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

<p><a name="index-concatenation-52"></a><a name="index-token-pasting-53"></a><a name="index-token-concatenation-54"></a><a name="index-g_t_0040samp_007b_0023_0023_007d-operator-55"></a>
It is often useful to merge two tokens into one while expanding macros.
This is called <dfn>token pasting</dfn> or <dfn>token concatenation</dfn>.  The
&lsquo;<samp><span class="samp">##</span></samp>&rsquo; preprocessing operator performs token pasting.  When a macro
is expanded, the two tokens on either side of each &lsquo;<samp><span class="samp">##</span></samp>&rsquo; operator
are combined into a single token, which then replaces the &lsquo;<samp><span class="samp">##</span></samp>&rsquo; and
the two original tokens in the macro expansion.  Usually both will be
identifiers, or one will be an identifier and the other a preprocessing
number.  When pasted, they make a longer identifier.  This isn't the
only valid case.  It is also possible to concatenate two numbers (or a
number and a name, such as <code>1.5</code> and <code>e3</code>) into a number.
Also, multi-character operators such as <code>+=</code> can be formed by
token pasting.

   <p>However, two tokens that don't together form a valid token cannot be
pasted together.  For example, you cannot concatenate <code>x</code> with
<code>+</code> in either order.  If you try, the preprocessor issues a warning
and emits the two tokens.  Whether it puts white space between the
tokens is undefined.  It is common to find unnecessary uses of &lsquo;<samp><span class="samp">##</span></samp>&rsquo;
in complex macros.  If you get this warning, it is likely that you can
simply remove the &lsquo;<samp><span class="samp">##</span></samp>&rsquo;.

   <p>Both the tokens combined by &lsquo;<samp><span class="samp">##</span></samp>&rsquo; could come from the macro body,
but you could just as well write them as one token in the first place.
Token pasting is most useful when one or both of the tokens comes from a
macro argument.  If either of the tokens next to an &lsquo;<samp><span class="samp">##</span></samp>&rsquo; is a
parameter name, it is replaced by its actual argument before &lsquo;<samp><span class="samp">##</span></samp>&rsquo;
executes.  As with stringification, the actual argument is not
macro-expanded first.  If the argument is empty, that &lsquo;<samp><span class="samp">##</span></samp>&rsquo; has no
effect.

   <p>Keep in mind that the C preprocessor converts comments to whitespace
before macros are even considered.  Therefore, you cannot create a
comment by concatenating &lsquo;<samp><span class="samp">/</span></samp>&rsquo; and &lsquo;<samp><span class="samp">*</span></samp>&rsquo;.  You can put as much
whitespace between &lsquo;<samp><span class="samp">##</span></samp>&rsquo; and its operands as you like, including
comments, and you can put comments in arguments that will be
concatenated.  However, it is an error if &lsquo;<samp><span class="samp">##</span></samp>&rsquo; appears at either
end of a macro body.

   <p>Consider a C program that interprets named commands.  There probably
needs to be a table of commands, perhaps an array of structures declared
as follows:

<pre class="smallexample">     struct command
     {
       char *name;
       void (*function) (void);
     };

     struct command commands[] =
     {
       { "quit", quit_command },
       { "help", help_command },
       ...
     };
</pre>
   <p>It would be cleaner not to have to give each command name twice, once in
the string constant and once in the function name.  A macro which takes the
name of a command as an argument can make this unnecessary.  The string
constant can be created with stringification, and the function name by
concatenating the argument with &lsquo;<samp><span class="samp">_command</span></samp>&rsquo;.  Here is how it is done:

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

     struct command commands[] =
     {
       COMMAND (quit),
       COMMAND (help),
       ...
     };
</pre>
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