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  20. <title>Undefining and Redefining Macros (The C Preprocessor)</title>
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  61. <a name="Undefining-and-Redefining-Macros"></a>
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  63. <p>
  64. 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> &nbsp; [<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>
  65. </div>
  66. <hr>
  67. <a name="Undefining-and-Redefining-Macros-1"></a>
  68. <h3 class="section">3.8 Undefining and Redefining Macros</h3>
  69. <a name="index-undefining-macros"></a>
  70. <a name="index-redefining-macros"></a>
  71. <a name="index-_0023undef"></a>
  72. <p>If a macro ceases to be useful, it may be <em>undefined</em> with the
  73. &lsquo;<samp>#undef</samp>&rsquo; directive. &lsquo;<samp>#undef</samp>&rsquo; takes a single argument, the
  74. name of the macro to undefine. You use the bare macro name, even if the
  75. macro is function-like. It is an error if anything appears on the line
  76. after the macro name. &lsquo;<samp>#undef</samp>&rsquo; has no effect if the name is not a
  77. macro.
  78. </p>
  79. <div class="smallexample">
  80. <pre class="smallexample">#define FOO 4
  81. x = FOO; &rarr; x = 4;
  82. #undef FOO
  83. x = FOO; &rarr; x = FOO;
  84. </pre></div>
  85. <p>Once a macro has been undefined, that identifier may be <em>redefined</em>
  86. as a macro by a subsequent &lsquo;<samp>#define</samp>&rsquo; directive. The new definition
  87. need not have any resemblance to the old definition.
  88. </p>
  89. <p>However, if an identifier which is currently a macro is redefined, then
  90. the new definition must be <em>effectively the same</em> as the old one.
  91. Two macro definitions are effectively the same if:
  92. </p><ul>
  93. <li> Both are the same type of macro (object- or function-like).
  94. </li><li> All the tokens of the replacement list are the same.
  95. </li><li> If there are any parameters, they are the same.
  96. </li><li> Whitespace appears in the same places in both. It need not be
  97. exactly the same amount of whitespace, though. Remember that comments
  98. count as whitespace.
  99. </li></ul>
  100. <p>These definitions are effectively the same:
  101. </p><div class="smallexample">
  102. <pre class="smallexample">#define FOUR (2 + 2)
  103. #define FOUR (2 + 2)
  104. #define FOUR (2 /* <span class="roman">two</span> */ + 2)
  105. </pre></div>
  106. <p>but these are not:
  107. </p><div class="smallexample">
  108. <pre class="smallexample">#define FOUR (2 + 2)
  109. #define FOUR ( 2+2 )
  110. #define FOUR (2 * 2)
  111. #define FOUR(score,and,seven,years,ago) (2 + 2)
  112. </pre></div>
  113. <p>If a macro is redefined with a definition that is not effectively the
  114. same as the old one, the preprocessor issues a warning and changes the
  115. macro to use the new definition. If the new definition is effectively
  116. the same, the redefinition is silently ignored. This allows, for
  117. instance, two different headers to define a common macro. The
  118. preprocessor will only complain if the definitions do not match.
  119. </p>
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  123. 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> &nbsp; [<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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