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| <h3 class="section">11.4 Differences from previous versions</h3> |
| |
| <p><a name="index-differences-from-previous-versions-117"></a> |
| This section details behavior which has changed from previous versions |
| of CPP. We do not plan to change it again in the near future, but |
| we do not promise not to, either. |
| |
| <p>The “previous versions” discussed here are 2.95 and before. The |
| behavior of GCC 3.0 is mostly the same as the behavior of the widely |
| used 2.96 and 2.97 development snapshots. Where there are differences, |
| they generally represent bugs in the snapshots. |
| |
| <ul> |
| <li>-I- deprecated |
| |
| <p>This option has been deprecated in 4.0. <samp><span class="option">-iquote</span></samp> is meant to |
| replace the need for this option. |
| |
| <li>Order of evaluation of ‘<samp><span class="samp">#</span></samp>’ and ‘<samp><span class="samp">##</span></samp>’ operators |
| |
| <p>The standard does not specify the order of evaluation of a chain of |
| ‘<samp><span class="samp">##</span></samp>’ operators, nor whether ‘<samp><span class="samp">#</span></samp>’ is evaluated before, after, or |
| at the same time as ‘<samp><span class="samp">##</span></samp>’. You should therefore not write any code |
| which depends on any specific ordering. It is possible to guarantee an |
| ordering, if you need one, by suitable use of nested macros. |
| |
| <p>An example of where this might matter is pasting the arguments ‘<samp><span class="samp">1</span></samp>’, |
| ‘<samp><span class="samp">e</span></samp>’ and ‘<samp><span class="samp">-2</span></samp>’. This would be fine for left-to-right pasting, |
| but right-to-left pasting would produce an invalid token ‘<samp><span class="samp">e-2</span></samp>’. |
| |
| <p>GCC 3.0 evaluates ‘<samp><span class="samp">#</span></samp>’ and ‘<samp><span class="samp">##</span></samp>’ at the same time and strictly |
| left to right. Older versions evaluated all ‘<samp><span class="samp">#</span></samp>’ operators first, |
| then all ‘<samp><span class="samp">##</span></samp>’ operators, in an unreliable order. |
| |
| <li>The form of whitespace between tokens in preprocessor output |
| |
| <p>See <a href="Preprocessor-Output.html#Preprocessor-Output">Preprocessor Output</a>, for the current textual format. This is |
| also the format used by stringification. Normally, the preprocessor |
| communicates tokens directly to the compiler's parser, and whitespace |
| does not come up at all. |
| |
| <p>Older versions of GCC preserved all whitespace provided by the user and |
| inserted lots more whitespace of their own, because they could not |
| accurately predict when extra spaces were needed to prevent accidental |
| token pasting. |
| |
| <li>Optional argument when invoking rest argument macros |
| |
| <p>As an extension, GCC permits you to omit the variable arguments entirely |
| when you use a variable argument macro. This is forbidden by the 1999 C |
| standard, and will provoke a pedantic warning with GCC 3.0. Previous |
| versions accepted it silently. |
| |
| <li>‘<samp><span class="samp">##</span></samp>’ swallowing preceding text in rest argument macros |
| |
| <p>Formerly, in a macro expansion, if ‘<samp><span class="samp">##</span></samp>’ appeared before a variable |
| arguments parameter, and the set of tokens specified for that argument |
| in the macro invocation was empty, previous versions of CPP would |
| back up and remove the preceding sequence of non-whitespace characters |
| (<strong>not</strong> the preceding token). This extension is in direct |
| conflict with the 1999 C standard and has been drastically pared back. |
| |
| <p>In the current version of the preprocessor, if ‘<samp><span class="samp">##</span></samp>’ appears between |
| a comma and a variable arguments parameter, and the variable argument is |
| omitted entirely, the comma will be removed from the expansion. If the |
| variable argument is empty, or the token before ‘<samp><span class="samp">##</span></samp>’ is not a |
| comma, then ‘<samp><span class="samp">##</span></samp>’ behaves as a normal token paste. |
| |
| <li>‘<samp><span class="samp">#line</span></samp>’ and ‘<samp><span class="samp">#include</span></samp>’ |
| |
| <p>The ‘<samp><span class="samp">#line</span></samp>’ directive used to change GCC's notion of the |
| “directory containing the current file”, used by ‘<samp><span class="samp">#include</span></samp>’ with |
| a double-quoted header file name. In 3.0 and later, it does not. |
| See <a href="Line-Control.html#Line-Control">Line Control</a>, for further explanation. |
| |
| <li>Syntax of ‘<samp><span class="samp">#line</span></samp>’ |
| |
| <p>In GCC 2.95 and previous, the string constant argument to ‘<samp><span class="samp">#line</span></samp>’ |
| was treated the same way as the argument to ‘<samp><span class="samp">#include</span></samp>’: backslash |
| escapes were not honored, and the string ended at the second ‘<samp><span class="samp">"</span></samp>’. |
| This is not compliant with the C standard. In GCC 3.0, an attempt was |
| made to correct the behavior, so that the string was treated as a real |
| string constant, but it turned out to be buggy. In 3.1, the bugs have |
| been fixed. (We are not fixing the bugs in 3.0 because they affect |
| relatively few people and the fix is quite invasive.) |
| |
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