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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 |
| ‘<samp><span class="samp">##</span></samp>’ preprocessing operator performs token pasting. When a macro |
| is expanded, the two tokens on either side of each ‘<samp><span class="samp">##</span></samp>’ operator |
| are combined into a single token, which then replaces the ‘<samp><span class="samp">##</span></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>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><span class="samp">##</span></samp>’ |
| in complex macros. If you get this warning, it is likely that you can |
| simply remove the ‘<samp><span class="samp">##</span></samp>’. |
| |
| <p>Both the tokens combined by ‘<samp><span class="samp">##</span></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><span class="samp">##</span></samp>’ is a |
| parameter name, it is replaced by its actual argument before ‘<samp><span class="samp">##</span></samp>’ |
| executes. As with stringification, the actual argument is not |
| macro-expanded first. If the argument is empty, that ‘<samp><span class="samp">##</span></samp>’ 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 ‘<samp><span class="samp">/</span></samp>’ and ‘<samp><span class="samp">*</span></samp>’. You can put as much |
| whitespace between ‘<samp><span class="samp">##</span></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><span class="samp">##</span></samp>’ 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 ‘<samp><span class="samp">_command</span></samp>’. 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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