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bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escape
From: |
Alan Mackenzie |
Subject: |
bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped. |
Date: |
Thu, 24 Sep 2020 19:57:42 +0000 |
Hello, Michael.
On Thu, Sep 24, 2020 at 14:52:16 -0400, Michael Welsh Duggan wrote:
> Alan Mackenzie <acm@muc.de> writes:
> > On Wed, Sep 23, 2020 at 11:01:59 +0200, Mattias Engdegård wrote:
> >> Sorry if I misunderstood, but since when do backslashes escape */ in C?
> > Since forever, but only in the CC Mode test suite. :-(
> > I just tried it out with gcc, and it seems that \*/ does indeed end a
> > block comment. But an escaped newline doesn't end a line comment,
> > instead continuing it to the next line. So I got confused. Thanks for
> > pointing out the mistake.
> > It seems that as well as the existing variable
> > comment-end-can-be-escaped, we need a new one, say
> > line-comment-end-can-be-escaped, too. In C and C++ modes, these would
> > be nil and t respectively.
> But where does it say that backslashes escape */ in C++?
Nowhere. :-(
There has been a test in the CC Mode test suite for many years which
assumed this (but was disabled for existing (X)Emacs versions, waiting
for a new Emacs version to be "fixed").
> The C++ 14 standard (and it hasn't changed through C++ 20) says:
> 2.7 Comments [lex.comment]
> The characters /* start a comment, which terminates with the
> characters */. These comments do not nest. The characters // start
> a comment, which terminates immediately before the next new-line
> character.
For all the difference it makes, Emacs assumes the comment ends _after_
the NL.
> If there is a form-feed or a vertical-tab character in such a
> comment, only white-space characters shall appear between it and
> the new-line that terminates the comment; no diagnostic is
> required.
I didn't know that. Emacs/CC Mode doesn't code up this subtlety. It
probably isn't worth bothering about.
> [ Note: The comment characters //, /*, and */ have no special
> meaning within a // comment and are treated just like other
> characters. Similarly, the comment characters // and /* have no
> special meaning within a /* comment. — end note ]
Additionally, an escaped newline continues a comment onto the next line.
This happens, notionally, at a very early stage of compilation where a
backslash followed by NL anywhere get replaced by a space. I think that
even two backslashes followed by NL would get replaced by backslash,
space.
> --
> Michael Welsh Duggan
> (md5i@md5i.com)
--
Alan Mackenzie (Nuremberg, Germany).
- bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped., (continued)
- bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped., Mattias Engdegård, 2020/09/23
- bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped., Alan Mackenzie, 2020/09/23
- bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped., Alan Mackenzie, 2020/09/24
- bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped., Stefan Monnier, 2020/09/24
- bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped., Alan Mackenzie, 2020/09/24
- bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped., Stefan Monnier, 2020/09/24
- bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped., Michael Welsh Duggan, 2020/09/24
- bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped.,
Alan Mackenzie <=
- bug#43558: [PATCH]: Fix (forward-comment 1) when end delimiter is escaped., Michael Welsh Duggan, 2020/09/24