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  71. <a name="Macros"></a>

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  74. Next: <a href="Conditionals.html#Conditionals" accesskey="n" rel="next">Conditionals</a>, Previous: <a href="Header-Files.html#Header-Files" accesskey="p" rel="prev">Header Files</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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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  77. <a name="Macros-1"></a>

  78. <h2 class="chapter">3 Macros</h2>

  79. 

  80. <p>A <em>macro</em> is a fragment of code which has been given a name.

  81. Whenever the name is used, it is replaced by the contents of the macro.

  82. There are two kinds of macros.  They differ mostly in what they look

  83. like when they are used.  <em>Object-like</em> macros resemble data objects

  84. when used, <em>function-like</em> macros resemble function calls.

  85. </p>

  86. <p>You may define any valid identifier as a macro, even if it is a C

  87. keyword.  The preprocessor does not know anything about keywords.  This

  88. can be useful if you wish to hide a keyword such as <code>const</code> from an

  89. older compiler that does not understand it.  However, the preprocessor

  90. operator <code>defined</code> (see <a href="Defined.html#Defined">Defined</a>) can never be defined as a

  91. macro, and C++&rsquo;s named operators (see <a href="C_002b_002b-Named-Operators.html#C_002b_002b-Named-Operators">C++ Named Operators</a>) cannot be

  92. macros when you are compiling C++.

  93. </p>

  94. <table class="menu" border="0" cellspacing="0">

  95. <tr><td align="left" valign="top">&bull; <a href="Object_002dlike-Macros.html#Object_002dlike-Macros" accesskey="1">Object-like Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  96. </td></tr>

  97. <tr><td align="left" valign="top">&bull; <a href="Function_002dlike-Macros.html#Function_002dlike-Macros" accesskey="2">Function-like Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  98. </td></tr>

  99. <tr><td align="left" valign="top">&bull; <a href="Macro-Arguments.html#Macro-Arguments" accesskey="3">Macro Arguments</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  100. </td></tr>

  101. <tr><td align="left" valign="top">&bull; <a href="Stringizing.html#Stringizing" accesskey="4">Stringizing</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  102. </td></tr>

  103. <tr><td align="left" valign="top">&bull; <a href="Concatenation.html#Concatenation" accesskey="5">Concatenation</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  104. </td></tr>

  105. <tr><td align="left" valign="top">&bull; <a href="Variadic-Macros.html#Variadic-Macros" accesskey="6">Variadic Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  106. </td></tr>

  107. <tr><td align="left" valign="top">&bull; <a href="Predefined-Macros.html#Predefined-Macros" accesskey="7">Predefined Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  108. </td></tr>

  109. <tr><td align="left" valign="top">&bull; <a href="Undefining-and-Redefining-Macros.html#Undefining-and-Redefining-Macros" accesskey="8">Undefining and Redefining Macros</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  110. </td></tr>

  111. <tr><td align="left" valign="top">&bull; <a href="Directives-Within-Macro-Arguments.html#Directives-Within-Macro-Arguments" accesskey="9">Directives Within Macro Arguments</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  112. </td></tr>

  113. <tr><td align="left" valign="top">&bull; <a href="Macro-Pitfalls.html#Macro-Pitfalls">Macro Pitfalls</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  114. </td></tr>

  115. </table>

  116. 

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