Re: [boost] [PREPROCESSOR] Possible improvement for BOOST_PP_VARIADIC_SIZE

Subject: Re: [boost] [PREPROCESSOR] Possible improvement for BOOST_PP_VARIADIC_SIZE
From: paul Fultz (pfultz2_at_[hidden])
Date: 2013-09-10 01:59:31

> Currently,
> 
> BOOST_VARIADIC_SIZE()
> 
> evaluates to 1. 

In the preprocessor, variadic arguments are always one or more. If you need
zero or more arguments instead should use a tuple or a sequence. I'm not sure
that Boost supports zero-sized tuples or sequences, yet.

> Based on http://gustedt.wordpress.com/2010/06/08/detect-empty-macro-arguments/
> it can be improved to return 0 for an empty argument list.

Heres an excerpt about tuples from Chaos PP documentation(I'm not sure there's
a link on the web for it), that explains it well(perhaps we should adapt this
to boost docs since this question comes up often):

Several problems arise from this situation when using tuples as data
structures. The first is that there is no general way of detecting the
difference between () and (1). It is possible to get fairly close in various
ways by restricting the types of elements, but a general solution is
impossible.

Even if such detection **was** possible, another problem surfaces. Consider
again the (,,) tuple, and consider an algorithm that operates on each element
of a tuple. If the algorithm was to attempt to individually process elements
one- by-one, it might use CHAOS_PP_TUPLE_HEAD and CHAOS_PP_TUPLE_TAIL.
Everything is fine for CHAOS_PP_TUPLE_TAIL((,,)) (which yields (,)). However,
CHAOS_PP_TUPLE_TAIL((,)) yields () (i.e. the theoretical nil tuple), and an
element has been silently skipped.

The underlying problem is that emptiness (or "nothing") is a useful and
perfectly valid form of data in its own right.

As a consequence, algorithms and primitives that operate on tuples
algorithmically need to use a distinct representation for nil that has no
ambiguity. This representation is CHAOS_PP_TUPLE_NIL() which simply expands to
nothing. This model always treats () as a unary tuple, and
CHAOS_PP_TUPLE_TAIL(()) expands to nothing.

Paul Fultz II