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84 lines
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84 lines
3.3 KiB
HTML
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<title>Conditional Uses - The C Preprocessor</title>
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<link rel="up" href="Conditionals.html#Conditionals" title="Conditionals">
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<link rel="next" href="Conditional-Syntax.html#Conditional-Syntax" title="Conditional Syntax">
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<a name="Conditional-Uses"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Conditional-Syntax.html#Conditional-Syntax">Conditional Syntax</a>,
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Up: <a rel="up" accesskey="u" href="Conditionals.html#Conditionals">Conditionals</a>
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</div>
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<h3 class="section">4.1 Conditional Uses</h3>
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<p>There are three general reasons to use a conditional.
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<ul>
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<li>A program may need to use different code depending on the machine or
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operating system it is to run on. In some cases the code for one
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operating system may be erroneous on another operating system; for
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example, it might refer to data types or constants that do not exist on
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the other system. When this happens, it is not enough to avoid
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executing the invalid code. Its mere presence will cause the compiler
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to reject the program. With a preprocessing conditional, the offending
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code can be effectively excised from the program when it is not valid.
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<li>You may want to be able to compile the same source file into two
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different programs. One version might make frequent time-consuming
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consistency checks on its intermediate data, or print the values of
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those data for debugging, and the other not.
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<li>A conditional whose condition is always false is one way to exclude code
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from the program but keep it as a sort of comment for future reference.
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</ul>
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<p>Simple programs that do not need system-specific logic or complex
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debugging hooks generally will not need to use preprocessing
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conditionals.
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