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[Units] Issue with user defined quantities
by Pieter B. 22 Jan '12

22 Jan '12

LS, I have some problems working with user defined quantities/units in combination with unitless quantities (such as the results of a sin() or cos() operations).Taking the example of the 'nautical mile', which is presented in the documentation: ---START CODE --- namespace nautical { struct length_base_unit : boost::units::base_unit<length_base_unit, length_dimension, 1> { static std::string name() { return "nautical mile"; } static std::string symbol() { return "nmi"; } }; typedef boost::units::make_system<length_base_unit>::type system; typedef unit<length_dimension,system> length; static const length mile,miles; } BOOST_UNITS_DEFINE_CONVERSION_FACTOR(nautical::length_base_unit, boost::units::si::meter_base_unit, double, 1.852e3); ---END CODE --- The following does not compile: ---START CODE --- typedef quantity<boost::units::si::plane_angle> TAngleInRad; typedef boost::units::quantity<nautical::length> TNauticalDistanceInNMI; TNauticalDistanceInNMI NauticalDistanceInNMI; TNauticalDistanceInNMI SumNauticalDistanceInNMI = NauticalDistanceInNMI + NauticalDistanceInNMI; // OK TNauticalDistanceInNMI ScaledSumNauticalDistanceInNMI = NauticalDistanceInNMI + NauticalDistanceInNMI*sin(AngleInRad); // Last line does not compile due to lack of a suitable operator+ ---END CODE --- It does work correctly when using the 'nautical mile' as it has been defined in the library itself in "nautical_mile.hpp" (as a scaled base unit related to the si::meter): ---START CODE --- namespace metric { typedef scaled_base_unit<boost::units::si::meter_base_unit, scale<1852, static_rational<1> > > nautical_mile_base_unit; } // definition of the nautical mile from "nautical_mile.hpp" typedef boost::units::metric::nautical_mile_base_unit::unit_type nautical_mile_unit; typedef quantity<nautical_mile_unit> TNauticalDistanceInNMI; ---END CODE --- However, I don't think I can specify my user-defined units/quantities in this manner as they use a scale factor represented by an arbitrary floating point value. My perception is that the example of the documentation does not work due to the mixing of quantities from the si system (the sin() returns a dimensionless quantity in that system) with user-defined quantities in another ('nautical') system, while the 'nautical mile' from 'nautical_mile.hpp' is a scaled version of the si::meter and thus defined within the realms of the si::system - but please correct me if I am wrong here. My problem can thus be summarized as follows: - the solution from the documentation does not compile - the solution from 'nautical_mile.hpp' uses a scaled base unit which is limited to scaling factors of the form scale<Base,Exponent> and my question is therefore: how can I define my user-defined units/quantities in such a way that they allow the required operations while being related to base units by an arbitrary scale factor? Help would be highly appreciated, Pieter ------------------- Nautical mile version #1 (from the included nautical_mile.hpp): namespace metric { typedef scaled_base_unit<boost::units::si::meter_base_unit, scale<1852, static_rational<1> > > nautical_mile_base_unit; } template<> struct base_unit_info<metric::nautical_mile_base_unit> { static const char* name() { return("nautical mile"); } static const char* symbol() { return("nmi"); } }; typedef quantity<boost::units::si::plane_angle> TAngleInRad; typedef quantity<boost::units::si::length> TDistanceInM; typedef boost::units::metric::nautical_mile_base_unit::unit_type nautical_mile_unit; typedef quantity<nautical_mile_unit> TDistanceInNMI; typedef boost::units::quantity<nautical::length> TNauticalDistanceInNMI; # this is OK: TAngleInRad Angle(0.5*radians); TDistanceInNMI DistanceInNMI; TDistanceInNMI SumDistanceInNMI = DistanceInNMI + DistanceInNMI; TDistanceInNMI ScaledSumDistanceInNMI = DistanceInNMI + DistanceInNMI*sin(AngleInRad); TNauticalDistanceInNMI NauticalDistanceInNMI; TNauticalDistanceInNMI SumNauticalDistanceInNMI = NauticalDistanceInNMI + NauticalDistanceInNMI; TNauticalDistanceInNMI ScaledSum NauticalDistanceInNMI = NauticalDistanceInNMI + NauticalDistanceInNMI*sin(AngleInRad); // DOES NOT COMPILE for lack of a suitable operator+

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[phoenix] [cmath] lazy pow not working in boost 1.48
by alfC 22 Jan '12

22 Jan '12

Hi, Is this a bug in phoenix? the boost::phoenix::pow lazy function doesn't seem to work. My version is Boost 1.48 or maybe 1.49 (after svn update) -- Thanks, Alfredo #include<iostream> #include <boost/phoenix.hpp> #include <boost/phoenix/stl/cmath.hpp> using std::clog; using std::endl; int main(){ using namespace boost::phoenix::arg_names; clog << boost::phoenix::sin(arg1)(1.) << " " << std::sin(1.) << endl; // ok clog << boost::phoenix::pow(arg1, arg1)(2.) << " " << std::pow(2.,2.) << endl; // ^^^^------ error: no matching function for call to ‘pow(const boost::phoenix::expression::argument<1>::type&, double) return 0; }

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Why does boost thread copy a functor?
by Chris Cleeland 22 Jan '12

