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[MP11] Initialize the Nth alternative of an std::variant based on a runtime index
by Dominique Devienne 01 Jul '21

01 Jul '21

Hi. I'm modernizing some code, to use std::variant instead of ad-hoc peudo-variant structs. These structs need to be encoded/decoded via avro::codec_traits specializations, but given that std::variant is variadic, I'm struggling a bit. I've done my research and found peter's [1] and also looked at [2], which I thought might work, but does not, and I don't understand the errors I'm getting with MSVC 2019 in C++17 mode. The code is below, with also a subset of the compiler errors I'm getting. [1] https://www.reddit.com/r/cpp/comments/f8cbzs/creating_stdvariant_based_on_i… [2] https://www.boost.org/doc/libs/develop/libs/mp11/doc/html/mp11.html#mp_with… template <> struct codec_traits<std::monostate> { static void encode(Encoder& e, const std::monostate&) { e.encodeNull(); } static void decode(Decoder& d, std::monostate&) { d.decodeNull(); } }; template <typename... Ts> struct codec_traits<std::variant<Ts...>> { static void encode(Encoder& e, const std::variant<Ts...>& var) { e.encodeUnionIndex(var.index()); std::visit( [&e](const auto& alternative) { avro::encode(e, alternative); }, var ); } static void decode(Decoder& d, std::variant<Ts...>& var) { const size_t index = d.decodeUnionIndex(); if (index >= sizeof...(Ts)) { throw avro::Exception("Invalid Union index"); } auto& alternative = boost::mp11::mp_with_index<sizeof...(Ts)>( index, [&var](auto I) { return var.emplace<I>(); // I is mp_size_t<index>{} here } ) ; avro::decode(d, alternative); } }; 1>Messages.cpp 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\Messages.hpp(70,1): error C2220: the following warning is treated as an error 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\Messages.hpp(70,1): warning C4239: nonstandard extension used: 'initializing': conversion from 'std::monostate' to 'std::monostate &' 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\Messages.hpp(70,1): message : A non-const reference may only be bound to an lvalue 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\Messages.hpp(70): message : while compiling class template member function 'void avro::codec_traits<T>::decode(avro::Decoder &,std::variant<std::monostate,int64_t,double> &)' 1> with 1> [ 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v 1> ] 1>D:\pdgm\kits\trunk\Avrocpp\1.9.2-Boost-1.74.0\Win_x64_10_v16_debug\include\avro/Specific.hh(341): message : see reference to function template instantiation 'void avro::codec_traits<T>::decode(avro::Decoder &,std::variant<std::monostate,int64_t,double> &)' being compiled 1> with 1> [ 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v 1> ] 1>D:\pdgm\kits\trunk\Avrocpp\1.9.2-Boost-1.74.0\Win_x64_10_v16_debug\include\avro/Specific.hh(333): message : see reference to class template instantiation 'avro::codec_traits<T>' being compiled 1> with 1> [ 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v 1> ] 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\codegen/Messages.i.ipp(1527): message : see reference to function template instantiation 'void avro::encode<pdgm::eml::etp12::Datatypes::IndexValue_item_v>(avro::Encoder &,const T &)' being compiled 1> with 1> [ 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v 1> ] 1>D:\pdgm\kits\trunk\boost\1.74.0\Win_x64_10_v16_debug\include\boost/mp11/detail/mp_with_index.hpp(87,1): error C2440: 'return': cannot convert from '__int64' to 'std::monostate' 1>D:\pdgm\kits\trunk\boost\1.74.0\Win_x64_10_v16_debug\include\boost/mp11/detail/mp_with_index.hpp(87,1): message : No constructor could take the source type, or constructor overload resolution was ambiguous 1>D:\pdgm\kits\trunk\boost\1.74.0\Win_x64_10_v16_debug\include\boost/mp11/detail/mp_with_index.hpp(371): message : see reference to function template instantiation 'std::monostate boost::mp11::detail::mp_with_index_impl_<3>::call<0,_Ty>(size_t,F &&)' being compiled 1> with 1> [ 1> _Ty=avro::codec_traits<pdgm::eml::etp12::Datatypes::IndexValue_item_v>::decode::<lambda_7e593df011d98e42859c6ecb357b3e3f>, 1> F=avro::codec_traits<pdgm::eml::etp12::Datatypes::IndexValue_item_v>::decode::<lambda_7e593df011d98e42859c6ecb357b3e3f> 1> ] 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\Messages.hpp(70): message : see reference to function template instantiation 'std::monostate boost::mp11::mp_with_index<3,avro::codec_traits<T>::decode::<lambda_7e593df011d98e42859c6ecb357b3e3f>>(size_t,F &&)' being compiled 1> with 1> [ 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v, 1> F=avro::codec_traits<pdgm::eml::etp12::Datatypes::IndexValue_item_v>::decode::<lambda_7e593df011d98e42859c6ecb357b3e3f> 1> ] 1>D:\pdgm\trunk\psc3\SharedComponents\src\lib\pdgm\eml\etp12\Messages.hpp(70): message : while compiling class template member function 'void avro::codec_traits<T>::decode(avro::Decoder &,std::variant<std::monostate,int64_t,double> &)' 1> with 1> [ 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v 1> ] 1>D:\pdgm\kits\trunk\Avrocpp\1.9.2-Boost-1.74.0\Win_x64_10_v16_debug\include\avro/Specific.hh(341): message : see reference to function template instantiation 'void avro::codec_traits<T>::decode(avro::Decoder &,std::variant<std::monostate,int64_t,double> &)' being compiled 1> with 1> [ 1> T=pdgm::eml::etp12::Datatypes::IndexValue_item_v 1> ]

