February 2009 - Boost-users

[multi_array] Why the precondition on assignment?
by Peter Barker 28 Feb '09

28 Feb '09

Hello, I wonder if anyone can give me the rationale behind boost::multi_index's assignment operator having the following precondition: std::equal(this->shape(),this->shape()+this->num_dimensions(),x.shape()); as mentioned in this document: http://www.boost.org/doc/libs/1_38_0/libs/multi_array/doc/reference.html ? I've got a multi_array as a data member of a class and would like to avoid having to create an assignment operator for my class do the resizing and assignment. Regards, Peter Barker

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[random] Problems generating pseudorandom 64-bit ints across multiple computers
by James Madison 27 Feb '09

27 Feb '09

I am currently developing a distributed application where many clients need to generate unique 64-bit random numbers which will be used as keys. I am having problems getting boost to do this. For my first attempt, I tried something like the following: boost::mt19937 RandomNumberGenerator::s_algorithm; boost::uniform_int<unsigned long long> RandomNumberGenerator::s_range( std::numeric_limits<unsigned long long>::min(), std::numeric_limits<unsigned long long>::max()); boost::variate_generator<boost::mt19937&, boost::uniform_int<unsigned long long> > RandomNumberGenerator::s_rng(RandomNumberGenerator::s_algorithm, RandomNumberGenerator::s_range); I had two issues though: 1. Is this really giving me random 64 bit numbers, or is it just generating 32-bit numbers and then doing bit expansion? 2. The values always started with the same number. Trying to find a fix for #2, I though of using a hashed GUID as a seed. The problem is I noticed the seed value takes a 32 bit int, which essentially means a 1/2^32 chance of the first value colliding even if chosen completely randomly, which is unacceptable for my application. I am back where I started. Is there any clean way to use boost to generate random 64-bit intergers, when the same application is going to be run across multiple computers?

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[Proto] Reducing domain check in operator
by Joel Falcou 27 Feb '09

27 Feb '09

Actually, Proto defiens operators that STATIC_ASSERT when the left and right operand domain types are no the same using is_same. How could I have those check be looser than is_same ? Should I just overload boost::is_same for my domain type and perform a proper test here or is there a way to incorporate this into Proto ? Thanks in advance

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Upgrade from Boost v1.37.0 to v1.38.0
by biolaser 27 Feb '09

27 Feb '09

I have Boost v1.37.0 installed on Windows XP with Visual Studio v7.1 - is there an upgrade path to v1.38.0, or must I remove v1.37.0 and install v1.38.0 from scratch? William

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Re: [Boost-users] make member functions (in)accessible
by Stefan Funck 27 Feb '09

27 Feb '09

This looks brilliant! Thanks a lot!! Stefan > > > > private: > > template<class Pwd, class ExpectedPwd> > > void set_impl( Type const& value > > , typename boost::enable_if<boost::is_same<Pwd, > > ExpectedPwd> >::type* enable =0 > > ) > > { > > value_ = value; > > } > > > > > Actually ExpectedPwd is not required as template parameter, but it helps to > understand the flow. It is possible to rewrite the Property as: > > > > template<class Type, class PasswordT=void> > class Property > { > > public: > Type const& get()const > { > return value_; > } > > > > template<class PwdT> > void set( Type const& value) > { > set_impl<PwdT>(value); > } > > void set(Type const& value) > { > set_impl<void>(value); > } > > > private: > template<class Pwd> > void set_impl( Type const& value > , typename boost::enable_if<boost::is_same<Pwd, PasswordT> > > >::type* enable =0 > ) > { > value_ = value; > } > > > > private: > Type value_; > };

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[lambda]How to use the member of placeholder in std algorithm?
by 钱晓明 27 Feb '09

27 Feb '09

I have a struct: struct Coordinate { double x, y, z; }; There is no any operator definition for it. I want to sort a array of this struct with 3 elements by comparing the Z value, so: Coordinate coords[3]; ....... std::sort(coords, coords+3, _1 < _2); What should I write to replace "_1 < _2" here? How can I reference the Z member of _1 and _2? I hope I can sort this array without defining any new function. thanks!

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Linking Boost in Code::Blocks
by mothdragon 27 Feb '09

