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diff --git a/share/doc/cpp/Undefining-and-Redefining-Macros.html b/share/doc/cpp/Undefining-and-Redefining-Macros.html new file mode 100644 index 0000000..f7e4ba1 --- /dev/null +++ b/share/doc/cpp/Undefining-and-Redefining-Macros.html @@ -0,0 +1,144 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1987-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation. A copy of +the license is included in the +section entitled "GNU Free Documentation License". + +This manual contains no Invariant Sections. 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Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>The C Preprocessor: Undefining and Redefining Macros</title> + +<meta name="description" content="The C Preprocessor: Undefining and Redefining Macros"> +<meta name="keywords" content="The C Preprocessor: Undefining and Redefining Macros"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="Macros.html#Macros" rel="up" title="Macros"> +<link href="Directives-Within-Macro-Arguments.html#Directives-Within-Macro-Arguments" rel="next" title="Directives Within Macro Arguments"> +<link href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators" rel="previous" title="C++ Named Operators"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Undefining-and-Redefining-Macros"></a> +<div class="header"> +<p> +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="previous">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> +</div> +<hr> +<a name="Undefining-and-Redefining-Macros-1"></a> +<h3 class="section">3.8 Undefining and Redefining Macros</h3> +<a name="index-undefining-macros"></a> +<a name="index-redefining-macros"></a> +<a name="index-_0023undef"></a> + +<p>If a macro ceases to be useful, it may be <em>undefined</em> with the +‘<samp>#undef</samp>’ directive. ‘<samp>#undef</samp>’ takes a single argument, the +name of the macro to undefine. You use the bare macro name, even if the +macro is function-like. It is an error if anything appears on the line +after the macro name. ‘<samp>#undef</samp>’ has no effect if the name is not a +macro. +</p> +<div class="smallexample"> +<pre class="smallexample">#define FOO 4 +x = FOO; → x = 4; +#undef FOO +x = FOO; → x = FOO; +</pre></div> + +<p>Once a macro has been undefined, that identifier may be <em>redefined</em> +as a macro by a subsequent ‘<samp>#define</samp>’ directive. The new definition +need not have any resemblance to the old definition. +</p> +<p>However, if an identifier which is currently a macro is redefined, then +the new definition must be <em>effectively the same</em> as the old one. +Two macro definitions are effectively the same if: +</p><ul> +<li> Both are the same type of macro (object- or function-like). +</li><li> All the tokens of the replacement list are the same. +</li><li> If there are any parameters, they are the same. +</li><li> Whitespace appears in the same places in both. It need not be +exactly the same amount of whitespace, though. Remember that comments +count as whitespace. +</li></ul> + +<p>These definitions are effectively the same: +</p><div class="smallexample"> +<pre class="smallexample">#define FOUR (2 + 2) +#define FOUR (2 + 2) +#define FOUR (2 /* <span class="roman">two</span> */ + 2) +</pre></div> +<p>but these are not: +</p><div class="smallexample"> +<pre class="smallexample">#define FOUR (2 + 2) +#define FOUR ( 2+2 ) +#define FOUR (2 * 2) +#define FOUR(score,and,seven,years,ago) (2 + 2) +</pre></div> + +<p>If a macro is redefined with a definition that is not effectively the +same as the old one, the preprocessor issues a warning and changes the +macro to use the new definition. If the new definition is effectively +the same, the redefinition is silently ignored. This allows, for +instance, two different headers to define a common macro. The +preprocessor will only complain if the definitions do not match. +</p> +<hr> +<div class="header"> +<p> +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="previous">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> +</div> + + + +</body> +</html> |