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From: shankar@hpclscu.HP.COM (Shankar Unni)
Newsgroups: comp.lang.c++
Subject: "//" comments and #define's : problems
Message-ID: <1000019@hpclscu.HP.COM>
Date: 11 Jul 89 17:06:50 GMT
Organization: Hewlett-Packard Calif. Language Lab
Lines: 53

When fixing up a "cpp" to handle "//"-style comments, I came across an
interesting problem.

Comments on #define's are preserved by cpp when sucking up the define, and
later, after substitution, subsequent rescans of the line simply strip out
the comment. (This is how it makes token-pasting, etc work).

When the comment is a "//" comment, the following happens:

    #define a b // comment
    x a y

turns into

    x b // comment y

and then into

    x b

(I.e. the "y" is stripped out).

This is an intrinsic problem with "//"-comments on #defines, because if you
use the "pass comments" option on cpp (usually -C on sysV-based systems),
the same thing will happen (you'll get stage two above, and then cfront
will strip out the "y" token).

Question:

How should a "//"-comment on a #define work?

  (a) Completely stripped out when processing #define. Not passed through
      even when requesting "pass comments". (This is my preference).

  (b) Leave it as is. I.e. specify that a "//"-comment on a #define will
      have "undefined" (i.e. nasty and unpredictable) effects. Warn the
      user when detected.

  (c) Do some fudge like have "cpp" insert a "/*" and "*/" around the
      "//"-style comment, as in

      x b /* // comment */ y

I'm interested in hearing opinions on this (esp. from AT&T, since they are
asking their cfront customers/VARs to provide their own cpp).  Shouldn't this
be standardized somewhere?

This is one more reason to adopt a *standard* cpp (preferably an ANSI C style
cpp), with well-defined "//"-comment semantics.
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Shankar Unni                                   E-Mail: 
Hewlett-Packard California Language Lab.     Internet: shankar@hpda.hp.com
Phone : (408) 447-5797                           UUCP: ...!hplabs!hpda!shankar