Is Boost using legal C++ preprocessing directive syntax?

Question

My (relatively old) C++ compiler choked on this file in Boost, which starts out as:

# /* Copyright (C) 2001
#  * Housemarque Oy
#  * http://www.housemarque.com
#  *
#  * Distributed under the Boost Software License, Version 1.0. (See
#  * accompanying file LICENSE_1_0.txt or copy at
#  * http://www.boost.org/LICENSE_1_0.txt)
#  */
#

Is this really legal C++? What's the rule on the syntax of preprocessor tokens?

Maggy

Oh no! Don't tell me you use Turbo C++ too!! – Pinsk

@CodyGray: Heavens, no! It was an old VC compiler. – Maggy

What version, VC 4.0? I'm curious why can't you upgrade – Boresome

@Viet: 6.0. It was just a compatibility test for a library; I've already upgraded. – Maggy

Answer 1

Yes, a line containing only # and whitespace is explicitly permitted by the standard §16 [cpp]:

control-line:
   # include pp-tokens new-line
   # define identifier replacement-list new-line
   # define identifier lparen identifier-list_opt) replacement-list new-line
   # define identifier lparen ... ) replacement-list new-line
   # define identifier lparen identifier-list, ... ) replacement-list new-line
   # undef identifier new-line
   # line pp-tokens new-line
   # error pp-tokens_opt new-line
   # pragma pp-tokens_opt new-line
   # new-line

Note that comments are replaced by whitespace at translation phase 3, that is before the preprocessor.

Unfounded

+1. The last one is all that I was looking for, and actually answers the question : # new-line. – Warrin

@Nawaz: the OP asks "What's the rule on the syntax of preprocessor tokens" so providing some context won't hurt. – Unfounded

@Nawaz: Haha take a look at the edit history on my post, it was amusing. :) – Maggy

@Mehrdad: Haha.. we were doing the same thing at the same time, hence SO merged them. – Warrin

Why is boost commenting and adding blank preprocessor token? Is there any advantage in doing that instead of using just simple regular block comment? – Pamphylia

@LieRyan: look at the file linked in question. The author decided that it is easier to start all lines in the file with #. It's just such a coding style. Personally I don't like it. – Unfounded