May 2005 - Boost-users - lists.preview.boost.org

Re: Metaprogramming woes
by David Abrahams 31 May '05

31 May '05

"White Wolf" <wolof(a)freemail.hu> writes: > Hi Dave, Hi Atilla, In future please post your questions to the boost-users list so that the whole community can benefit. > I am trying to do something very simple, but I am going bananas. I have > types, each of them has a "static member" (enum label) called nr_. I > make a boost::mpl::list of them, and I would like to add them all > together. But if I write _2::nr_, it says there is no nr_. Well, the author of _2 can't anticipate all the nested values you might be using. _2 is not magic; it's just a type. // !! untested code !! #include <boost/mpl/size_t.hpp> #include <boost/mpl/plus.hpp> using namespace mpl::placeholders; // A metafunction to get an Element's ::nr_ value template <class Element> struct get_nr_ { // all metadata are types, so wrap it in a type. typedef mpl::size_t<Element::nr_> type; }; typedef mpl::fold< some_list, mpl::size_t<0> , mpl::plus< _1 , get_nr_<_2> > >::type sum; // sum::value contains the value you're interested in. > I fugured it wants a type. So I have made a typedef inside called > type, which has a boost::mpl::integer_c<size_t, nr_> in it. But it > does not work, it gives me back the type of the last list element as > a result. I try to use mpl::accumulate. Yes, that's a synonym for fold if you have a version of the MPL that includes accumulate. > Finally I guess I will have to use transform in some way, because I > need a list which has an mpl::pair (if there is one) http://www.boost.org/libs/mpl/doc/refmanual/pair.html > of the original type and the sum of all nr_'s before it. Seems like it's still a job for fold. In this case I'd use mpl::vector because lists can only be push_front-ed and you probably don't want to end up with your elements in "reverse" order. typedef mpl::fold< some_list , mpl::pair< // initial state mpl::vector0 // initial sequence , mpl::size_t<0> // initial sum > , mpl::pair< // the state at each step is a pair containing mpl::push_front< // added to the front of... mpl::first<_1> // the sequence from the previous step , mpl::pair< // a new element, a pair containing _2 // the original type , mpl::plus< // and the sum of mpl::second<_1> // the sum from previous step , get_nr_<_2> // the ::nr_ of the original type > > > , mpl::plus< // and the sum of mpl::second<_1> // the sum from previous step , get_nr_<_2> // the ::nr_ of the original type > > >::type pair_seq_sum; typedef pair_seq_sum::first new_sequence; So this is pretty verbose using lambdas because the sum is repeated, among other things. You might do better to build a custom metafunction that gets you the new state: template <class PrevState, class Element> struct next_state { typedef typename PrevState::first prev_sequence; typedef typename PrevState::second prev_sum; typedef mpl::size_t<(prev_sum::value + Element::nr_)> new_sum; typedef typename mpl::push_back< prev_sequence , mpl::pair<Element, new_sum> >::type new_sequence; typedef mpl::pair<new_sequence, new_sum> type; }; typedef mpl::fold< some_list, mpl::fold<_,_> >::type pair_seq_sum; typedef pair_seq_sum::first new_sequence; > Can you help me? I have > something like this: > > struct T1 { > enum x { > label1, > label2, > nr_; > }; > }; > > // Same for T2, possibly less or more labels etc. > > And I would like to end up with a type which looks like this: > > struct something { > int get(T1::x n) { > return arr_[n]; > } > int get(T2::x n) { > return arr_[n+2]; Where does n+2 come from? > } > // etc. > private: > int arr_[sum_of_all_nr_]; > }; ?? that doesn't look anything like what you just described. Sheesh. The above is still a job for fold. I suggest you start with a base type: template <class Derived> struct base { enum { sum = 0 }; void get(); // never used. }; then build layers: template <class Base, class T, class MostDerived> struct layer : mpl::apply<Base, MostDerived>::type { typedef typename mpl::apply<Base, MostDerived>::type super; enum { sum = T::nr_ + super::sum }; using Base::get; int get(T::x n) { return static_cast<MostDerived*>(this)->arr_[n]; } }; template <class Folded> struct outer : Folded { int arr_[Folded::sum]; }; Well, the above "solution" won't work and I don't have time to write something that does for you right now. It would take maybe another 15 minutes and I've already spent 30 on this email. I guess you'll probably do best to make 2 passes over the sequence: one to compute the sums and the other to build the result type. Good luck! -- Dave Abrahams Boost Consulting www.boost-consulting.com

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RE: [Boost-users] Re: building and installing boost
by Sohail Somani 31 May '05

