February 2009 - Boost - lists.boost.org

[graph] kolmogorov_max_flow color map, gray state
by Sebastian Nowozin 01 Mar '09

01 Mar '09

Hello everybody, I use the boost graph library to find the linear min-cut of a directed graph with exactly one source and one sink node (s-t min-cut). For this, I setup a linear max-flow problem and solve it using kolmogorov_max_flow. The color_map property is supposed to be usable to recover the edge cut set from the max-flow solution, see the description on http://www.boost.org/doc/libs/1_38_0/libs/graph/doc/kolmogorov_max_flow.html However, as long as there are no vertices colored "gray" I can simply tell which side of the cut a node belongs to by doing something like: std::vector<default_color_type> color(num_vertices(g_ab)); double flow = kolmogorov_max_flow(g_ab, alpha_vertex, beta_vertex, color_map(&color[0])); bool is_left = (color[node_desc] == tc_color_traits::white()); bool is_right = (color[node_desc] == tc_color_traits::black()); And this works just fine. For some problems I am solving there are gray nodes, and I do not know how to determine their position relative to the edge cut set. Is there a code example available in the boost repository or has anyone had this problem? It seems surprising to me that obtaining the edge cut set of a linear min-cut problem seems to be non-trivial using the boost graph library. I must be doing something wrong. Thanks, Sebastian

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Bug in intrusive_ptr,scoped_ptr,scoped_array
by Hansi 28 Feb '09

28 Feb '09

Hello, at least with VS2005 there is a bug in the three smart pointers... If you want to check if the pointer is empty (with unspecified bool) then you get also with NULL a true back. For this the following fix should work: #if defined( _MANAGED ) static void unspecified_bool( this_type*** ) { } typedef void (*unspecified_bool_type)( this_type*** ); operator unspecified_bool_type() const // never throws { return px == 0? 0: unspecified_bool; } #else .... #endif Best regards Hansjörg

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[move] Library uploaded to sandbox
by Ion Gaztañaga 28 Feb '09

28 Feb '09

Hi to all, I've uploaded a new version of the move emulation library to sandbox. I've put it temporarily in "move_semantics" folder to avoid overwriting current move code. I've put some quickbook documentation whose introduction is based in the article "A Brief Introduction to Rvalue References" by Hinnant, Stroustrup, Kozicki. http://svn.boost.org/svn/boost/sandbox/libs/move_semantics/index.html The library needs specially modified Interprocess containers (and Intrusive because of dependences), which I've also uploaded to sandbox. The examples show how movable only values can be inserted into containers. For those that don't like svn sandbox (I'm one of them) I've also put the code and the documentation here: http://www.drivehq.com/web/igaztanaga/move_semantics.zip And online documentation also here: http://www.drivehq.com/web/igaztanaga/libs/move_semantics/ The documentation is not in english, it's in my own english dialect so be careful when reading ;-) Patches and corrections welcome. I've documented some functions to create a little boostbook reference. I've invested some precious time in the library and testing different approaches to see if it plays well with containers but I would like to hear some comments on whether the direction is acceptable and if I should spend more time on it. If someone else with more time wants to continue the library, please do it! ;-) I've personally added to the library all that I've seen useful for move-aware containers, so it might lack some features (move aware algorithms, for example). Anyway, my opinion is that we need some move library, better now than later even if it's too basic, so that we can unify all redundant move emulation that we already have in boost libraries. I hope this is a step in the right direction. Best, Ion

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[threadpool] new version - with rescheduling of current task
by k-oli＠gmx.de 28 Feb '09

28 Feb '09

Hi, I've uploaded a new version of boost.threadpool. It contains a free function this_task_reschedule_until() which lets the current task reschedule until a condition (passed to the fucntion) becomes true. This allows to synchronize with other async events without blocking the worker-thread (thx to Vicente). I've also added two function in order to wait for multiple tasks (it uses Anthonies future lib wait_for_any()/wait_for_all() ). Oliver

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[python] Making a Python tuple from a Fusion sequence
by Bruno Lalande 27 Feb '09

