September 2009 - Boost - lists.preview.boost.org

Interest in flat_iterator for nested containers?
by Christoph Heindl 01 Oct '09

01 Oct '09

Hi, is there any interest in an iterator that provides a flat view of nested containers? For example ---------- using namespace boost::assign; typedef std::list<int> inner_container_type; typedef std::vector<inner_container_type> outer_container_type; outer_container_type elements(6); elements[1] += 1,2,3; elements[2] += 4,5; elements[4] += 6; BOOST_CHECK(std::equal( make_flat_iterator(elements.begin(), elements.end()), make_flat_iterator(elements.end(), elements.end()), list_of(1)(2)(3)(4)(5)(6).begin()) ); ---------- If so, I'll be glad to share my code with the boost community. I attach my current state of development and a single test suite. The current state of development contains a class named flat_iterator, a make_flat_iterator function and a set of meta-functions to derive required types. flat_iterator derives from boost::iterator_adaptor. It derives an inner_iterator type from the passed iterator to be adapted (outer_iterator type). The value_type of outer_iterator is required to model STL container concept. The value_type passed to boost::iterator_adaptor is const-qualified if the reference type of the outer_iterator or inner_iterator type is const-qualified (please see question 1 below). The traversal category of flat_iterator is bidirectional if the the outer_iterator and inner_iterator type are at least bidirectional. It models forward traversal else (question 2). flat_iterators can be stacked over each other to recursively flatten a hierarchy of arbitrary levels. ---------- using namespace boost::assign; typedef std::list<int> level_0; typedef std::vector<level_0> level_1; typedef std::vector<level_1> level_2; level_2 elements(10); elements[2] += list_of(1)(2); elements[4] += list_of(3)(4); typedef flat_iterator<level_2::iterator> flat_iterator_outer; typedef flat_iterator<flat_iterator_outer> flat_iterator_inner; flat_iterator_outer outer_begin(elements.begin(), elements.end()); flat_iterator_outer outer_end(elements.end(), elements.end()); flat_iterator_inner inner_begin(outer_begin, outer_end); flat_iterator_inner inner_end(outer_end, outer_end); BOOST_CHECK(std::equal(inner_begin, inner_end, list_of(1)(2)(3)(4).begin())); ---------- Open questions: * Currently the the value_type passed to boost::iterator_adaptor is const qualified based on required reference type of the outer_iterator, which renders the code for input iterators unusable, right? What's the best way to achieve const correctness? * The traversal category of flat_iterator is supposed to be the 'greatest common divisor' of the inner and outer iterator type. Is there a meta-function in boost that provides that type? Currently bidirectional traversal is chosen if both, inner and outer iterator, are at least of bidirectional traversal. Else forward traversal is chosen (which is of course incorrect when incrementable is passed). * Stacking flat_iterators to flatten out a hierarchy of arbitrary levels is possible but currently requires a lot of typing. I think that applying some recursive meta-functions to generate a correct iterator types for such situations should be possible but is beyond my current knowledge of meta-programming. Kind regards, Christoph

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boost::variant and "Current Approach: Temporary Heap Backup" vs "An Initial Solution: Double Storage"
by Peter Foelsche 01 Oct '09

01 Oct '09

1) boost::variant and "Current Approach: Temporary Heap Backup" vs "An Initial Solution: Double Storage" Please provide both solutions. A potential heap allocation may prevent certain application areas. 2) I also do not understand that exception safety requires that the variant is never empty (e.g. empty() returning always false). What is the use of calling the constructor (of which type btw?) if somebody creates an unintialized variant variable? Or: In the following piece of code is the std::string constructor being called? boost::variant<std::string, int> s; Thanks Peter

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Re: [boost] On the Road to Aspen (BoostCon 2010)
by Pete Bartlett 29 Sep '09

