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| <title>The C Preprocessor: Deleted Code</title> |
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| <a name="Deleted-Code-1"></a> |
| <h3 class="section">4.3 Deleted Code</h3> |
| <a name="index-commenting-out-code"></a> |
| |
| <p>If you replace or delete a part of the program but want to keep the old |
| code around for future reference, you often cannot simply comment it |
| out. Block comments do not nest, so the first comment inside the old |
| code will end the commenting-out. The probable result is a flood of |
| syntax errors. |
| </p> |
| <p>One way to avoid this problem is to use an always-false conditional |
| instead. For instance, put <code>#if 0</code> before the deleted code and |
| <code>#endif</code> after it. This works even if the code being turned |
| off contains conditionals, but they must be entire conditionals |
| (balanced ‘<samp>#if</samp>’ and ‘<samp>#endif</samp>’). |
| </p> |
| <p>Some people use <code>#ifdef notdef</code> instead. This is risky, because |
| <code>notdef</code> might be accidentally defined as a macro, and then the |
| conditional would succeed. <code>#if 0</code> can be counted on to fail. |
| </p> |
| <p>Do not use <code>#if 0</code> for comments which are not C code. Use a real |
| comment, instead. The interior of <code>#if 0</code> must consist of complete |
| tokens; in particular, single-quote characters must balance. Comments |
| often contain unbalanced single-quote characters (known in English as |
| apostrophes). These confuse <code>#if 0</code>. They don’t confuse |
| ‘<samp>/*</samp>’. |
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