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| <title>The C Preprocessor: Undefining and Redefining Macros</title> |
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| <a name="Undefining-and-Redefining-Macros"></a> |
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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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| <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="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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