27 Feb '09

Hello, I have written a little function that converts any Boost.Fusion sequence into a Python tuple (boost::python::tuple). If a sub-sequence is nested in the sequence, the result will also be a nested tuple (for instance, boost::make_tuple(0, std::make_pair(1, 2), 3) will give (0, (1, 2), 3) in Python). The source code is attached to this mail. The principle is that any sequence previously adapted to Boost.Fusion will become a tuple in Python. So, by including the right boost/fusion/adapted/xxx header, one can convert a pair, a tuple, a boost::array, and obviously any of the sequences provided by Boost.Fusion. For example: #include <boost/python.hpp> #include <boost/fusion/adapted/std_pair.hpp> #include <boost/fusion/adapted/boost_tuple.hpp> #include <boost/fusion/container/generation/make_vector.hpp> #include "make_tuple_from_fusion_sequence.hpp" using namespace boost::python; tuple from_sequence() { return make_tuple_from_fusion( boost::fusion::make_vector( 1, std::make_pair("first", "second"), 2, boost::make_tuple('a', 'b', 'c'), 3 ) ); } BOOST_PYTHON_MODULE(mymodule) { def("from_sequence", &from_sequence); } In Python we get: >>> import mymodule >>> mymodule.from_sequence() (1, ('first', 'second'), 2, ('a', 'b', 'c'), 3) Is there any interest in adding this function into Boost.Python? If yes, I can write the doc and tests, clean the source and maybe improve the implementation (for example, I feel that I could avoid the use of m_iteration with a better use of Boost.Fusion...). Regards Bruno

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another approach to memory management that solves the referential cycles problem
by Achilleas Margaritis 27 Feb '09

27 Feb '09

Dear boost members, I have placed in the boost Vault, under the section "memory", the file 'mm.zip' which contains: 1) a file "readme.txt" with a little documentation. 2) a file 'main.cpp' which contains a reference implementation and test. This (I wouldn't not call it new, but certainly interesting) approach to memory management solves the problem of cycles and it can also provide a sort of garbage collection, without the issues of other approaches (explained in the readme). Suggestions, comments, ideas, etc are welcomed.

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Re: [boost] [geometry] area, length, centroid, behaviour
by Mika Heiskanen 27 Feb '09

27 Feb '09

I wrote: > However, I'd hate to have to produce a certain winding rule for the > exterior and the holes in order to use the same data for more > complicated GIS calculations such as logical operations and buffering. > IMHO it should be just as easy as choosing to use the even-odd filling > rule instead of the winding rule. To which Barend Gehrels replied: > It cannot be called from the algorithms themselves: to > verify the winding rule the area has to be calculated beforehand. This > will drastically decrease performance. Certainly area calculation is significantly faster than logical operations and buffering. > The solution might be a traits class telling the winding rule the > polygon type has. Agreed. [Sorry about the unorthodox quoting, I rebuilt the reply by googling] --> Mika Heiskanen

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Interest check: factory patterns
by Tor Brede Vekterli 27 Feb '09

27 Feb '09

Hi all I've been working on a "Modern C++ Design"-inspired factory/abstract factory pattern implementation for a while now, and was wondering if there was any interest in having it as a Boost library at some point? If my understanding of the existing Boost.Functional/Factory is correct, then this should not really clash with its area of operation. Factory features - Configurable identifier, mapping, error policy types - Function storage type can be overridden by the user on a per-factory basis Abstract factory features: - Auto-generation of class hierarchies (a-la Loki)--allows passing as individual factory fields - Passing of construction-arguments to the concrete type "creators" (operator new by default, but extensible) - Prototypes - Easy (re-)binding of constructor arguments - Boost.Functions if none of the above are appropriate or flexible enough - Mixing and matching creators for concrete factories. i.e., allows to use a prototype for one concrete type, Boost.Function for another, operator new for the third and so on - Easily specify if the created object should be returned as a raw pointer or a smart pointer (any type with a nested element_type is automatically recognized) - Almost everything is policy-specifyable Somewhat contrived example: typedef abstract_factory< // Want object created with no args, returned as raw pointer abstract_foo* // Want object created with std::string const ref, float by value args; returned as auto_ptr , std::auto_ptr<abstract_bar>(const std::string&, float) > abstract_factory_t; concrete_factory< // Concrete factory for abstract_factory_t abstract_factory_t // Concrete creator for abstract_foo; prototype , prototype<> // Concrete creator for abstract_bar; operator new creator (default when it's a regular type) , bar_impl > cf; // Set prototype for abstract_foo (takes ownership of raw pointers) cf.prototype<abstract_foo>(new fancy_foo_impl); const abstract_factory_t& af = cf; // By default invoke clone() on the stored prototype abstract_foo* foobj = af.create<abstract_foo>(); // Forward args to creator, in this case operator new --> constructor of bar_impl std::auto_ptr<abstract_bar> barobj = af.create<abstract_bar>("hello boost", 1234.5f); Preliminary documentation and download link can be found here: http://arcticinteractive.com/static/boost/libs/factory/doc/html/index.html I've yet to request any vault/sandbox access, because I first would really like to get some input on what people who are far more knowledgeable and experienced in C++ and software design than myself think of it :). Critique, comments, suggestions etc. are very welcome on all aspects of the library (code, design, documentation,...) Regards, Tor Brede Vekterli

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Sandbox structure in subversion (was Re: [move] Library uploaded to sandbox)
by Daniel James 27 Feb '09

