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| <title>The C Preprocessor: Concatenation</title> |
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| Next: <a href="Variadic-Macros.html#Variadic-Macros" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Stringification.html#Stringification" accesskey="p" rel="prev">Stringification</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</a> [<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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| <a name="Concatenation-1"></a> |
| <h3 class="section">3.5 Concatenation</h3> |
| <a name="index-concatenation"></a> |
| <a name="index-token-pasting"></a> |
| <a name="index-token-concatenation"></a> |
| <a name="index-_0023_0023-operator"></a> |
| |
| <p>It is often useful to merge two tokens into one while expanding macros. |
| This is called <em>token pasting</em> or <em>token concatenation</em>. The |
| ‘<samp>##</samp>’ preprocessing operator performs token pasting. When a macro |
| is expanded, the two tokens on either side of each ‘<samp>##</samp>’ operator |
| are combined into a single token, which then replaces the ‘<samp>##</samp>’ 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> |
| <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 ‘<samp>##</samp>’ |
| in complex macros. If you get this warning, it is likely that you can |
| simply remove the ‘<samp>##</samp>’. |
| </p> |
| <p>Both the tokens combined by ‘<samp>##</samp>’ 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 ‘<samp>##</samp>’ is a |
| parameter name, it is replaced by its actual argument before ‘<samp>##</samp>’ |
| executes. As with stringification, the actual argument is not |
| macro-expanded first. If the argument is empty, that ‘<samp>##</samp>’ has no |
| effect. |
| </p> |
| <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 ‘<samp>/</samp>’ and ‘<samp>*</samp>’. You can put as much |
| whitespace between ‘<samp>##</samp>’ 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 ‘<samp>##</samp>’ appears at either |
| end of a macro body. |
| </p> |
| <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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">struct command |
| { |
| char *name; |
| void (*function) (void); |
| }; |
| </pre><pre class="smallexample"> |
| </pre><pre class="smallexample">struct command commands[] = |
| { |
| { "quit", quit_command }, |
| { "help", help_command }, |
| … |
| }; |
| </pre></div> |
| |
| <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 ‘<samp>_command</samp>’. Here is how it is done: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define COMMAND(NAME) { #NAME, NAME ## _command } |
| |
| struct command commands[] = |
| { |
| COMMAND (quit), |
| COMMAND (help), |
| … |
| }; |
| </pre></div> |
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| Next: <a href="Variadic-Macros.html#Variadic-Macros" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Stringification.html#Stringification" accesskey="p" rel="prev">Stringification</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</a> [<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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