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iterator_facade and distance()
by Andrej van der Zee 17 Apr '11

17 Apr '11

Hi, I am trying to implement an iterator using boost::iterator_facade. It works fine but it is very slow, I think because the distance() function takes lineair time as it is seems to be implemented in terms of increment(). I would like to implement the distance() function myself but I am not sure how. I tried to implement distance_to() like this: class const_data_stream_iterator : public boost::iterator_facade<const_data_stream_iterator, char const, boost::forward_traversal_tag> { public: typedef boost::iterator_facade<const_data_stream_iterator, char const, boost::forward_traversal_tag> facade_t; private: friend class boost::iterator_core_access; facade_t::difference_type difference_to(const_data_stream_iterator & to) const { ... } }; But this does not work. How can I do this? Thank you, Andrej

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[BGL] Modifying graph in BFS
by Michal Sekletar 16 Apr '11

16 Apr '11

Hello everyove, I have a question about BGL. I want to traverse through graph using BFS and before actual algorithm I want to remove from graph structure (adjacency_list) some vertices and all edges connected to these vertices, so I wrote my own visitor class and implemented initliaze_vertex method to do check on every vertex (it's property) if it's satisfied then I iterate through all edges in graph and i store iterators to edges - edge must be connected to current vertex. Then I want to remove these edges and vertex itself. I store edge iterators for edges connected to vertex I want to remove, because I understand that calling remove_edge directly, while iterating through edges will lead to undefined behavior (if edges are stored in vector - removing invalidates iterators). My problem is that I can't compile source file, compiler is complaining about template instantiation(error message is very long and I don't understand what is cause of the problem) Any ideas, suggestions, code snippets, how to solve this issue would be much appreciated. Thank you Michal Sekletar

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[phoenix][spirit] Is is possible to use phoenix v3 and spirit in parallel?
by alfC 16 Apr '11

16 Apr '11

Hi, I have a program that uses Phoenix and Spirit. Spirit uses indirectly Phoenix2. With some effort I modified the part that use Phoenix to phoenix3 but now when I want to put all together it complains. The obivous warnings are: usr/include/boost/phoenix/version.hpp:16:1: warning: "BOOST_PHOENIX_VERSION" redefined usr/include/boost/phoenix/core/limits.hpp:23:1: warning: "PHOENIX_LIMIT" redefined and the most serious is: usr/include/boost/phoenix/core/limits.hpp:112:5: error: #error "BOOST_PROTO_MAX_LOGICAL_ARITY < BOOST_PHOENIX_COMPOSITE_LIMIT" The question is how can I use Phoenix3 and Spirit in the same code (in parallel). thank you, Alfredo

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[phoenix] function object that is also lazy
by alfC 16 Apr '11

16 Apr '11

Hi, Is this possible at all? struct A : boost::phoenix::function<A const&>{ double operator()(double const& d) const{return 5.;} } such that an object function it is not only a function but also a lazy function of itself? std::clog << a(4.) << std::endl; std::clog << a(arg1 + 1.)(4.) << std::endl; the code above doesn't work because A is not complete when the type function<A> is instatiated. However after some experimentation I got the following. The question is, does it really work in more exotic context? Is there a possible simplification? Am I reinventing the wheel? template<class Self> struct lazy{ boost::phoenix::function<Self const&> const* const lazy_; lazy(Self const& self) : lazy_(new boost::phoenix::function<Self const&>(self)){} template<class Arg> const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tag::terminal, boost::proto::argsns_::term<Self>, 0l>, Arg >, 2l> > operator()(Arg const& arg) const{return (*lazy_)(arg);} ~lazy(){delete(lazy_);} }; struct A : lazy<A>{ using lazy<A>::operator(); A() : lazy<A>(*this){} A(A const& other) : lazy<A>(*this){} typedef double result_type; result_type operator()(double const& d) const{return d;} }; so, any function object B can be a lazy function of it self provided that we use the correct CRTP argument (lazy<B>) and manually add the code: using lazy<A>::operator(); B() : lazy<A>(*this){} B(A const& other) : lazy<A>(*this){} inside the class. This feature seems to make the usage of the function object b more flexible, since it can we used in many different contexts. For example, as integrate ( b, 1., 2.); or as integrate ( 2.*b(arg1 + 1.) , 1., 2.) Thanks, Alfredo