31 May '05

> -----Original Message----- > From: boost-users-bounces(a)lists.boost.org > [mailto:boost-users-bounces@lists.boost.org] On Behalf Of > David Abrahams > Sent: Tuesday, May 31, 2005 10:09 AM > To: boost-users(a)lists.boost.org > Subject: [Boost-users] Re: building and installing boost > > > Rene Rivera <grafik.list(a)redshift-software.com> writes: > > > TBoafo(a)aol.com wrote: > >> Dear All, > >> its working now. All I did was download the boost executable and > >> run it to install boost and then run the bjam command with > >> parameters for boost install and it worked. > > > > So it was the extraction that failed originally? If so, > could you tell > > us what program you used, so we can warn people when they > have similar > > problems. > > And please clarify what you mean by "the boost executable." He might mean he followed the instructions on the boost wiki (http://www.crystalclearsoftware.com/cgi-bin/boost_wiki/wiki.pl?Installi ngBoost)

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Re: [Boost-users] Re: building and installing boost
by TBoafo＠aol.com 31 May '05

31 May '05

Dear All, its working now. All I did was download the boost executable and run it to install boost and then run the bjam command with parameters for boost install and it worked. Many thanks to all who helped Regards Theo

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simple_ls compile error
by Tord Holmqvist 31 May '05

31 May '05

Just tried to install boost on my laptop and I can't seem to get it to work. (slack 10.1) installed it with bjam "-sTOOLS=gcc" install and it seems to have added itself in /usr/local/include and so on. but when I try to build simple_ls.cpp I only get: todelod@todelod2:~/hemma/my_little_project$ gcc simple_ls.cpp -lboost_filesystem /usr/lib/gcc-lib/i486-slackware-linux/3.3.4/../../../../i486-slackware-linux/bin/ld: cannot find -lboost_filesystem collect2: ld returned 1 exit status what am I doing wrong?? (going mad...) /t

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numeric/conversion library question
by Toon Knapen 31 May '05

31 May '05

Following small program does not give the expected result. It prints some big negative value if compiled with my gcc 3.4.3 on linux or intel-linux 8.1. However commenting out either (one suffices) of the commented out lines makes it work correctly. I don't understand what is happening here, can anybody what is wrong with this function 'convert' or is the use of the conversion library wrong? <code> #include <boost/numeric/conversion/cast.hpp> #include <iostream> template<typename Target, typename Source> inline typename ::boost::numeric::converter<Target,Source>::result_type //convert ( const Source& arg ) // using this signatures makes it work convert ( Source arg ) { return ::boost::numeric::converter< Target, Source >::convert( arg ) ; } // { Target temp = ::boost::numeric::converter< Target, Source >::convert( arg ) ; return temp ; } int main() { int big = 900 ; ptrdiff_t diff = 16 ; std::cout << std::min( big, convert< int >( diff ) ) << std::endl ; return 0 ; } </code>

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Questions about Boost.ProgramOptions
by Elisha Berns 31 May '05

31 May '05

Hi, I have a few questions about Boost.ProgramOptions pertaining to parsing an argv that is of wchar_t data type. The first thing that isn't clear from the examples is whether it's possible at all. The second thing that isn't clear is how to do it and store string values in std::wstring variables. Here are the specifics: 1) How do you use program_options with an argv argument that is a wchar_t data type? Like this? variables_map vm; store(parse_command_line<wchar_t>(argc, argv, desc), vm); but then the question is how do you store any of the command line values in wstring variables? Like this? options_description desc("allowed options"); desc.add_options() // First parameter describes option name/short name // The second is parameter to option // The third is description ("help,h", "print usage message") ("validate,v", value(&strFile), "validate xml file"); // strFile is a wstring Well that doesn't work. 2) I am getting 3 link errors when trying the above: ToolsTest.obj : error LNK2019: unresolved external symbol "void __cdecl boost::program_options::store(class boost::program_options::basic_parsed_options<unsigned short> const &,class boost::program_options::variables_map &)" (?store@program_options@boost@@YAXABV?$basic_parsed_options@G@12@AAVvari ables_map@12@@Z) referenced in function _main ToolsTest.obj : error LNK2019: unresolved external symbol "public: __thiscall boost::program_options::basic_parsed_options<unsigned short>::basic_parsed_options<unsigned short>(class boost::program_options::basic_parsed_options<char> const &)" (??0?$basic_parsed_options@G@program_options@boost@@QAE@ABV?$basic_parse d_options@D@12@@Z) referenced in function "public: class boost::program_options::basic_parsed_options<unsigned short> __thiscall boost::program_options::basic_command_line_parser<unsigned short>::run(void)const " (?run@?$basic_command_line_parser@G@program_options@boost@@QBE?AV?$basic _parsed_options@G@23@XZ) ToolsTest.obj : error LNK2019: unresolved external symbol "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl boost::program_options::to_internal(class std::basic_string<unsigned short,struct std::char_traits<unsigned short>,class std::allocator<unsigned short> > const &)" (?to_internal@program_options@boost@@YA?AV?$basic_string@DU?$char_traits @D@std@@V?$allocator@D@2@@std@@ABV?$basic_string@GU?$char_traits@G@std@@ V?$allocator@G@2@@4@@Z) referenced in function "class std::vector<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class std::allocator<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > > > __cdecl boost::program_options::to_internal<class std::basic_string<unsigned short,struct std::char_traits<unsigned short>,class std::allocator<unsigned short> > >(class std::vector<class std::basic_string<unsigned short,struct std::char_traits<unsigned short>,class std::allocator<unsigned short> >,class std::allocator<class std::basic_string<unsigned short,struct std::char_traits<unsigned short>,class std::allocator<unsigned short> > > > const &)" (??$to_internal@V?$basic_string@GU?$char_traits@G@std@@V?$allocator@G@2@ @std@@@program_options@boost@@YA?AV?$vector@V?$basic_string@DU?$char_tra its@D@std@@V?$allocator@D@2@@std@@V?$allocator@V?$basic_string@DU?$char_ traits@D@std@@V?$allocator@D@2@@std@@@2@@std@@ABV?$vector@V?$basic_strin g@GU?$char_traits@G@std@@V?$allocator@G@2@@std@@V?$allocator@V?$basic_st ring@GU?$char_traits@G@std@@V?$allocator@G@2@@std@@@2@@3@@Z) Thanks for any help here, Elisha Berns e.berns(a)computer.org tel. (310) 556 - 8332 fax (310) 556 - 2839

