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108 lines
4.8 KiB
HTML
108 lines
4.8 KiB
HTML
4 years ago
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<title>Undefining and Redefining Macros - The C Preprocessor</title>
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<meta name="description" content="The C Preprocessor">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Macros.html#Macros" title="Macros">
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<link rel="prev" href="Predefined-Macros.html#Predefined-Macros" title="Predefined Macros">
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<link rel="next" href="Directives-Within-Macro-Arguments.html#Directives-Within-Macro-Arguments" title="Directives Within Macro Arguments">
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Copyright (C) 1987-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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the license is included in the
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section entitled ``GNU Free Documentation License''.
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This manual contains no Invariant Sections. The Front-Cover Texts are
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(a) (see below), and the Back-Cover Texts are (b) (see below).
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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</head>
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<body>
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<div class="node">
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<a name="Undefining-and-Redefining-Macros"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Directives-Within-Macro-Arguments.html#Directives-Within-Macro-Arguments">Directives Within Macro Arguments</a>,
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Previous: <a rel="previous" accesskey="p" href="Predefined-Macros.html#Predefined-Macros">Predefined Macros</a>,
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Up: <a rel="up" accesskey="u" href="Macros.html#Macros">Macros</a>
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<hr>
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</div>
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<h3 class="section">3.8 Undefining and Redefining Macros</h3>
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<p><a name="index-undefining-macros-68"></a><a name="index-redefining-macros-69"></a><a name="index-g_t_0023undef-70"></a>
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If a macro ceases to be useful, it may be <dfn>undefined</dfn> with the
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‘<samp><span class="samp">#undef</span></samp>’ directive. ‘<samp><span class="samp">#undef</span></samp>’ takes a single argument, the
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name of the macro to undefine. You use the bare macro name, even if the
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macro is function-like. It is an error if anything appears on the line
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after the macro name. ‘<samp><span class="samp">#undef</span></samp>’ has no effect if the name is not a
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macro.
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<pre class="smallexample"> #define FOO 4
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x = FOO; ==> x = 4;
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#undef FOO
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x = FOO; ==> x = FOO;
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</pre>
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<p>Once a macro has been undefined, that identifier may be <dfn>redefined</dfn>
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as a macro by a subsequent ‘<samp><span class="samp">#define</span></samp>’ directive. The new definition
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need not have any resemblance to the old definition.
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<p>However, if an identifier which is currently a macro is redefined, then
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the new definition must be <dfn>effectively the same</dfn> as the old one.
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Two macro definitions are effectively the same if:
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<ul>
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<li>Both are the same type of macro (object- or function-like).
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<li>All the tokens of the replacement list are the same.
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<li>If there are any parameters, they are the same.
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<li>Whitespace appears in the same places in both. It need not be
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exactly the same amount of whitespace, though. Remember that comments
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count as whitespace.
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</ul>
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<p class="noindent">These definitions are effectively the same:
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<pre class="smallexample"> #define FOUR (2 + 2)
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#define FOUR (2 + 2)
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#define FOUR (2 /* <span class="roman">two</span> */ + 2)
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</pre>
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<p class="noindent">but these are not:
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<pre class="smallexample"> #define FOUR (2 + 2)
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#define FOUR ( 2+2 )
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#define FOUR (2 * 2)
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#define FOUR(score,and,seven,years,ago) (2 + 2)
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</pre>
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<p>If a macro is redefined with a definition that is not effectively the
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same as the old one, the preprocessor issues a warning and changes the
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macro to use the new definition. If the new definition is effectively
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the same, the redefinition is silently ignored. This allows, for
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instance, two different headers to define a common macro. The
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preprocessor will only complain if the definitions do not match.
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</body></html>
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