You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
108 lines
4.8 KiB
HTML
108 lines
4.8 KiB
HTML
<html lang="en">
|
|
<head>
|
|
<title>Undefining and Redefining Macros - The C Preprocessor</title>
|
|
<meta http-equiv="Content-Type" content="text/html">
|
|
<meta name="description" content="The C Preprocessor">
|
|
<meta name="generator" content="makeinfo 4.13">
|
|
<link title="Top" rel="start" href="index.html#Top">
|
|
<link rel="up" href="Macros.html#Macros" title="Macros">
|
|
<link rel="prev" href="Predefined-Macros.html#Predefined-Macros" title="Predefined Macros">
|
|
<link rel="next" href="Directives-Within-Macro-Arguments.html#Directives-Within-Macro-Arguments" title="Directives Within Macro Arguments">
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
|
<!--
|
|
Copyright (C) 1987-2015 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. The Front-Cover Texts are
|
|
(a) (see below), and the Back-Cover Texts are (b) (see below).
|
|
|
|
(a) The FSF's Front-Cover Text is:
|
|
|
|
A GNU Manual
|
|
|
|
(b) The FSF's Back-Cover Text is:
|
|
|
|
You have freedom to copy and modify this GNU Manual, like GNU
|
|
software. Copies published by the Free Software Foundation raise
|
|
funds for GNU development.
|
|
-->
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
|
<style type="text/css"><!--
|
|
pre.display { font-family:inherit }
|
|
pre.format { font-family:inherit }
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
|
pre.smallexample { font-size:smaller }
|
|
pre.smalllisp { font-size:smaller }
|
|
span.sc { font-variant:small-caps }
|
|
span.roman { font-family:serif; font-weight:normal; }
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
|
--></style>
|
|
</head>
|
|
<body>
|
|
<div class="node">
|
|
<a name="Undefining-and-Redefining-Macros"></a>
|
|
<p>
|
|
Next: <a rel="next" accesskey="n" href="Directives-Within-Macro-Arguments.html#Directives-Within-Macro-Arguments">Directives Within Macro Arguments</a>,
|
|
Previous: <a rel="previous" accesskey="p" href="Predefined-Macros.html#Predefined-Macros">Predefined Macros</a>,
|
|
Up: <a rel="up" accesskey="u" href="Macros.html#Macros">Macros</a>
|
|
<hr>
|
|
</div>
|
|
|
|
<h3 class="section">3.8 Undefining and Redefining Macros</h3>
|
|
|
|
<p><a name="index-undefining-macros-68"></a><a name="index-redefining-macros-69"></a><a name="index-g_t_0023undef-70"></a>
|
|
If a macro ceases to be useful, it may be <dfn>undefined</dfn> with the
|
|
‘<samp><span class="samp">#undef</span></samp>’ directive. ‘<samp><span class="samp">#undef</span></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><span class="samp">#undef</span></samp>’ has no effect if the name is not a
|
|
macro.
|
|
|
|
<pre class="smallexample"> #define FOO 4
|
|
x = FOO; ==> x = 4;
|
|
#undef FOO
|
|
x = FOO; ==> x = FOO;
|
|
</pre>
|
|
<p>Once a macro has been undefined, that identifier may be <dfn>redefined</dfn>
|
|
as a macro by a subsequent ‘<samp><span class="samp">#define</span></samp>’ directive. The new definition
|
|
need not have any resemblance to the old definition.
|
|
|
|
<p>However, if an identifier which is currently a macro is redefined, then
|
|
the new definition must be <dfn>effectively the same</dfn> as the old one.
|
|
Two macro definitions are effectively the same if:
|
|
<ul>
|
|
<li>Both are the same type of macro (object- or function-like).
|
|
<li>All the tokens of the replacement list are the same.
|
|
<li>If there are any parameters, they are the same.
|
|
<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.
|
|
</ul>
|
|
|
|
<p class="noindent">These definitions are effectively the same:
|
|
<pre class="smallexample"> #define FOUR (2 + 2)
|
|
#define FOUR (2 + 2)
|
|
#define FOUR (2 /* <span class="roman">two</span> */ + 2)
|
|
</pre>
|
|
<p class="noindent">but these are not:
|
|
<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>
|
|
<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.
|
|
|
|
</body></html>
|
|
|