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144 lines
6.5 KiB
HTML
144 lines
6.5 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1987-2018 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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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Undefining and Redefining Macros (The C Preprocessor)</title>
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<meta name="description" content="Undefining and Redefining Macros (The C Preprocessor)">
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<meta name="keywords" content="Undefining and Redefining Macros (The C Preprocessor)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Macros.html#Macros" rel="up" title="Macros">
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<link href="Directives-Within-Macro-Arguments.html#Directives-Within-Macro-Arguments" rel="next" title="Directives Within Macro Arguments">
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<link href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators" rel="prev" title="C++ Named Operators">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Undefining-and-Redefining-Macros"></a>
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<div class="header">
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<p>
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Next: <a href="Directives-Within-Macro-Arguments.html#Directives-Within-Macro-Arguments" accesskey="n" rel="next">Directives Within Macro Arguments</a>, Previous: <a href="Predefined-Macros.html#Predefined-Macros" accesskey="p" rel="prev">Predefined Macros</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</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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</div>
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<hr>
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<a name="Undefining-and-Redefining-Macros-1"></a>
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<h3 class="section">3.8 Undefining and Redefining Macros</h3>
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<a name="index-undefining-macros"></a>
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<a name="index-redefining-macros"></a>
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<a name="index-_0023undef"></a>
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<p>If a macro ceases to be useful, it may be <em>undefined</em> with the
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‘<samp>#undef</samp>’ directive. ‘<samp>#undef</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>#undef</samp>’ has no effect if the name is not a
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macro.
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</p>
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<div class="smallexample">
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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></div>
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<p>Once a macro has been undefined, that identifier may be <em>redefined</em>
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as a macro by a subsequent ‘<samp>#define</samp>’ directive. The new definition
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need not have any resemblance to the old definition.
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</p>
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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 <em>effectively the same</em> as the old one.
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Two macro definitions are effectively the same if:
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</p><ul>
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<li> Both are the same type of macro (object- or function-like).
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</li><li> All the tokens of the replacement list are the same.
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</li><li> If there are any parameters, they are the same.
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</li><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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</li></ul>
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<p>These definitions are effectively the same:
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</p><div class="smallexample">
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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></div>
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<p>but these are not:
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</p><div class="smallexample">
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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></div>
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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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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Directives-Within-Macro-Arguments.html#Directives-Within-Macro-Arguments" accesskey="n" rel="next">Directives Within Macro Arguments</a>, Previous: <a href="Predefined-Macros.html#Predefined-Macros" accesskey="p" rel="prev">Predefined Macros</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</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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</div>
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</body>
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</html>
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