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| <h3 class="section">10.2 Traditional macros</h3> |
| |
| <p>The major difference between traditional and ISO macros is that the |
| former expand to text rather than to a token sequence. CPP removes |
| all leading and trailing horizontal whitespace from a macro's |
| replacement text before storing it, but preserves the form of internal |
| whitespace. |
| |
| <p>One consequence is that it is legitimate for the replacement text to |
| contain an unmatched quote (see <a href="Traditional-lexical-analysis.html#Traditional-lexical-analysis">Traditional lexical analysis</a>). An |
| unclosed string or character constant continues into the text |
| following the macro call. Similarly, the text at the end of a macro's |
| expansion can run together with the text after the macro invocation to |
| produce a single token. |
| |
| <p>Normally comments are removed from the replacement text after the |
| macro is expanded, but if the <samp><span class="option">-CC</span></samp> option is passed on the |
| command line comments are preserved. (In fact, the current |
| implementation removes comments even before saving the macro |
| replacement text, but it careful to do it in such a way that the |
| observed effect is identical even in the function-like macro case.) |
| |
| <p>The ISO stringification operator ‘<samp><span class="samp">#</span></samp>’ and token paste operator |
| ‘<samp><span class="samp">##</span></samp>’ have no special meaning. As explained later, an effect |
| similar to these operators can be obtained in a different way. Macro |
| names that are embedded in quotes, either from the main file or after |
| macro replacement, do not expand. |
| |
| <p>CPP replaces an unquoted object-like macro name with its replacement |
| text, and then rescans it for further macros to replace. Unlike |
| standard macro expansion, traditional macro expansion has no provision |
| to prevent recursion. If an object-like macro appears unquoted in its |
| replacement text, it will be replaced again during the rescan pass, |
| and so on <em>ad infinitum</em>. GCC detects when it is expanding |
| recursive macros, emits an error message, and continues after the |
| offending macro invocation. |
| |
| <pre class="smallexample"> #define PLUS + |
| #define INC(x) PLUS+x |
| INC(foo); |
| ==> ++foo; |
| </pre> |
| <p>Function-like macros are similar in form but quite different in |
| behavior to their ISO counterparts. Their arguments are contained |
| within parentheses, are comma-separated, and can cross physical lines. |
| Commas within nested parentheses are not treated as argument |
| separators. Similarly, a quote in an argument cannot be left |
| unclosed; a following comma or parenthesis that comes before the |
| closing quote is treated like any other character. There is no |
| facility for handling variadic macros. |
| |
| <p>This implementation removes all comments from macro arguments, unless |
| the <samp><span class="option">-C</span></samp> option is given. The form of all other horizontal |
| whitespace in arguments is preserved, including leading and trailing |
| whitespace. In particular |
| |
| <pre class="smallexample"> f( ) |
| </pre> |
| <p class="noindent">is treated as an invocation of the macro ‘<samp><span class="samp">f</span></samp>’ with a single |
| argument consisting of a single space. If you want to invoke a |
| function-like macro that takes no arguments, you must not leave any |
| whitespace between the parentheses. |
| |
| <p>If a macro argument crosses a new line, the new line is replaced with |
| a space when forming the argument. If the previous line contained an |
| unterminated quote, the following line inherits the quoted state. |
| |
| <p>Traditional preprocessors replace parameters in the replacement text |
| with their arguments regardless of whether the parameters are within |
| quotes or not. This provides a way to stringize arguments. For |
| example |
| |
| <pre class="smallexample"> #define str(x) "x" |
| str(/* <span class="roman">A comment</span> */some text ) |
| ==> "some text " |
| </pre> |
| <p class="noindent">Note that the comment is removed, but that the trailing space is |
| preserved. Here is an example of using a comment to effect token |
| pasting. |
| |
| <pre class="smallexample"> #define suffix(x) foo_/**/x |
| suffix(bar) |
| ==> foo_bar |
| </pre> |
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