January 2016 - Boost-users - lists.preview.boost.org

[Boost::Python]
by Eugenio Bargiacchi 03 Jan '16

03 Jan '16

Hello everyone, I have a member function with signature const Matrix2D & foo const (); Which returns an internal data structure. Matrix2D is using Matrix2D = Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor | Eigen::AutoAlign>; I have explicitly set a to_python converter for Matrix2D, and I have exported the function with a call policy of return_internal_reference. However, when this function is called from Python I get TypeError: No Python class registered for C++ class Eigen::Matrix<double, -1, -1, 1, -1, -1> This happens also if the reference is not const. At the same time, if I modify the call policy to return_value_policy<copy_const_reference>() Python can decode the matrix just fine. The same holds true if I return the Matrix2D by value. However I really need to have a reference to the matrix in order store it into other classes. How can I make Python see the data structure through the reference? The converter in case it is needed: boost::python::list toPythonList(const double * data, size_t num) { boost::python::list list; for (size_t i = 0; i < num; ++i) list.append(data[i]); return list;} struct Matrix2DToPython{ static PyObject* convert(Matrix2D const & m) { boost::python::list list; for (int i = 0; i < m.rows(); ++i) { list.append(toPythonList(m.row(i).data(), m.cols())); } return boost::python::incref(list.ptr()); }}; Thanks, Eugenio

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[units] Adding two absolute units
by Itzik 03 Jan '16

03 Jan '16

Hello, <obligatory appreciation on first post> I started using Boost just for boost::units and I find new ways boost can help me almost every day. So thanks for the great stuff! </obligatory appreciation on first post> My question to the board is why two absolute quantities (si::temperature (K) in my case) can't be summed in boost::units? I realized that this is a rule enforced by-design (gmane.comp.lib.boost.user/71747), and I understand that it is probably not a very common case, but I wanted my code to solve a simple formula which requires adding up two absolute temperatures (quantity<absolute<temperature>>). Each temperature represents an Inlet temperature in a Heat Exchange system. So even if converting one temperature to a relative type (quantity<temperature>) might be a workaround, it seems like a forced misuse to me. Eventually it turned out that I was mistaken with the formula, and subtraction was required instead, so there's no problem there. Alas, there's also a much simpler use case: What if I want to average the temperature of the last N readouts? Shouldn't boost::units support this simple operation? I have no formal physics education, but intuitively, adding two absolute temperatures doesn't seem really like a mistake to me, even if it's an odd thing to do. Will appreciate your thoughts on the matter.

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Modifying all matching elements in a Boost.MultiIndex Container
by Amit Prakash Ambasta 01 Jan '16

01 Jan '16

Hi, I am trying to modify members of all elements matching value for a given index in a boost.multiindex container index.modify( index.find(value), [](T& element) { ... } ) seems to update a singular element(first match) On the other hand, std::pair<index::iterator, index::iterator> iter = index.equal_range(value); while(iter.first != iter.second) { index.modify( iter.first, [](T& element) { ... } ); iter.first++; } leads to segfaults and unexpected behavior. What would be right way to approach this?

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Pointer to objects in Boost.MultiIndex
by Amit Prakash Ambasta 01 Jan '16

01 Jan '16

Consider the following code snippet: #include <iostream> #include <boost/multi_index_container.hpp> #include <boost/multi_index/ordered_index.hpp> #include <boost/multi_index/member.hpp> using namespace std; using namespace boost::multi_index; struct LinkList; struct LinkList { size_t index; LinkList* parent; LinkList(size_t index, LinkList* parent) : index(index), parent(parent) {} }; struct llindex {}; typedef ordered_unique<tag<llindex>, member<LinkList, size_t, &LinkList::index> > LinkListIndex; typedef boost::multi_index_container<LinkList, indexed_by<LinkListIndex> > IndexedLinkList; template <typename T> void show(T& iterable) { for(auto const& element: iterable) { if(element.parent != nullptr) { std::cout << "Index<" << element.index << ">: " << element.parent->index << std::endl; } else { std::cout << "Index<" << element.index << ">: " << "NULL" << std::endl; } } } int main() { IndexedLinkList ill; auto &index_ll = ill.get<0>(); ill.insert(LinkList{0, nullptr}); ill.insert(LinkList{1, (LinkList*)(&(*index_ll.find(0)))}); ill.insert(LinkList{2, (LinkList*)(&(*index_ll.find(0)))}); ill.insert(LinkList{3, (LinkList*)(&(*index_ll.find(2)))}); show(ill); return 0; } Would (LinkList*)(&(*index__ll.find(value))) be the recommended way to fetch the pointer to parent?

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[BGL] Are normal and reverse undirected edges the same or different?
by Ireneusz Szcześniak 01 Jan '16

01 Jan '16

Hi, In an undirected graph, we create an undirected edge: add_edge(a, b, g); We get the reverse of that edge: edge(b, a, g); Now, are these two edges the same or different? vIf I compare them with the == operator, they are equal, but they have different source and target vertexes. Can I count on this behavior? Will it stick? I'm asking because I couldn't find it in the documentation. Please find attached my unit test. On Debian Jessie, I'm compiling it with: g++ test.cc -l boost_test_exec_monitor -l boost_unit_test_framework -o test Thanks & best, Irek

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