------- Comment #3 from b07584 at freescale dot com 2009-04-08 17:46 -------
(In reply to comment #0)
> This testcase:
>
> #define debug(format, ...) format, ## __VA_ARGS__)
> debug(X);
> debug(Y, 1, 2);
> debug(Y, 1, 2 );
> debug(Z, );
> debug(W,);
>
> Should preprocess to:
>
> X);
> Y, 1, 2);
> Y, 1, 2);
> Z, );
> W, );
>
> not:
>
> X);
> Y, 1, 2);
> Y, 1, 2);
> Z,);
> W,);
>
>
> Z/W should have spaces.
>
Chris, I want to make sure I understand what you are saying. With:
#define debug(format, ...) format, ## __VA_ARGS__)
For the the invocation:
debug(X);
- the argument is omitted and hence the comma preceeding ## in the body is
removed in accordance with GNU CPP's extended semantics, yielding:
X);
In contrast, for the invocations:
debug(Z, );
debug(W,);
when you say, the output ought to be:
Z, );
W, );
- is it because a space preceeds the ## in the macro definition (i.e. body)?
So, if the definition were to be:
#define debug(format, ...) format,## __VA_ARGS__)
instead, then:
debug(Z, );
debug(W,);
preprocessing to:
Z,);
W,);
would be OK? This reasoning about the appearance (or non-appearance) of space
is similar to Neil's explanation in the Cpplib Internals manual (for GCC
version 4.4.0) in the section Token Spacing:
#define add(x, y, z) x + y +z
sum = add (1,2, 3)
preprocesses to:
sum = 1 + 2 +3
However in the case at hand, per ISO/IEC 9899:TC2 Committee Draft May 6,
2005 WG14/N1124, 6.10.3.3 "The ## operator", constraint 2: "... however, if an
argument consists of no preprocessing tokens, the parameter is replaced by a
placemarker preprocessing token instead." Further, in constraint 3:
"... concatenation of a placemarker with a non-placemarker preprocessing token
results in the non-placemarker preprocessing token."
By that, the expansion sequence for (say):
debug(Z, );
(with the first definition - i.e. the one with a space before the ##) would
be:
debug(Z, );
->
format, ## __VA_ARGS__)
(CPP_COMMA gets the PASTE_LEFT flag set, and <placemarker> is only conceptually
depicted per constraint 2)
->
CPP_NAME CPP_COMMA[PASTE_LEFT] <placemarker> CPP_CLOSE_PAREN CPP_SEMICOLON
(per constraint 3, CPP_COMMA and <placemarker> concatenate to CPP_COMMA)
->
CPP_NAME CPP_COMMA CPP_CLOSE_PAREN CPP_SEMICOLON
So, the space before the ## does not make it to the expansion because this is
the paste operation in operation - as it proceeds normally.
Or perhaps, I am missing something? Kindly explain. Thanks.
--
b07584 at freescale dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |b07584 at freescale dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35010