29 Sep '09

I'm sorry somehow my message was sent before I was ready... I wrote: >Many users of Boost are in corporate environments. Many corporate environments block access to Facebook and/or >Twitter (mine blocks both) domains. >It'd be great therefore if Boost didn't move too much content there if equivalent .. functionality is available on boost.org. I understand that many will think the problems lie with unenlightened corporate policies rather than boost, but the latter has a rich and noble history of BOOST_WORKAROUNDing the mistakes of others :-). Pete

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[smart_ptr] comparison of intrusive_ptr objects
by Dmitry Vinogradov 29 Sep '09

29 Sep '09

Does anybody know why intrusive_ptr has ==, != and < operators, but the rest ones are missing (i.e. >, <=, >=)? It makes hard to use intrusive_ptr in generic code.

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Using some of Boost on i5/OS but still problems
by Jon Brisbin 29 Sep '09

29 Sep '09

I was able to use Boost FOREACH in my tests, now I'm trying to use Boost optionals in a C++ ILE module. The compiler is yelling at me, though: "/home/jbrisbin/include/boost/optional/optional.hpp", line 434.66: CZP0210(30) "T" is not a base class of "basic_string<char,std::char_traits<char>,std::allocator<char> >". "/home/jbrisbin/include/boost/optional/optional.hpp", line 434.10: CZP0700(0) The previous message was produced while processing "boost::optional_detail::optional_base<std::basic_string<char,std::char_ traits<char>,std::allocator<char> > >::destroy_impl(is_not_reference_tag)". "/home/jbrisbin/include/boost/optional/optional.hpp", line 408.9: CZP0700(0) The previous message was produced while processing "boost::optional_detail::optional_base<std::basic_string<char,std::char_ traits<char>,std::allocator<char> > >::destroy()". "/home/jbrisbin/include/boost/optional/optional.hpp", line 237.24: CZP0700(0) The previous message was produced while processing "boost::optional_detail::optional_base<std::basic_string<char,std::char_ traits<char>,std::allocator<char> > >::~optional_base()". "/home/jbrisbin/include/boost/optional/optional.hpp", line 145.7: CZP0700(0) The previous message was produced while processing "boost::optional_detail::optional_base<std::basic_string<char,std::char_ traits<char>,std::allocator<char> > >::__dftbdt()". "/home/jbrisbin/include/boost/optional/optional.hpp", line 522.5: CZP0700(0) The previous message was produced while processing "boost::optional<std::basic_string<char,std::char_traits<char>,std::allo cator<char> > >::~optional()". "/home/jbrisbin/include/boost/optional/optional.hpp", line 454.7: CZP0700(0) The previous message was produced while processing "boost::optional<std::basic_string<char,std::char_traits<char>,std::allo cator<char> > >::__dftdt()". "/home/jbrisbin/src/boosttest.cpp", line 6.5: CZP0700(0) The previous message was produced while processing "main(int, char **)". Any hints on what I need to hack here? This seems to be a problem with: void destroy_impl ( is_not_reference_tag ) { get_ptr_impl()->T::~T () ; m_initialized = false ; } I understand what's happening, I'm just not sure how to make the i5/OS compiler happy with this. I keep running into little quirks like this where the AS400 needs a little different treatment. IBM says Boost won't work on the AS400, which I'm finding to be inaccurate at best. It DOES work, just not everything straight out of the box. Lots of things work fine, but some need a little bit of tweaking. What else can I try to get the destructor called? Jon Brisbin Portal Webmaster NPC International, Inc.

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On the Road to Aspen (BoostCon 10)
by Joel de Guzman 29 Sep '09