27 Feb '09

Okay, I've been trying to figure this out for a couple of weeks now... I'm using Boost 1.37 on Ubuntu Hardy Heron with Code::Blocks 8.02, Programming in C++.... Now, I'm rather new to Ubuntu, and just as much so programming in Ubuntu... Well, I've been working on this project, and I've been cross-developing it on Windows and Linux, but I've hit a hiccup once i've started using Boost... All works great on Windows using MinGW and Code::Blocks, however I cannot get the same code to run in Linux. The best I can think of is that there's a problem with the linking.... I've added the include path, and the lib path to the search directories, and I've added the libboost_serialization-gcc42-mt-1_37.a to the list of libraries but for some reason it still doesn't work... I get the following error when I try to compile: obj/Debug/Griffin RPG V2.o||In function `extended_type_info_typeid_1':| /usr/include/boost/serialization/extended_type_info_typeid.hpp|77|undefined reference to `boost::serialization::extended_type_info::self_register()'| /usr/include/boost/serialization/extended_type_info_typeid.hpp|77|undefined reference to `boost::serialization::extended_type_info::self_register()'| /usr/include/boost/serialization/extended_type_info_typeid.hpp|77|undefined reference to `boost::serialization::extended_type_info::self_register()'| ]+0x8)||undefined reference to `boost::serialization::detail::extended_type_info_typeid_0::less_than(boost::serialization::extended_type_info const&) const'| ]+0x8)||undefined reference to `boost::serialization::detail::extended_type_info_typeid_0::less_than(boost::serialization::extended_type_info const&) const'| ]+0x8)||undefined reference to `boost::serialization::detail::extended_type_info_typeid_0::less_than(boost::serialization::extended_type_info const&) const'| ||=== Build finished: 6 errors, 0 warnings ===| I've looked everywhere I can think of and chatted on a few IRC's but I haven't been able to get anywhere with this... Because of this I've had to develop only on Windows (Yuck!) and not do any testing in Linux... I take comfort in knowing though that once I get Boost operational, my code will work in Linux too... ( probably better since Linux is so nice to me! ) I'd appreciate any help tat can be offered! -- View this message in context: http://www.nabble.com/Linking-Boost-in-Code%3A%3ABlocks-tp21867302p21867302… Sent from the Boost - Users mailing list archive at Nabble.com.

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[filesystem] How to tell if a file is locked.
by Andrew Maclean 26 Feb '09

26 Feb '09

Is it possible using boost::filesystem functions to determine if a file is locked? I have a program which scans a directory for files and I want it to ignore locked files because other process are writing to them. Basically I want to do the following: get the file if (is_locked()) continue else process the file So I just want to know if it is locked. This must work for both Linux and Windows Vista. Has anyone any ideas or example code? Thanks Andrew -- ___________________________________________ Andrew J. P. Maclean Centre for Autonomous Systems The Rose Street Building J04 The University of Sydney 2006 NSW AUSTRALIA Ph: +61 2 9351 3283 Fax: +61 2 9351 7474 URL: http://www.acfr.usyd.edu.au/ ___________________________________________

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make member functions (in)accessible
by Stefan Funck 26 Feb '09

26 Feb '09

Dear all, I'm trying to find some magic that makes a member function of some object x accessible if and only if it were called from within a class that has object x as a member. Is there a (boost)-way of doing this? As this may seem a bit obscure, I'm trying to give an example: template <typename T, bool Writable> class Property { public: T get() const; void set(const T& val); private: T m_val; }; is a simple class holding a value of T. I have some classes holding potentially many Properties. I'd like to have these properties publically available, some as only readable, others writable as well, so that others (e.g. gui-classes) can access them: class A { public: Property<double, true > propA; Property<int , false> propB; }; so I may do: A a; a.propA.get(); a.propA.set(0.5); a.propB.get(); but NOT: a.propB.set(1); because propB was defined with Writable=false. BUT, from within class A, I would like to do propB.set(1); because otherwise this property is useless. (How) can this be achieved (with boost)? I could make set() private in the Writable=false - specialisation of Property and declare A as a friend, but that's prohibitive since Property shouldn't know about A and it won't work for derived classes of A. Another option would be class A { public: A() : propA(propA_priv), propB(propB_priv) {} PropertyWrapper<double, true > propA; PropertyWrapper<int , false> propB; private: Property<double> propA_priv; Property<int > propB_priv; }; so the actual properties are private and the wrapper provides the actual interface to the outside world. But as there may be many properties I'd like to get away without the wrapper objects ... Thanks for any ideas! Stefan

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BGL question about districting
by Barend Gehrels 26 Feb '09

26 Feb '09

Hi, We're about an automated districting tool. We want to create Y districts from X polygons, all having a specific value (e.g. area or inhabitants), where the total value of each district is as similar as possible and each district is also as similar as possible. Let's call the value the "potential". Of course this problem is NP-hard so we're glad with the first solution or e.g. the first ten solutions. We thought to implement this by the Boost Graph Library. First we detect neighbour relations with our own Geometry Library (using the still-to-build "touches" relationship). We've a graph then and can divide the graph into subgraphs, each having connected nodes and an (nearly) equal "potential". I've some experience with BGL, having used the "connected_components". But I cannot find an appropriate algorithm for this problem, if it is available. Actually it even could be a "network flow" algorithm dividing the total potential by Y, and starting at some point flowing the potential through the graph. But all network flow algorithms seem to be on directed graphs. This is an undirected graph. It it is not possible with a standard BGL algorithm I think we can still solve it using BGL and e.g. breadth-sort-first, but just want to verify if it is possible or maybe by an unexpected workaround. Thanks, Barend

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