130 lines
6.2 KiB
HTML
130 lines
6.2 KiB
HTML
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<title>The C Preprocessor: Conditionals</title>
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<a name="Conditionals"></a>
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<p>
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Next: <a href="Diagnostics.html#Diagnostics" accesskey="n" rel="next">Diagnostics</a>, Previous: <a href="Macros.html#Macros" accesskey="p" rel="prev">Macros</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="Conditionals-1"></a>
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<h2 class="chapter">4 Conditionals</h2>
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<a name="index-conditionals"></a>
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<p>A <em>conditional</em> is a directive that instructs the preprocessor to
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select whether or not to include a chunk of code in the final token
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stream passed to the compiler. Preprocessor conditionals can test
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arithmetic expressions, or whether a name is defined as a macro, or both
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simultaneously using the special <code>defined</code> operator.
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</p>
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<p>A conditional in the C preprocessor resembles in some ways an <code>if</code>
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statement in C, but it is important to understand the difference between
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them. The condition in an <code>if</code> statement is tested during the
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execution of your program. Its purpose is to allow your program to
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behave differently from run to run, depending on the data it is
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operating on. The condition in a preprocessing conditional directive is
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tested when your program is compiled. Its purpose is to allow different
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code to be included in the program depending on the situation at the
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time of compilation.
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</p>
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<p>However, the distinction is becoming less clear. Modern compilers often
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do test <code>if</code> statements when a program is compiled, if their
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conditions are known not to vary at run time, and eliminate code which
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can never be executed. If you can count on your compiler to do this,
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you may find that your program is more readable if you use <code>if</code>
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statements with constant conditions (perhaps determined by macros). Of
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course, you can only use this to exclude code, not type definitions or
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other preprocessing directives, and you can only do it if the code
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remains syntactically valid when it is not to be used.
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</p>
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<p>GCC version 3 eliminates this kind of never-executed code even when
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not optimizing. Older versions did it only when optimizing.
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</p>
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<table class="menu" border="0" cellspacing="0">
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<tr><td align="left" valign="top">• <a href="Conditional-Uses.html#Conditional-Uses" accesskey="1">Conditional Uses</a>:</td><td> </td><td align="left" valign="top">
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<tr><td align="left" valign="top">• <a href="Conditional-Syntax.html#Conditional-Syntax" accesskey="2">Conditional Syntax</a>:</td><td> </td><td align="left" valign="top">
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Next: <a href="Diagnostics.html#Diagnostics" accesskey="n" rel="next">Diagnostics</a>, Previous: <a href="Macros.html#Macros" accesskey="p" rel="prev">Macros</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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