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[Asio] boost::system symbols visible despite BOOST_ASIO_DISABLE_VISIBILITY being defined
by Angela Breuer 30 Jun '21

30 Jun '21

Hello, I'm developing a C++11 shared library that uses Boost.Asio (with Boost version 1.72) as a header-only library. Exceptions are disabled as the library is intended to be used on embedded devices that do not support exceptions and, to ensure this is the case, all interfaces in my library have the noexcept operator and I have BOOST_EXCEPTION_DISABLE and BOOST_NO_EXCEPTIONS defined. Additionally, all public interfaces in the library do not expose any Boost / Boost.Asio types. When attempting to hide private symbols in my shared library with the GCC visibility flags and annotations (i.e. -fvisibility=hidden), I learned that I needed to define BOOST_ASIO_DISABLE_VISIBILITY to disable the visibility pragmas in Boost.Asio so the boost::asio symbols will also be hidden. Although I was able to hide the boost::asio symbols with this strategy, this still results in a subset of boost::system symbols being visible. I do have some internal helper free functions that accept a boost::system::error_code. I suspect that they may have something to do with these visible symbols, but they are completely internal to the library and the symbols for those helper functions are not visible when I inspect the library's symbols with nm -DC. I will note that I was able to hide the boost::system symbols by using a linker version script, but I do not wish to maintain an unwieldy list of symbols to export if I can help it. Is there a supported way to hide the boost::system symbols aside from using a linker version script? I have attached nm -DC output along with gcc / g++ invocations for reference. Thank you, -Angela

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MSVC compiler error with boost ver 1.71
by John Emmas 24 Jun '21

24 Jun '21

Hi there - I'm using boost ver 1.71 and I'm seeing a strange problem with VS2019. Simply #including this line at the top of a source file will generate a compiler error if I try to build in Debug mode (Release builds are okay...) #include <boost/property_tree/json_parser.hpp> When I say a "compiler error" I'm not referring to one of the usual CXXXX messages - what happens is this:- >INTERNAL COMPILER ERROR in 'C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30037\bin\HostX86\x64\CL.exe' > Please choose the Technical Support command on the Visual C++ > Help menu, or open the Technical Support help file for more information >Done building project "ardour.vcxproj" -- FAILED. >========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ========== Just wondering if anyone else has encountered this..? Thanks, John

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Making multiprecision::int saturate
by Emilio Paolini 24 Jun '21

24 Jun '21

Hi everyone, i have the following problem: i need the integer type of boost::multiprecision to saturate, instead of wrapping. Is it possible to change its behaviour? Thanks!