29 Sep '09

Hi, The road to Aspen starts now. I've been to all three BoostCon events since its inaugural conference in 2007. I can tell you, the experience is definitely amazing. I've been a Boost developer since 2001. Boost is like a virtual family to me. From 2001, I've known and collaborated with countless people, many of which I haven't actually met, until the first BoostCon in 2007. Amazing people like Hartmut Kaiser, Dan Marsden, Eric Niebler, to name a few, were just names. Before BoostCon, I often wished to get to see the faces behind the names. It was exhilarating to finally meet these folks, have a couple of beer (and Pizza!), talk about C++, library development, the wondrous mountains of Aspen, the trees, the bears and coyotes (hah!). If this looks too much like a friendly post, well, it is! I'd like to break a bit from the highly technical discussions taking place in these mailing lists and point you to a FaceBook group named, ermm, "BoostCon" :-) http://www.facebook.com/group.php?gid=125839305706 My task is to invite you guys to join in. You do not have to be a former BoostCon attendee. Everyone here is invited to join. If you want to see the "faces" behind the names, get to know more of the people behind Boost, get to know more friends that share your passion for Boost, C++, templates, etc., in a more relaxed venue, this is the place. If you have even the slightest inkling to attend a future conference, well, join in. Here we can discuss things like sharing rooms to cut cost, where to find the best food in town, car pooling etc. In preparation time prior to the conference, we can discuss matters like which kind of talks you want to see, keynote speakers you'd be interested in seeing, etc. For past BoostCon attendees, here’s a chance to meet up again. Hey, even if you are not planning to attend, I'd like to invite you anyway. Consider this group as a friendly face of Boost :-). Hopefully, the friendly atmosphere will make you attend anyway ;-) Cheers, -- Joel de Guzman http://www.boostpro.com http://spirit.sf.net

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Pretty type_info::name string?
by Emil Dotchevski 28 Sep '09

28 Sep '09

Do we have in Boost something that would attempt to return the pretty version (meaning, it'd fall back to returning the ugly version) of the std::type_info::name string? Emil Dotchevski Reverge Studios, Inc. http://www.revergestudios.com/reblog/index.php?n=ReCode

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Trunk tests broken, missing file
by Belcourt, Kenneth 27 Sep '09

27 Sep '09

error: Unable to find file or target named error: 'algorithm/nview.cpp' error: referred from project at error: '../libs/fusion/test'

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[fusion] inconsistency in size metafunction
by Jean-Louis Leroy 27 Sep '09

27 Sep '09

Consider : #include <boost/fusion/include/vector.hpp> #include <boost/fusion/include/push_back.hpp> #include <boost/fusion/include/size.hpp> #include <boost/mpl/assert.hpp> #include <boost/type_traits/is_same.hpp> int main() { using namespace boost; using namespace fusion; typedef vector<int, int> seq1; typedef result_of::push_back<vector<int>, int>::type seq2; BOOST_MPL_ASSERT((is_same<result_of::size<seq1>::type, result_of::size<seq2>::type>)); } One would not expect this to fail, however it does : 'boost::mpl::assertion_failed' : cannot convert parameter 1 from 'boost::mpl::failed ************boost::is_same<T,U>::* ***********' to 'boost::mpl::assert<false>::type' 1> with 1> [ 1> T=boost::mpl::int_<2>, 1> U=boost::mpl::integral_c<int,2> 1> ] J-L

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Boost.Iterator filter_iterator issues
by Christoph Heindl 27 Sep '09

27 Sep '09

Hi, I think I've found an issue with boost::filter_iterator when decrementing a valid iterator without having any predicate evaluating to true on its way back. filter_iterator.hpp line# 93 states void decrement() { while(!this->m_predicate(*--(this->base_reference()))){}; } which will ?clearly? pass beyond the initial start iterator if no predicate evaluates to true. Is it a requirement for the iterator being adapted by filter_iterator to catch decrements past the containers begin range? The following test will iterate 'iter' past containers begin iterator which will trigger a debug assertion on MSVC. template<class _Iterator> void dec_iterator(_Iterator iter) { BOOST_CHECK(*iter == 2); --iter; // iterates past begin } BOOST_AUTO_TEST_CASE(test_filter_iterator) { using namespace boost::assign; typedef std::vector<int> container_type; container_type values = list_of(0)(1)(2); dec_iterator(boost::make_filter_iterator(std::bind2nd(std::greater<int>(), 1), values.begin(), values.end())); } Please help me clarifying this issue. Best regards, christoph

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