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| <h3 class="section">2.4 Once-Only Headers</h3> |
| |
| <p><a name="index-repeated-inclusion-28"></a><a name="index-including-just-once-29"></a><a name="index-wrapper-_0040code_007b_0023ifndef_007d-30"></a> |
| If a header file happens to be included twice, the compiler will process |
| its contents twice. This is very likely to cause an error, e.g. when the |
| compiler sees the same structure definition twice. Even if it does not, |
| it will certainly waste time. |
| |
| <p>The standard way to prevent this is to enclose the entire real contents |
| of the file in a conditional, like this: |
| |
| <pre class="smallexample"> /* File foo. */ |
| #ifndef FILE_FOO_SEEN |
| #define FILE_FOO_SEEN |
| |
| <var>the entire file</var> |
| |
| #endif /* !FILE_FOO_SEEN */ |
| </pre> |
| <p>This construct is commonly known as a <dfn>wrapper #ifndef</dfn>. |
| When the header is included again, the conditional will be false, |
| because <code>FILE_FOO_SEEN</code> is defined. The preprocessor will skip |
| over the entire contents of the file, and the compiler will not see it |
| twice. |
| |
| <p>CPP optimizes even further. It remembers when a header file has a |
| wrapper ‘<samp><span class="samp">#ifndef</span></samp>’. If a subsequent ‘<samp><span class="samp">#include</span></samp>’ specifies that |
| header, and the macro in the ‘<samp><span class="samp">#ifndef</span></samp>’ is still defined, it does |
| not bother to rescan the file at all. |
| |
| <p>You can put comments outside the wrapper. They will not interfere with |
| this optimization. |
| |
| <p><a name="index-controlling-macro-31"></a><a name="index-guard-macro-32"></a>The macro <code>FILE_FOO_SEEN</code> is called the <dfn>controlling macro</dfn> or |
| <dfn>guard macro</dfn>. In a user header file, the macro name should not |
| begin with ‘<samp><span class="samp">_</span></samp>’. In a system header file, it should begin with |
| ‘<samp><span class="samp">__</span></samp>’ to avoid conflicts with user programs. In any kind of header |
| file, the macro name should contain the name of the file and some |
| additional text, to avoid conflicts with other header files. |
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