27 Feb '09

2009/2/17 Ion Gaztañaga <igaztanaga(a)gmail.com>: > > http://svn.boost.org/svn/boost/sandbox/libs/move_semantics/index.html [snip] > > For those that don't like svn sandbox (I'm one of them) I've also put the > code and the documentation here: Most people nowadays are putting their libraries into subdirectories of sandbox, instead of mixing it up with the stuff that's already there. It's much easier to check out and browse online. For example, there's chrono at: https://svn.boost.org/svn/boost/sandbox/chrono https://svn.boost.org/trac/boost/browser/sandbox/chrono It is messy having this in the middle of the old sandbox layout. We could separate out the two although doing this without disrupting development would be tricky. Maybe we could create a new space in subversion and let people migrate to it if they wish. We could use a more traditional subversion layout, perhaps something like: /svn/boost/projects/chrono/trunk /svn/boost/projects/chrono/branches /svn/boost/projects/move_semantics/trunk /svn/boost/projects/move_semantics/branches Daniel

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[iostreams] detecting io errors
by Johan Grafstrom 27 Feb '09

27 Feb '09

Hello, I use boost:iostreams for reading and writing a binary data format. There is currently no use of filters, but the reason for using boost was to try using compression by adding a filter. I have run into some problems/questions regarding the boost:iostreams::stream class though, and was hoping that someone here might give me a hint. The files I save is project files, and the users use a versioning system to lock the files while editing. Occasionally they forget to lock and would get an error when trying to save, since the files in write protected unless locked. My first (and worst) problem was that i did not get any error at all when the write failed, I found out that this was my own fault since I used: bio::stream<bio::file_sink> outs(tstFileName, std::ios_base::out); // ... write etc... if (outs.is_open()) // error ... instead of if (outs->is_open()) // ^^ '->', not '.' must be used here // error ... but it seem like an easy pitfall for newbies like me. A different method name or at least a warning in the documentation maybe? The second problem is that I cannot find any way to get information on the nature of the write error. E g is it an access permission problem or a hardware error? The third problem is that seem to be no way to detect an error when closing the file. I have attached a short c-file to illustrate what I would want to know. I pause and remove the USB-stick with the open file before closing. c-runtime streams will give me error on close, but I cant get the same information out of boost::iostreams. Any help, directions or opinions would be appreciated. Best regards, Johan Grafström /* -*- mode: C++; coding: utf-8-dos; -*- */ /* ========================================================================== * boost_test2.cpp * * Problems: * * * Boost only reports false for outs->is_open() after error but gives no * details (write failure, invalid handle, access violation) * * * An error on file close cannot be detected * * * The worst problem was my own: * using outs.is_open() (always returns true) * instead of outs->is_open() returns false after error * * ========================================================================== */ #include <stdio.h> #include <conio.h> #include <boost/iostreams/device/file.hpp> #include <boost/iostreams/stream.hpp> void boostWriteTestFile(); void streamsWriteTestFile(); void userInteract(); namespace bio = boost::iostreams; char tstFileName[] = "write_test.txt"; int main(int argc, char *argv[]) { // DebugBreak(); if (argc == 2 && strcmp(argv[1],"streams") == 0) { printf("Write using c-runtime stream i/o\n"); streamsWriteTestFile(); } else { printf("Write using boost:iostreams\n"); boostWriteTestFile(); } return 0; } void boostWriteTestFile() { char streamText[] = "Written by boost iostreams api\n"; int msgSize = strlen(streamText); // write using boost bio::stream<bio::file_sink> outs(tstFileName, std::ios_base::out | std::ios_base::binary); int bytesWritten = outs->write(streamText, msgSize); if (bytesWritten != msgSize) printf("Error boost writing\n"); if (!outs->is_open()) printf("Error boost file closed\n"); userInteract(); // Problem: how to get error status from outs.close() ?? // returns void // in boost/iostreams/device/file.hpp:175 // template<typename Ch> // void basic_file<Ch>::close() { pimpl_->file_.close(); } // returns void thus ignoring the return value from // basic_filebuf.close() in c++ runtime <fstream> outs.close(); } void streamsWriteTestFile() { char streamText[] = "Written by c-runtume streams api\n"; int msgSize = strlen(streamText); // write using c-runtime streams FILE *fh = fopen(tstFileName, "w"); if (fh == NULL) { perror("on fopen()"); return; } size_t countWritten = fwrite( streamText, // const void *buffer, 1, // size_t size, msgSize, // size_t count, fh // FILE *stream ); if (countWritten != msgSize) { perror("On fwrite()"); return; } userInteract(); int status = fclose(fh); if (status == 0) { // ok } else { // status == EOF perror("on fclose()"); return; } } void userInteract() { printf("Please remove the file %s, in some apropriate manner e g \n" "remove the floppy or usb-drive, delete with Unlocker or equivalent.\n" "Then press any key to continue.\n", tstFileName); _getch(); }

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