On Tue, Jul 12, 2011 at 3:43 AM, Brian Wood <woodbrian77@gmail.com> wrote:

B"H

I've been testing my marshalling support for sub_range and have a question.а
In reviewing some old posts and notes, it looks like the following used to
compile:

boost::sub_range<std::list<int32_t> >а rng;

boost::sub_range<std::list<int32_t> > still works on Boost 1.47.0. I retested this explicitly after your report and found no defects.

Today though I get the following error message:

g++а -Os -s -W -Wall -I. -I ./loki-0.1.7/include/loki/flex -I /home/brian/boost_1_47_0_beta1аа -c -o local_messages_d.cg.o local_messages_d.cg.cc
In file included from /home/brian/boost_1_47_0_beta1/boost/range/iterator_range.hpp:13:0,
аааааааааааааааа from /home/brian/boost_1_47_0_beta1/boost/range/sub_range.hpp:23,
аааааааааааааааа from ./local_messages_d.cg.hh:5,
аааааааааааааааа from local_messages_d.cg.cc:1:
/home/brian/boost_1_47_0_beta1/boost/range/iterator_range_core.hpp: In member function Сboost::iterator_range<IteratorT>::difference_type boost::iterator_range<IteratorT>::size() const [with IteratorT = std::_List_iterator<int>, boost::iterator_range<IteratorT>::difference_type = int]Т:
/home/brian/boost_1_47_0_beta1/boost/range/sub_range.hpp:117:58:аа instantiated from Сboost::sub_range<ForwardRange>::difference_type boost::sub_range<ForwardRange>::size() const [with ForwardRange = std::list<int>, boost::sub_range<ForwardRange>::difference_type = int]Т
local_messages_d.cg.cc:22:33:аа instantiated from here
/home/brian/boost_1_47_0_beta1/boost/range/iterator_range_core.hpp:252:32: error: no match for Сoperator-Т in С((const boost::iterator_range<std::_List_iterator<int> >*)this)->boost::iterator_range<std::_List_iterator<int> >::m_End - ((const boost::iterator_range<std::_List_iterator<int> >*)this)->boost::iterator_range<std::_List_iterator<int> >::m_BeginТ
/usr/lib/gcc/i686-redhat-linux/4.5.1/../../../../include/c++/4.5.1/bits/stl_bvector.h:179:3: note: candidate is: ptrdiff_t std::operator-(const std::_Bit_iterator_base&, const std::_Bit_iterator_base&)
/home/brian/boost_1_47_0_beta1/boost/range/iterator_range_core.hpp:253:13: warning: control reaches end of non-void function

Is this to be expected?а I don't know if a specialization for std::list
in this regard would be useful, but seems possible.

This compiler error is to be expected because your code is calling the size() member function. The size() member function requires that the underlying range is a model of the RandomAccessRange Concept. Since you are using a sub_range of a std::list and the std::list has bidirectional iterators, the sub_range is a model of the BidirectionalRange Concept.

This has changed between boost versions. The boost::size(Range&) function was modified to only work with ranges that model the RandomAccessRange Concept because the performance complexity was inconsistent with typical expectations. For example, in your case the sub_range size would have been O(N) despite the container probably having an O(1) implementation of size(). The complexity of std::list size() is not mandated by the standard to be O(1), even though it is recommended. If one wishes to pay the price and wishes to have a more general 'size' calculation I recommend using boost::distance(Range&).

If I change the list to a vector, everything compiles fine.
boost::sub_range<std::vector<int32_t> >а rng;

Yes it would, since the sub_range is now a model of the RandomAccessRange Concept.

The following sentence from the docs doesn't make sense to me.
"counting_range is a function to generator that generates an iterator_range wrapping a counting_iterator (from Boost.Iterator). "

That's because it doesn't make any sense! Thanks for highlighting this documentation defect. I meant to state that counting_range wraps the range passed as a parameter using counting_iterator instances. The counting_iterator implementation comes from Boost.Iterator.

I hope this helps,

Neil Groves

Regards,
Brian Wood
Ebenezer Enterprises
http://webEbenezer.net