August 2015 - Boost-users - lists.preview.boost.org

Emulating Python class variables in C++
by Will Hall 03 Aug '15

03 Aug '15

Hi there, I am porting a Python library to C++ - and re-exposing back to Python with boost (suffice to say the Python API is fixed). I am having problems emulating class variables from boost as C++ static data members using inheritance. Here's an example: using namespace boost::python; using namespace std; struct A { static string value; }; struct B : public A { static string value; }; string A::value("A"); string B::value("B"); BOOST_PYTHON_MODULE(test) { class_<A>("A") .def_readonly("value", &A::value); class_<B>("B") .def_readonly("value", &B::value); } If I don't specify inheritance using bases<>, then this works. But I need inheritance, so I write: BOOST_PYTHON_MODULE(test) { class_<A>("A") .def_readonly("value", &A::value); class_<B, bases<A> >("B"); } But now in Python, I only have access to A: >>> from test import A, B >>> A.value 'A' >>> B.value 'A' If try and overload B::value, I get an error: BOOST_PYTHON_MODULE(test) { class_<A>("A") .def_readonly("value", &A::value); class_<B, bases<A> >("B") .def_readonly("value", &B::value); } >>> from test import A, B Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: can't set attribute I've also tried switching .def_readwrite, but now the error is: >>> from test import A, B Traceback (most recent call last): File "<stdin>", line 1, in <module> Boost.Python.ArgumentError: Python argument types in None.None(Boost.Python.StaticProperty) did not match C++ signature: None(std::string) So, to summarise, any idea how to provide polymorphic Python class variables in boost::python? Thanks in advance for your help.

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Structure size when compiling with and without -std=c++11
by Miloslav Marik 03 Aug '15

03 Aug '15

Hi, I would like to share my experience with boost when combining with c++11. I hope it might help someone. Suppose a simple program: #include <boost/asio.hpp> #include <iostream> using boost::asio::detail::atomic_count; struct Test { // Whether the task has been interrupted. bool task_interrupted_; // The count of unfinished work. atomic_count outstanding_work_; }; int main () { std::cout << "Size is " << sizeof(Test) << std::endl; return 0; } And here is the story: $ /opt/gcc-4.8.4/bin/g++ -std=c++11 -I /opt/boost-1.58.0/include/ -L /opt/boost-1.58.0/lib -lboost_system b.cpp $ ./a.out Size is 16 $ /opt/gcc-4.8.4/bin/g++ -I /opt/boost-1.58.0/include/ -L /opt/boost-1.58.0/lib -lboost_system b.cpp $ ./a.out Size is 8 It means that c++11 program cannot be linked with boost libraries compiled without "-std=c++11" option. It was asio in my case and the program was linked with libboost_log.so.1.58.0. I would really appreciate a short warning somewhere at "Getting Started on Unix Variants" page. It may safe someone a few hours of life. Best Regards, Slavek

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Boost 1_59_0_b1_rc1 is available for testing
by Marshall Clow 02 Aug '15

02 Aug '15

Release candidate files for 1.59.0 beta 1 are available at http://boost.cowic.de/rc/ As always, the release managers would appreciate it if you download the candidate of your choice and give building it a try. Please report both success and failure, and anything else that is noteworthy. This helps ensure the candidates build OK before we push them out to SourceForge. The files (and associated md5s) are: MD5 (boost_1_59_0_b1_rc1.7z) = f54a97e5ee7f8e3d04cd9af8396e30f9 MD5 (boost_1_59_0_b1_rc1.tar.bz2) = 9804305aae0c9de9f8cfc02e3de75167 MD5 (boost_1_59_0_b1_rc1.tar.gz) = e584770bd76885c123a0426d24466fe2 MD5 (boost_1_59_0_b1_rc1.zip) = ee34f223aa789a6b3f727cb9480651ed Thanks! -- The release managers

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Need sample for CMakelist.txt with BOOST.test
by Abhay Mhatre 01 Aug '15

01 Aug '15

Hi, I am newbie, and struggling with testing project that is build in CMakeList.txt for using Boost.Test. project builds successfully and runs too, only problem is when unit test fails BOOST_REQUIRE exits without printing fatal exception message on console. It also doesn't show next failure. I tired compiling same library with g++ command and everything works as expected. however with CMakeList project there is no output I tried setting explicit boost::unit_test::unit_test_log_t::instance().set_threshold_level(boost::unit_test::log_all_errors) also tried with setting # define BOOST_TEST_LOG_LEVEL all but there is no output except fatal error on console. also it doesn't show fatal error for multiple tests/ Can some one please suggest some way to investigate this further/ provide sample CMakeList.txt file that I can compare and see if I am missing anything thanks in advance, Abhay

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cannot get graph object deserialized
by breadbread1984 01 Aug '15

01 Aug '15

I am using boost graph library. I need to serialize a graph object and read it out later. But I found my program cant work properly with xml archiver. It always throws exception saying XML start/end tag mismatch - position. I did not define a tag named position in my serialization code. I dont know how to solve it. [Here] (http://www.rapidshare.com.cn/i1cvbXM) is a demo of the problem. Please help me. Thanks!

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