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Re: Re: Serialization when using virtual inheritance
by Eric 30 May '05

30 May '05

Thanks Robert. This partially worked... Your comment about putting the BOOST_CLASS_EXPORT macro into the header keyed me into something. I am using the BOOST_CLASS_EXPORT(derived) macro, but I've been placing them into the source (cpp) file instead of the header (h) file. With my system, when I put BOOST_CLASS_EXPORT(derived) into the header file, I get a multiply-defined error at link time (the offending code is boost::archive::detail::guid_initializer<class,...> since the header is included multiple times when I use the class. As a test, I setup a sandbox with a stripped down system and was able to get it to work when I put the BOOST_CLASS_EXPORT in the header, but not when it is in the cpp file. Q: Why does the location of the BOOST_CLASS_EXPORT matter? Is it doing order-depended initialization upon bringing up the system? Q: How do I avoid the mutiple defined error in the linker? Thanks again for your help and the library :) -Eric

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Re: date time library question !!
by Saurabh Sood 30 May '05

30 May '05

thanks. Both work but i might finally decide in favor lexical_cast -ss

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Serialization when using virtual inheritance
by Eric 29 May '05

29 May '05

Boost version: 1.32.0 I am trying to use the serialization class to serialize data pointed to by the virtual base class. Note that the code works great if I don't use virtual inheritance off of the base class. Here is a quick run-down of the hierarchy and code: class Base { virtual void OneOfManyVirtuals() = 0; // Serialization Interface template<class Archive> void serialize(Archive &ar, const unsigned int version) { //empty } }; BOOST_IS_ABSTRACT(Base); class Derived : virtual public Base { virtual void OneOfManyVirtuals() { } // Serialization Interface template<class Archive> void serialize(Archive &ar, const unsigned int version) { //empty ar & BOOST_SERIALIZATION_BASE_OBJECT_NVP(Base); } } // later in the main code Base* pBase = new Derived; boost::archive::xml_oarchive oa(ofs); // doesn't work oa << BOOST_SERIALIZATION_NVP(pBase); // works oa << BOOST_SERIALIZATION_NVP(*pBase); When if fails, it fails in the oseralizer.hpp at line 419 with the comment: // convert pointer to more derived type. if this is thrown // it means that the base/derived relationship hasn't be registered I tried registering the pairs with void_cast_register, but I wasn't successful. Any real examples of how to use this method would be appreciated. So, does anyone know why the pointer to the base doesn't work when using virtual inheritance? It works find when I remove the virtual from the inheritance. Unfortunately, with the software architecture that I have, there are several cases when the virtual inheritance off of interfaces is required. Any help with the matter is greatly appreciated. -Eric

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date time library question !!
by Saurabh Sood 29 May '05

29 May '05

Hi all, i am totally new to boost but more i am feeling good and bad at same time ... good coz boost is cross-platform and powerful .. however sometimes its hard to do simple things ... Take following for instance .. ------------------------------------------------------------------------------------- using namespace boost::posix_time; ptime p = = microsec_clock::local_time(); ... ... (some calculations) ... time_duration td = microsec_clock::local_time() - p; std::cout << std::string(" Total Milliseconds: ") << td.total_milliseconds() << std::endl; // Works fine ------------------------------------------------------------------------------------- But how can the value present in td.total_milliseconds() could be put inside a std::string ? It might sound simple but i guess its tricky . ... thanks for the reply in advance. cheers saurabh

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