Re: [Boost-users] [preprocessor] BOOST_PP_VARIADIC_SIZE() and g++ in 1.52

Subject: Re: [Boost-users] [preprocessor] BOOST_PP_VARIADIC_SIZE() and g++ in 1.52
From: Nathan Ridge (zeratul976_at_[hidden])
Date: 2012-12-12 11:59:09

> > > In fact, when I change the definition of the intercept() macro below to:
> > >
> > > #define intercept(FIRST, ARGS...) \
> > > targetfunc(transform(FIRST) \
> > > , boost::lexical_cast<std::string>(BOOST_PP_VARIADIC_SIZE(ARGS)))
> > >
> > > I see that BOOST_PP_VARIADIC_SIZE() expands to 1 for
> > > intercept("first") as well as for intercept("first", "second").
>
> > As I understand it, it is impossible to reliably detect an empty
> > argument list in all cases, so BOOST_PP_VARIADIC_SIZE() returns
> > 1 for an empty argument list. Take a look at this thread [1] for
> > an explanation and a workaround.
>
> Hmm, am I trying to solve the wrong problem?
>
> The briefest explanation of what I want is to take a macro argument
> list with zero or more arguments and construct a function argument
> list with one more (initial) argument than that.
>
> I just need MACRO() to expand to function(first), but MACRO(anything)
> to expand to function(first, anything).
>
> Is there some more reliable way than
> BOOST_PP_COMMA_IF(BOOST_PP_VARIADIC_SIZE()) to make the comma appear
> only when required?

The preprocessor only really understand commas and parentheses. So,
as far as it's concerned, the definition of "the size of a variadic
argument list" is "the number of (top-level) commas in the list 
plus one".

You want to make an additional distinction - in the case where there
are no top-level commas, you want to distinguish between the case
where there are some tokens present, and the case where there are 
none. As Paul Mensonides explains in the thread I linked to, this
is not possible in the general case. 

It *is* possible to write a version of BOOST_PP_VARIADIC_SIZE() that
works the way you want in many cases, quite possibly including your 
use cases. I believe Boost.Preprocessor does not provide such a
version on principle (because it would not work in all cases).
However, nothing stops you from using your own version that does this.
Gennadiy Rozental posted such a version in the first post of the
thread I linked to.

Regards,
Nate