Should this succession of two lines compile?

#define COMMENT_START /##*
COMMENT_START /**/

Thanks,

Andrei



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Andrei Alexandrescu wrote in message <36d6cf73.0@10.1.1.65>...
>
>Should this succession of two lines compile?
>
>#define COMMENT_START /##*
>COMMENT_START /**/

No, it need not.  It should produce this token sequence
    /    *
This is because comments are replaced by one space
character at translation phase 3 as defined at section
2.1 of the C++ standard.  Processing of ## occurs at
phase 4.  There is no mention of comment removal
for any translation phase other than 3.

16.3.1 states that if the result of the ## concatenation "is
not a valid preprocessing token, the behavior is undefined."

Section 2.4 on preprocessing tokens, at paragraph 2
clearly indicates that comments are preprocessing
token separators rather than preprocessing tokens.
Paragraph 1 further indicates this, requiring that
each preprocessing token "shall have the lexical
form of a keyword, an identifier, a literal, an operator
or a punctuator."

As a practical matter, due to the semi-recursive
nature of lexical processing, (required because
of the possibility of the ## and # operators), a
comment nominally found during phase 4 will
likely also be converted to a space.  But it is
foolishness to count on this.  The C++ standard
does not require your code to compile.

--Larry Brasfield
Above opinions may be mine alone.
X-Replace-Address
(Humans may reply at unundered larry_br@sea_net.com )
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