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Byte stream with multiple internal buffers
by Andrej van der Zee 16 Apr '11

16 Apr '11

Hi, I am writing a network sniffer that receives a variable number of TCP packets with payload strings of variable length that i need to concatenate. Using std string concatination is too expensive because of extra copying and possible reallocation of the internal buffer when it runs out of capacity. Better would be a special byte stream that stores its data in multiple variable-length buffers with an interface that works as if the buffers are just one buffer, with operations similar to std string. I was wondering if boost provides a supporting library for this. Thanks, Andrej

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[karma] auto_ + default floating point precisions
by Brian Budge 15 Apr '11

15 Apr '11

Hi all - I'm using boost karma to generically generate strings from many different types use auto_. I've noticed that if I specify some double precision numbers like 1.123456, I will only get "1.123" out as a string. While looking around the documentation, it seems that it is possible to specify the precision up to numeric_limits<real>::digits10 + 1. I couldn't find an example for how to do this with double_ let alone auto_. Can anyone point me in the right direction? Is there documentation that google couldn't find for me? Thanks, Brian

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[thread] : pause/resume thread
by Kazz 15 Apr '11

15 Apr '11

Hi All, I have just started learning boost. I want my thread to support following functions Pause(); Resume(); Stop();. I have come up with following code, which ends up as dead lock. Can some one please guide me as how to achieve the desired functionality. Would really appreciate any help. Thanks. Kaz class qthread { public: boost::thread* mThread; boost::condition_variable mCond; boost::mutex mMutex; bool mExit; qthread(): mExit(false) { // create function pointer to local Execute function boost::function<void (qthread*)> fptr = &qthread::Execute; fptr(this); // pass on function pointer along with this reference to boost thread boost::thread th0( fptr, this); // assign temp thread to local variable mThread = &th0; } void Resume() { mMutex.unlock(); mCond.notify_all(); } void Stop() { mThread->interrupt(); mExit = true; Resume(); } void Pause() { // todo } virtual void Execute() { std::cout<<"Entering thread Execution"<<std::endl; boost::unique_lock<boost::mutex> lock(mMutex); mCond.notify_all(); mCond.wait(lock); //<< DEAD LOCK int i = 0; while ( !mExit ) { try { while ( 1 ) { std::cout<<"Executing thread, cycle i = "<<++i<<std::endl; boost::this_thread::interruption_point(); } } catch( boost::thread_interrupted&) { std::cout<<"thread_interrupt exception"<<std::endl; mCond.wait(lock); } } std::cout<<"Exiting thread"<<std::endl; } }; //-------------------------- int main() { qthread qt; ///<< DEAD LOCK boost::this_thread::sleep(boost::posix_time::seconds(3) ); qt.Resume(); boost::this_thread::sleep(boost::posix_time::seconds(3) ); qt.Stop(); }

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All-pairs SP algorithms -> Determine the path
by GOO Creations 15 Apr '11

15 Apr '11

Hi again I've noticed that the all-pairs SP algorithms (eg: Johnson, Floyd-Marshall) only "return" a distance matrix that hold the shortest distances between any node n and n'. Is there a way to determine the actual path (eg: n2-n4-n5-n1-n0) between these nodes using the all-pairs algorithms? Chris

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Longest path from u to v in a directed weighted graph
by Shaun Jackman 15 Apr '11

15 Apr '11

Hi, I would like to find the longest path from vertex u to vertex v through a directed graph with positive edge weights. I know that u and v are choke points in the graph, in that if you start from vertex u and start following random edges, you will end up at vertex v in pretty short order. Is dijkstra_shortest_paths the best function for this purpose? The vertices reachable from u without travelling through v form a small subgraph of the total graph. I want to avoid exploring beyond v, and I definitely don't need to know the distances to vertices beyond v. Cheers, Shaun

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Floyd-Warshall Example
by GOO Creations 15 Apr '11

15 Apr '11

Hi all I'm trying to figure out how to use the Floyd-Warshall algorithm. It seems like there is no example (like with the other algorithms); can someone maybe point me to an example. I'm having problems with the parameters, especially with the DistanceMatrix. Thank you Chris

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