Re: [Boost-users] [preporcessor]Can we use BOOST_PP_SEQ_FOLD_LEFT on sequences with one element?

11 Jul 2008

      ----- Original Message ----- 
From: "JOAQUIN M. LOPEZ MUÑOZ" joaquin@tid.es
________________________________________
De: boost-users-bounces@lists.boost.org 
[boost-users-bounces@lists.boost.org] En nombre de vicente.botet 
[vicente.botet@wanadoo.fr]
Asunto: [Boost-users] [preporcessor]Can we use BOOST_PP_SEQ_FOLD_LEFT 
onsequences with one element?
...
I'm a beginer with the preprocessor library. My goal is to generate from a
sequence (a)(b)(c) => a::b::c and from (a) => a
[...]
...
BOOST_PP_SEQ_FOLD_LEFT(NAME, BOOST_PP_SEQ_HEAD((a)),
BOOST_PP_SEQ_TAIL((a)))
Here lies the problem: (a) does not have a tail (there is no such thing as
an empty PP sequence, unfortunately). I'm afraid you'll have to treat 
sequences
of size 1 separately from the general case.

--
Thanks Joaquin,

the question now is how to treat sequences of size 1 separately from the 
general case.
I have tried with BOOST_PP_IF, but it is no better.

#include 
#include 
#include 

#define XX_SIZE_GREATER_THAN_1(S) BOOST_PP_GREATER(BOOST_PP_SEQ_SIZE(S), 1)
#define XX_QNAME(s, state, x) state::x
#define XX_SEQ_TO_QNAME_N(S) BOOST_PP_SEQ_FOLD_LEFT(XX_QNAME \
    , BOOST_PP_SEQ_HEAD(S) \
    , BOOST_PP_SEQ_TAIL(S))
#define XX_SEQ_TO_QNAME_1(S) BOOST_PP_SEQ_HEAD(S)
#define XX_SEQ_TO_QNAME(S) BOOST_PP_IF(XX_SIZE_GREATER_THAN_1(S)\
    , XX_SEQ_TO_QNAME_N(S) \
    , XX_SEQ_TO_QNAME_1(S))

XX_SEQ_TO_QNAME((a)(b))
XX_SEQ_TO_QNAME((a))

Has the following preprocessor result:
a::b
test.cpp:73:1: macro "BOOST_PP_SEQ_ELEM_III" requires 2 arguments, but only 
1 given
a

And do not compile neither. Any hint?

Vicente