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MPI_ERR_TRUNCATE with Boost.MPI and only blocking communication
by Alex Cumberworth 21 Jun '21

21 Jun '21

I am trying to run some Monte Carlo simulation code I wrote in C++ with MPI via the Boost.MPI wrapper. I use only blocking send and receive calls (i.e. send and recv, never isend or irecv), but after running the program for a few days, I inevitably end up with the following error terminate called after throwing an instance of boost::exception_detail::clone_impl >' what(): MPI_Recv: MPI_ERR_TRUNCATE: message truncated I have seen that this can happen with non-blocking calls are being made, but I cannot see how it can happen with only blocking calls. I am sending and receiving a vector of structs: struct Chain { bool operator==(Chain chain_2); int index; int identity; vector positions; vector orientations; private: friend class boost::serialization::access; template void serialize(Archive& ar, const unsigned int version) { ar& index; ar& identity; ar& positions; ar& orientations; } }; using Chains = vector; where VectorThree is declared elsewhere as class VectorThree { public: VectorThree(int x, int y, int z): m_container {{x, y, z}} {}; VectorThree(): m_container {{0, 0, 0}} {}; VectorThree operator-(); VectorThree operator+(const VectorThree& v_2) const; VectorThree operator-(const VectorThree& v_2) const; bool operator!=(const VectorThree& v_2) const; int& operator[](const size_t& i) { return m_container[i]; }; const int& at(const size_t& i) const { return m_container.at(i); }; VectorThree rotate_half(VectorThree axis); VectorThree rotate(VectorThree origin, VectorThree axis, int turns); VectorThree rotate(VectorThree axis, int turns); int sum(); int abssum(); VectorThree absolute(); VectorThree sort(); private: array m_container; friend class boost::serialization::access; template void serialize(Archive& arch, const unsigned int) { arch& m_container; } }; It sends and receives instances of this object many times before the error occurs. I have narrowed down the calls where the error occurs by printing statements before and after both sending and receiving. The sending code: Chains chains_send {m_us_sim->get_chains()}; cout << "Win " << m_rank << ": Sending chains (size " << chains_send.size() << ") to " << win_i << "\n"; m_world.send(win_i, swap_i, chains_send); cout << "Win " << m_rank << ": Sent chains to " << win_i << "\n"; The receiving code: Chains chains_rec; cout << "Win " << m_rank << ": Recieving chains from " << win_to_win[0] << "\n"; m_world.recv(win_to_win[0], swap_i, chains_rec); cout << "Win " << m_rank << ": Recieved chains (size " << chains_rec.size() << ") from " << win_to_win[0] << "\n"; In the output file before the crash I have Win 0: Recieving chains from 1 Win 1: Sending chains (size 2) to 0 Win 1: Sent chains to 0 I am using version 1.65 of Boost and OpenMPI 2.0.1.

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newbie, Getting Started on Unix Variants
by John 15 Jun '21

15 Jun '21

Please someone help me work through the Getting Started on Unix Variants. I've completed the steps up until 6 Link Your Program to a Boost Library. However, the library shown in the step 6 example, libboost_regex-gcc34-mt-d-1_36.a, doesn't seem to be present in my system. So, it looks like something was not right with the building of this library. What can I do to ensure that the library, libboost_regex-gcc34-mt-d-1_36.a, is made. Thank You, John

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[Serialization] archive_exception from bad interaction serialization() and load_construct_data()
by Boylan, Ross 14 Jun '21

