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  26. <title>Concatenation (The C Preprocessor)</title>

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

  72. <div class="header">

  73. <p>

  74. Next: <a href="Variadic-Macros.html#Variadic-Macros" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Stringizing.html#Stringizing" accesskey="p" rel="prev">Stringizing</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>

  75. </div>

  76. <hr>

  77. <a name="Concatenation-1"></a>

  78. <h3 class="section">3.5 Concatenation</h3>

  79. <a name="index-concatenation"></a>

  80. <a name="index-token-pasting"></a>

  81. <a name="index-token-concatenation"></a>

  82. <a name="index-_0023_0023-operator"></a>

  83. 

  84. <p>It is often useful to merge two tokens into one while expanding macros.

  85. This is called <em>token pasting</em> or <em>token concatenation</em>.  The

  86. &lsquo;<samp>##</samp>&rsquo; preprocessing operator performs token pasting.  When a macro

  87. is expanded, the two tokens on either side of each &lsquo;<samp>##</samp>&rsquo; operator

  88. are combined into a single token, which then replaces the &lsquo;<samp>##</samp>&rsquo; and

  89. the two original tokens in the macro expansion.  Usually both will be

  90. identifiers, or one will be an identifier and the other a preprocessing

  91. number.  When pasted, they make a longer identifier.  This isn&rsquo;t the

  92. only valid case.  It is also possible to concatenate two numbers (or a

  93. number and a name, such as <code>1.5</code> and <code>e3</code>) into a number.

  94. Also, multi-character operators such as <code>+=</code> can be formed by

  95. token pasting.

  96. </p>

  97. <p>However, two tokens that don&rsquo;t together form a valid token cannot be

  98. pasted together.  For example, you cannot concatenate <code>x</code> with

  99. <code>+</code> in either order.  If you try, the preprocessor issues a warning

  100. and emits the two tokens.  Whether it puts white space between the

  101. tokens is undefined.  It is common to find unnecessary uses of &lsquo;<samp>##</samp>&rsquo;

  102. in complex macros.  If you get this warning, it is likely that you can

  103. simply remove the &lsquo;<samp>##</samp>&rsquo;.

  104. </p>

  105. <p>Both the tokens combined by &lsquo;<samp>##</samp>&rsquo; could come from the macro body,

  106. but you could just as well write them as one token in the first place.

  107. Token pasting is most useful when one or both of the tokens comes from a

  108. macro argument.  If either of the tokens next to an &lsquo;<samp>##</samp>&rsquo; is a

  109. parameter name, it is replaced by its actual argument before &lsquo;<samp>##</samp>&rsquo;

  110. executes.  As with stringizing, the actual argument is not

  111. macro-expanded first.  If the argument is empty, that &lsquo;<samp>##</samp>&rsquo; has no

  112. effect.

  113. </p>

  114. <p>Keep in mind that the C preprocessor converts comments to whitespace

  115. before macros are even considered.  Therefore, you cannot create a

  116. comment by concatenating &lsquo;<samp>/</samp>&rsquo; and &lsquo;<samp>*</samp>&rsquo;.  You can put as much

  117. whitespace between &lsquo;<samp>##</samp>&rsquo; and its operands as you like, including

  118. comments, and you can put comments in arguments that will be

  119. concatenated.  However, it is an error if &lsquo;<samp>##</samp>&rsquo; appears at either

  120. end of a macro body.

  121. </p>

  122. <p>Consider a C program that interprets named commands.  There probably

  123. needs to be a table of commands, perhaps an array of structures declared

  124. as follows:

  125. </p>

  126. <div class="smallexample">

  127. <pre class="smallexample">struct command

  128. {

  129.   char *name;

  130.   void (*function) (void);

  131. };

  132. </pre><pre class="smallexample">

  133. 

  134. </pre><pre class="smallexample">struct command commands[] =

  135. {

  136.   { &quot;quit&quot;, quit_command },

  137.   { &quot;help&quot;, help_command },

  138.   &hellip;

  139. };

  140. </pre></div>

  141. 

  142. <p>It would be cleaner not to have to give each command name twice, once in

  143. the string constant and once in the function name.  A macro which takes the

  144. name of a command as an argument can make this unnecessary.  The string

  145. constant can be created with stringizing, and the function name by

  146. concatenating the argument with &lsquo;<samp>_command</samp>&rsquo;.  Here is how it is done:

  147. </p>

  148. <div class="smallexample">

  149. <pre class="smallexample">#define COMMAND(NAME)  { #NAME, NAME ## _command }

  150. 

  151. struct command commands[] =

  152. {

  153.   COMMAND (quit),

  154.   COMMAND (help),

  155.   &hellip;

  156. };

  157. </pre></div>

  158. 

  159. <hr>

  160. <div class="header">

  161. <p>

  162. Next: <a href="Variadic-Macros.html#Variadic-Macros" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Stringizing.html#Stringizing" accesskey="p" rel="prev">Stringizing</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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