(cpp.info.gz) Differences from previous versions

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 11.4 Differences from previous versions
 =======================================
 
 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.
 
    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.
 
    * -I- deprecated
 
      This option has been deprecated in 4.0.  `-iquote' is meant to
      replace the need for this option.
 
    * Order of evaluation of `#' and `##' operators
 
      The standard does not specify the order of evaluation of a chain of
      `##' operators, nor whether `#' is evaluated before, after, or at
      the same time as `##'.  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.
 
      An example of where this might matter is pasting the arguments `1',
      `e' and `-2'.  This would be fine for left-to-right pasting, but
      right-to-left pasting would produce an invalid token `e-2'.
 
      GCC 3.0 evaluates `#' and `##' at the same time and strictly left
      to right.  Older versions evaluated all `#' operators first, then
      all `##' operators, in an unreliable order.
 
    * The form of whitespace between tokens in preprocessor output
 
       Preprocessor Output, 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.
 
      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.
 
    * Optional argument when invoking rest argument macros
 
      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.
 
    * `##' swallowing preceding text in rest argument macros
 
      Formerly, in a macro expansion, if `##' 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 (*not* the preceding token).  This
      extension is in direct conflict with the 1999 C standard and has
      been drastically pared back.
 
      In the current version of the preprocessor, if `##' 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
      `##' is not a comma, then `##' behaves as a normal token paste.
 
    * `#line' and `#include'
 
      The `#line' directive used to change GCC's notion of the
      "directory containing the current file", used by `#include' with a
      double-quoted header file name.  In 3.0 and later, it does not.
       Line Control, for further explanation.
 
    * Syntax of `#line'
 
      In GCC 2.95 and previous, the string constant argument to `#line'
      was treated the same way as the argument to `#include': backslash
      escapes were not honored, and the string ended at the second `"'.
      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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(cpp.info.gz) Obsolete Features

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