessor</title>
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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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Next: <a rel="next" accesskey="n" href="Computed-Includes.html#Computed-Includes">Computed Includes</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><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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