Variadic-Macros.html 6.8 KB

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  68. <a name="Macros-with-a-Variable-Number-of-Arguments_002e"></a>
  69. <h3 class="section">6.20 Macros with a Variable Number of Arguments.</h3>
  70. <a name="index-variable-number-of-arguments"></a>
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  73. <a name="index-variadic-macros"></a>
  74. <p>In the ISO C standard of 1999, a macro can be declared to accept a
  75. variable number of arguments much as a function can. The syntax for
  76. defining the macro is similar to that of a function. Here is an
  77. example:
  78. </p>
  79. <div class="smallexample">
  80. <pre class="smallexample">#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)
  81. </pre></div>
  82. <p>Here &lsquo;<samp>&hellip;</samp>&rsquo; is a <em>variable argument</em>. In the invocation of
  83. such a macro, it represents the zero or more tokens until the closing
  84. parenthesis that ends the invocation, including any commas. This set of
  85. tokens replaces the identifier <code>__VA_ARGS__</code> in the macro body
  86. wherever it appears. See the CPP manual for more information.
  87. </p>
  88. <p>GCC has long supported variadic macros, and used a different syntax that
  89. allowed you to give a name to the variable arguments just like any other
  90. argument. Here is an example:
  91. </p>
  92. <div class="smallexample">
  93. <pre class="smallexample">#define debug(format, args...) fprintf (stderr, format, args)
  94. </pre></div>
  95. <p>This is in all ways equivalent to the ISO C example above, but arguably
  96. more readable and descriptive.
  97. </p>
  98. <p>GNU CPP has two further variadic macro extensions, and permits them to
  99. be used with either of the above forms of macro definition.
  100. </p>
  101. <p>In standard C, you are not allowed to leave the variable argument out
  102. entirely; but you are allowed to pass an empty argument. For example,
  103. this invocation is invalid in ISO C, because there is no comma after
  104. the string:
  105. </p>
  106. <div class="smallexample">
  107. <pre class="smallexample">debug (&quot;A message&quot;)
  108. </pre></div>
  109. <p>GNU CPP permits you to completely omit the variable arguments in this
  110. way. In the above examples, the compiler would complain, though since
  111. the expansion of the macro still has the extra comma after the format
  112. string.
  113. </p>
  114. <p>To help solve this problem, CPP behaves specially for variable arguments
  115. used with the token paste operator, &lsquo;<samp>##</samp>&rsquo;. If instead you write
  116. </p>
  117. <div class="smallexample">
  118. <pre class="smallexample">#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)
  119. </pre></div>
  120. <p>and if the variable arguments are omitted or empty, the &lsquo;<samp>##</samp>&rsquo;
  121. operator causes the preprocessor to remove the comma before it. If you
  122. do provide some variable arguments in your macro invocation, GNU CPP
  123. does not complain about the paste operation and instead places the
  124. variable arguments after the comma. Just like any other pasted macro
  125. argument, these arguments are not macro expanded.
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