145 lines
6.5 KiB
HTML
145 lines
6.5 KiB
HTML
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any later version published by the Free Software Foundation. A copy of
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<head>
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<title>The C Preprocessor: Undefining and Redefining Macros</title>
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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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</head>
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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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