Re: [Boost-bugs] [Boost C++ Libraries] #4455: clean up preprocessor/seq/fold_left.hpp

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #4455: clean up preprocessor/seq/fold_left.hpp
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2010-07-20 16:05:41

#4455: clean up preprocessor/seq/fold_left.hpp
----------------------------------+-----------------------------------------
  Reporter:  anonymous            |       Owner:  no-maintainer
      Type:  Library Submissions  |      Status:  new          
 Milestone:  Boost 1.44.0         |   Component:  preprocessor 
   Version:  Boost 1.44.0         |    Severity:  Optimization 
Resolution:                       |    Keywords:               
----------------------------------+-----------------------------------------

Comment (by steven_watanabe):

 Replying to [ticket:4455 anonymous]:
 > step 1:
 >
 > The definition of BOOST_PP_SEQ_FOLD_LEFT entails a call to
 BOOST_PP_AUTO_REC. In short, this call turns out to be completely
 pointless, as it will always return 1 as result, and can be simply
 replaced by this constant.
 >
 > This can be seen as follows:
 >
 > BOOST_PP_AUTO_REC operates on macros P(n), that
 > - take integers from 1 to 256 as paramater;
 > - are binary, i.e. yields only 0 or 1 as a result;
 > - are monotonely rising, i.e. there is an integer n such that P(i) == 0
 for i < n and == 1 otherwise.
 > BOOST_PP_AUTO_SEC is designed to find the 'leap position' where P
 assumes 1 for the first time.
 >
 > BOOST_PP_SEQ_FOLD_LEFT_P is such a P(n). Although the replacement of
 BOOST_PP_SEQ_FOLD_LEFT_P looks complex, a closer look at
 >
 >
 > {{{
 > BOOST_PP_SEQ_FOLD_LEFT_I_ ## n(BOOST_PP_SEQ_FOLD_LEFT_O, BOOST_PP_NIL,
 (nil), 1)
 > }}}
 >
 >
 > reveals, that this expression, independently of n, will always expand to
 BOOST_PP_NIL, and after some more replacing, BOOST_PP_SEQ_FOLD_LEFT_P
 turns out to be constantly 1. Consequently, BOOST_PP_AUTO_REC, applied to
 this (constant) macro, will yield 1 as a result.

 Nope.  BOOST_PP_SEQ_FOLD_LEFT_P expands to 1 iff. **we are not currently
 in an expansion of BOOST_PP_SEQ_FOLD_LEFT_I_ ## n**.  If
 BOOST_PP_SEQ_FOLD_LEFT_I_ ## n is being expanded, then it will not be
 expanded again, since recursive calls to a macro are not expanded.
 BOOST_PP_AUTO_REC is used to find the first available index that is not on
 the macro expansion stack.  This allows nested calls to
 BOOST_PP_SEQ_FOLD_LEFT.  Note that the s parameter to the macro is not the
 index in the sequence.  It is the next available index for reentering
 BOOST_PP_FOLD_LEFT.  See
 http://www.boost.org/libs/preprocessor/doc/ref/seq_fold_left.html.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/4455#comment:5>
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