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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 ‘<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. |
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