14 Jun '21

In short, combining a class implementation of serialize() with load/save_construct_data as described in the docs writes and reads data twice. Varying the pattern can produce exceptions. My solution is to implement serialize() as a no-op and provide paired load/save_construct_data() functions. Is that actually a solution? Is there a better way? The code at the bottom of this message yields Microsoft C++ exception: boost::archive::archive_exception at memory location 0x000000B8189AC300. When attempting to read in what was just written out. Debugging shows that code attempts to read in the object twice, first in the class-specific load_construct_data function I wrote, and then again in the instance method A::serialize. Since the save operation only wrote it out once, there is nothing, or at least nothing expected, to read and things go badly. The documentation (https://www.boost.org/doc/libs/1_76_0/libs/serialization/doc/serialization.…) says "If there is no such default constructor, the function templates load_construct_data and perhaps save_construct_data will have to be overridden." "perhaps" seems to imply that it is not essential to do so, and since the lack of a default constructor only matters when materializing an object I only wrote a load_construct_data function. The example in the documentation does show both a save_construct_data and a load_construct_data function and if I uncomment save_construct_data in the code below, it runs without error and reconstructs the object. HOWEVER, this only works because the data are written out twice, and then read in twice. I think the problem is that TestBed.exe!boost::archive::detail::pointer_iserializer<boost::archive::text_iarchive,A>::load_object_ptr(boost::archive::detail::basic_iarchive & ar, void * t, const unsigned int file_version) Line 340 at J:\Programs\boost_1_76_0\boost\archive\detail\iserializer.hpp(340) first materializes the object and then--after it is already constructed--passes it off to ar_impl >> boost::serialization::make_nvp(NULL, * static_cast<T *>(t)); at the end, triggering the call to A::serialize and the 2nd read from the archive. For now I have kept the save_construct_data function and made A::serialize a no-op; the serialization framework seem to require that it exist. This appears to work. Boost 1.76 Microsoft Visual Studio Community 2019 v16.10.1 with Language Level /std:c++latest. // TestBed.cpp : This file contains the 'main' function. Program execution begins and ends there. // #include <iostream> #include <fstream> #include <string> #include <boost/archive/text_oarchive.hpp> #include <boost/archive/text_iarchive.hpp> class A : boost::noncopyable { public: int a1; A(const int x1) : a1(x1) {}; virtual ~A() {}; template<class Archive> void serialize(Archive& ar, const unsigned int version) { ar& a1; }; }; namespace boost { namespace serialization { #if 0 template<class Archive> void save_construct_data(Archive& ar, const A* pa, const unsigned int version) { ar& pa->a1; }; #endif template<class Archive> inline void load_construct_data(Archive& ar, A* pa, const unsigned int file_version) { int x1; ar >> x1; ::new(pa)A(x1); } } } /* these are separate functions partly so RAII will close files*/ void write_out(const std::string& fpath, const A& a) { std::ofstream ofs(fpath); boost::archive::text_oarchive oa(ofs); oa << &a; } A* read_in(const std::string& fpath) { A* pa; std::ifstream ifs(fpath); boost::archive::text_iarchive ia(ifs); ia >> pa; return pa; } int main(int argc, char** argv) { const std::string fname("test.srl"); A a(810); write_out(fname, a); A* aa = read_in(fname); std::cout << "A(" << aa->a1 << std::endl; return 0; }

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[Boost::asio] How to set the name of work_scheduler thread started by boost::asio?
by Marco Guest 09 Jun '21

09 Jun '21

Hello, I'm looking to set custom names to std::threads started indirectly when using boost:asio library (on Linux operating system). I have seen this stack trace for such thread by gdb: pthread_cond_wait () from /lib64/libpthread.so.0 void boost::asio::detail::posix_event::wait<boost::asio::detail::conditionally_enabled_mutex::scoped_lock>(boost::asio::detail::conditionally_enabled_mutex::scoped_lock&) ()2 boost::asio::detail::conditionally_enabled_event::wait(boost::asio::detail::conditionally_enabled_mutex::scoped_lock&) () boost::asio::detail::scheduler::do_run_one(boost::asio::detail::conditionally_enabled_mutex::scoped_lock&, boost::asio::detail::scheduler_thread_info&, boost::system::error_code const&) () boost::asio::detail::scheduler::run(boost::system::error_code&) () boost::asio::detail::resolver_service_base::work_scheduler_runner::operator()() () boost::asio::detail::posix_thread::func<boost::asio::detail::resolver_service_base::work_scheduler_runner>::run() () boost_asio_detail_posix_thread_function () start_thread () from /lib64/libpthread.so.0 clone () from /lib64/libc.so.6 Setting names would be useful to debug my application behaviour in high throughput conditions and to be able to easily check the CPU usage of each thread using “top –H”. Is there a way to do so? Thanks

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[QUESTION] what's meaning of boost::asio::require()?
by ZheNing Hu 06 Jun '21

06 Jun '21

Hi, I ran into difficulties when I was reading boost::asio http server example[1], I see: work_.push_back( boost::asio::require(io_context->get_executor(), boost::asio::execution::outstanding_work.tracked)); I don't know what is the meaning of this code? Or what is boost::asio::require()? I did not find useful information to introduce it. [1] https://www.boost.org/doc/libs/1_76_0/doc/html/boost_asio/example/cpp03/htt… -- ZheNing Hu

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[Parameter Library] How to define a constexpr function
by pavel b 04 Jun '21

04 Jun '21

Hi all, Is it possible to define a constexpr function using BOOST_PARAMETER_FUNCTION? what is the correct syntax? Best regards, P

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