22 Jan '12

I had code similar to the following... Functor object: struct Task { operator()() { // thread main } smartfd p; }; Note that smartfd is a wrapper around a file descriptor. In its dtor it closes the fd. Then, elsewhere I set up an instance of Task and spawn it with boost::thread... Task t; t.p = // open file descriptor boost::thread thr(t); The spawned thread gets an instance of Task, but it's not the same instance as from the main. What is the rationale? -- Chris Cleeland

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Vote Boost Up for SourceForge Project of the Month
by Dave Abrahams 20 Jan '12

20 Jan '12

Folks, I was just informed that Boost is a top contender for SourceForge's February Project Of The Month. The vote is happening on TwtPoll, here: http://twtpoll.com/7vt4lq It's pretty close, so if you could vote Boost up, it could be good for our profile and our future. Thanks! -- Dave Abrahams BoostPro Computing http://www.boostpro.com

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Re: [Boost-users] Is there any functions that convert Chrono's time_point and duration to their Ptime equivalents?
by Iron Bug 20 Jan '12

20 Jan '12

I found such functions somewhere in boost's sandbox, in chrono development. There're some templates that can be used as examples and adopted for all such conversions. I think, they will be added to chrono, possibly. Look here: http://svn.boost.org/svn/boost/sandbox/chrono/boost/conversion/boost/

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[asio] Error "File exists" at connect
by Arash Abghari 20 Jan '12

20 Jan '12

Hi everybody, I am running a server and client using boost::asio. There are 10 clients sending requests simultaneously to the server. Sometimes one of the clients returns the error "File exist" when ip::tcp::socket::connect() is called. Do you have any idea what possibly could cause this error? Thanks, Arash

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Using filesystem::remove with wildcards
by toran 20 Jan '12

20 Jan '12

I am having trouble deleting files with boost::filesystem. Following code does not work for me (and I can't find the doc that explains how to do it). boost::filesystem::path pt("somepath"); pt /= ("*.data"); boost::filesystem::remove(pt); pt variable points to the valid set of files, but removal does not happened. I will appreciate an advise and may be pointing me to the right place in the docs. Thanks.

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Using shared_ptr with reference_wrapper
by Kelvin Chung 19 Jan '12

19 Jan '12

I am currently in the middle of a wrapper conundrum. I currently have the following: template <class Key, class Container> class ContainerWrapper { Container container; public: typedef Key key_type; protected: virtual const typename Container::key_type& mapKey(const key_type&) = 0; }; The idea of ContainerWrapper is that I want to disguise one Key type as another key type. But the following wrapper, intended to present a container of Key pointers as a container of Keys, presents a dangling reference problem: template <class Key, class Container> class HeapKeyWrapper : public ContainerWrapper<Key, Container> { // Container::key_type == boost::shared_ptr<Key> public: typedef Key key_type; protected: // Ignore the lack of boost::const_pointer_cast<Key>() below. The dangling reference is from the fact // that shared_from_this() doesn't return a reference. const boost::shared_ptr<Key>& mapKey(const key_type& key) { return key.shared_from_this(); } }; If I try to fix this by having mapKey() return by value, then this other wrapper breaks: template <class Key, class Container> class IdentityWrapper : public ContainerWrapper<Key, Container> { public: typedef Key key_type; protected: // Assume Key extends boost::enable_shared_from_this<Key>. // Returning by value would mean the returned value is a copy of key, and not the original // so calling shared_from_this() from the mapped key would break, say const Key& mapKey(const key_type& key) { return key; } }; I'm trying to fix the code so that both wrappers work (short of refactoring the mapKey() functionality to a functor of some sort), which leads me thinking about using boost::reference_wrapper in some way. (The problem that I have is that I could have IdentityWrapper::mapKey() return a reference_wrapper, but that violates the mapKey() interface.) This has also led me down to a related path - what if I changed my backing container to use reference_wrapper instead of shared_ptr? It seems to be that it is ill-advised: the container technically isn't holding the object, and so the objects in the container could be deallocated in the meantime, leaving me with containers full of dangling references. I have valid uses of both HeapKeyWrapper and IdentityWrapper elsewhere in my code, so what do you think should be the right choice given this dangling reference problem currently present in HeapKeyWrapper?

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[Chrono] Is there any functions that convert Chrono's time_point and duration to their Ptime equivalents?
by Matthew Jenks 19 Jan '12

19 Jan '12

Hello, I have been trying to find in the boost libraries if there are any interoperability functions for Boost::Chrono and Boost::Posix_Time? Does anyone know if they exist and what class they are in? Thanks, Matt

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adjacency lists
by Neiderer, Andrew M CIV (US) 19 Jan '12

19 Jan '12

I am still learning graphs, BGL 1.48.0 so beginner questions: 1. not sure what adjacency list/matrix are used for other than building a graph for traversal. Or maybe this is it? 2. I need to build a sparse network. So I am using adjacency_list_io.cpp by Francois Faure (2001). In this example I can read(g2), but when outputting std::cout << write(g2) I get lots of translation errors :-( I know I do not give details here but maybe someone could guess a -D??? to make adjacency_list_io.hpp happy. (The read(g2) works fine) Or I can certainly provide some of the error message. Also has Dr. Nick Edmonds completed (or near completion) of "active-message" based release (MPI)? Thank you. Andrew M. Neiderer US Army Research Laboratory

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