There's a few things wrong with this.
Firstly you write BOOST_PP_IIF but include
- you probably meant BOOST_PP_IF.
Well, includes which includes
,
and I thought I stay at the top level of the boost/preprocessor
directory
as the content of subdirectories may become subject of change.
But more seriously your "condition" is not a number known at
preprocessor-time - i.e. it won't evaluate to a number at
pre-processor time, only at compile-time when the
boost::is_convertible template magic kicks in. Thus
BOOST_PP_IF wouldn't expand as you expect.
Hmm, but then, what is the advantage compared to an ordinary #if
statement? Actually, I thought that BOOST_PPP_IF is a wrapper for
some template "if", e.g. like
template struct IF {typedef Then
RET;};
template struct IF {typedef
Else RET; };
Was this assumption wrong?
However, you aren't even seeing an error related to that problem.
BOOST_PP_COMMA() is indeed being evaluated before the
BOOST_PP_IIF() and thus the compiler is complaining about 4
arguments to the latter macro - compilers do vary in how they
expand macros - search the developer's list for Paul
Mensonides' essay "How macro expansion works" for more - but
you will be safe if your _force_ the COMMA macro to be
expanded after the IF one. The following trick occurs over
and over in writing preprocessor code:
BOOST_PP_IF( X , BOOST_PP_COMMA , BOOST_PP_EMPTY )()
Because BOOST_PP_COMMA and BOOST_PP_EMPTY are function-like
macros they cannot possibly expand until they are "next to"
the (). This can only happen once the BOOST_PP_IF has been
evaluated. I.e. the above example will evaluate to a comma if
X is non-zero and nothing otherwise....
Thanks for this explaination, that's really an interesting trick. It's
a pity that it doesn't help in this case, but it may be useful
otherwise.
I'd like to know if there is common name for this trick as I
find '"macro name inside the IF, macro arguments
outside"-trick' a bit verbose when discussing with colleagues.
Pete
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