May 2013 - Boost - lists.boost.org

JSON Parser + Boost Fusion + adapted structs
by Stephan Bourgeois 28 May '13

28 May '13

Hi everybody, I am writing a prototype for the JSON parser for the GSoC with the support of Bjorn Reese. When discussing the proposal, Michael Marcin suggested an interface that allows the user to define a struct reflecting the JSON data structure. I have been looking at Boost.Fusion to enable the parser to "fill in" the values in the struct. It looks like I'm after a solution described in the following post: http://stackoverflow.com/questions/13830792/simulate-compile-time-reflectio… Could anyone tell me if this is a good starting point, and/or if there are other ways of achieving this. Thank you, Stephan.

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[context] make_fcontext stack pointer unclear from documentation
by Michael Marcin 28 May '13

28 May '13

The function make_fcontext takes a stack pointer. However it is not clear at all from the documentation that the stack pointer should actually point to the end of the stack buffer. AFAICT it takes close examination of the simple_stack_allocator helper in the examples folder to figure this out. This led to several hours of debugging when I naively passed in the pointer to the start of the stack buffer and random application memory got stomped. I guess you're supposed to glean it from the note at the bottom of the stack allocation page [1]. This should at least be mentioned in the make_fcontext reference [2]. [1] http://www.boost.org/doc/libs/1_53_0/libs/context/doc/html/context/stack.ht… [2] http://www.boost.org/doc/libs/1_53_0/libs/context/doc/html/context/context/…

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Re: [boost] JSON Parser + Boost Fusion + adapted structs
by Niall Douglas 28 May '13

28 May '13

> Then, there's boost.mirror that hasn't given much news recently, but that > had one thing right: you need something to generate the metadata to avoid > the pitfalls of writing the macro by hand. I had in mind a tool based on > clang to do that but never had time to care about it since my very naive > perl code was enough for my small needs. Easiest of all is to expose the compiler's internal state via a magic namespace in the form of pseudo-template definitions. Then one has, at metaprogramming stage, access to everything the compiler knows at that point. I posited that idea to Chandler Carruth for clang at C++ Now. He didn't seem adamantly opposed. Third party libraries (e.g. a Boost one) could wrap up each compiler's internal magic namespace into something portable and therefore useful. Niall

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[MSM] Compile-time error using SubMachines and events that carries some data
by Fernando Pelliccioni 28 May '13

28 May '13

Hi, I have a Main State Machine (state_machine_) and two SubMachines. I want to start the first SubMachine and when it finished I want to start the second SubMachine. Here is the code. http://pastebin.com/sBdS1wvJ I have a compile-time error on some States of the second SubMachine. error: no member named 'element' in 'exit_introduction' std::cout << "event.element(): " << event.element() << std::endl; ~~~~~ ^ This is on state "StaringGame" and according the Transition Table of GameSM, only the event "character_selected" is used. I don't know why is trying to use the event "exit_introduction". Is this a bug of MSM? Am I doing something wrong? Thanks and regards, Fernando Pelliccioni.

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GSoC: Seven proposals accepted
by Boris Schaeling 28 May '13

28 May '13

Google just announced the seven proposals which were accepted for Boost. The (entire) list can be found here: <http://www.google-melange.com/gsoc/projects/list/google/gsoc2013> Congratulations to the successful students! Good luck and much fun in this summer's Boost C++ projects! :) For those students who were not selected: Feel free to contact me for the details! We actually asked Google for more than seven slots because we got more than seven amazing proposals. Unfortunately the number of slots is just limited. And so we had to make some tough decisions. :/ I also thank everyone who had volunteered to mentor a project! I know that not everyone could get the project they wanted. But maybe there is still enough motivation to move forward with some projects outside of the Google Summer of Code program. :) Boris

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Algorithmic Skeletons, Parallelism Patterns & Actors
by Suminda Dharmasena 28 May '13

28 May '13

Hi, Is it possible to introduce Algorithmic Skeletons, Parallelism Patterns & Actors in Boost? Any plans for this? Suminda -- Suminda Sirinath Salpitikorala Dharmasena, B.Sc. Comp. & I.S. (Hon.) Lond., P.G.Dip. Ind. Maths. J'Pura, MIEEE, MACM, CEO Sakrīō! ▣ *Address*: 6G • 1st Lane • Pagoda Road • Nugegoda 10250 • Sri Lanka. ▣ *Mobile* : +94-(0)711007945 ▣ *Tele*: +94-(0)11-5 864614 / 5 875614 / 2 825908 ▣ *Web *: http://www.sakrio.com ▣ This email is subjected to the email Terms of Use and Disclaimer: http://www.sakrio.com/email-legal. Please read this first. --

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[type_erasure] compile error with gcc 4.8
by Christophe Henry 27 May '13

27 May '13

Hi Steven, the following code used to compile with older gcc versions (I just tried with 4.5 and I used the same construct with 4.7) but no more with gcc 4.8, but I don't know whether it's a gcc or type_erasure problem (or mine). The problem seems to be with rvalue refs. If I use the commented out version, it compiles fine on both compilers. Any idea? If you have no gcc 4.8, I can attach the error. Thanks, Christophe #include <iostream> #include <boost/type_erasure/any.hpp> #include <boost/type_erasure/builtin.hpp> #include <boost/type_erasure/operators.hpp> #include <boost/type_erasure/constructible.hpp> #include <boost/type_erasure/relaxed.hpp> #include <boost/type_erasure/member.hpp> #include <boost/mpl/vector.hpp> #include <boost/function.hpp> BOOST_TYPE_ERASURE_MEMBER((has_post), post); struct any_post_concept: ::boost::mpl::vector< //has_post<void(boost::function<void()>), const boost::type_erasure::_self>, has_post<void(boost::function<void()>&&), const boost::type_erasure::_self>, boost::type_erasure::relaxed, boost::type_erasure::copy_constructible<> >{} ; typedef boost::type_erasure::any<any_post_concept> any_post; struct any_impl { //void post(boost::function<void()> f)const {f();} void post(boost::function<void()> && f)const {f();} }; int main() { any_impl s; any_post p (s); boost::function<void()> f = [](){std::cout << "ok" << std::endl;}; //p.post(f); p.post(std::move(f)); return 0; }

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[thread] countdown_latch
by Michael Marcin 27 May '13

27 May '13

I recently ran across the need to spawn a thread and wait for it to finish its setup before continuing. The accepted answer seems to be using a mutex and condition variable to achieve this. However that work clutters up the code quite a bit with the implementation details. I came across Java's CountDownLatch which does basically the same work but bundles it up into a tidy package. It seems to be a fairly trivial but useful abstraction. implementation: http://codepad.org/E8kd2Eb8 usage: class widget { public: widget() : latch( 1 ) , thread_( [&]{ thread_func(); } ) { latch.wait(); } private: void setup(); void run(); void thread_func() { setup(); latch.count_down(); run(); } countdown_latch latch; std::thread thread_; };

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Proposal for a new assert macro which stores the failed values ("informative assert")
by Viktor Sehr 27 May '13

27 May '13

I'd like some opinions about this assert implementation which I'd like to propose as an update of the old BOOST_ASSERT. Basically it stores the failed values next to the expression string, instead of just the expression string: "INFORMATIVE_ASSERT(5*5 < 2*2);" "5*5 < 2*2 | 25 | 4" "INFORMATIVE_ASSERT(std::vector<int>(10).size() == std::vector<int>(12).size());" "std::vector<int>(10).size() > std::vector<int>(12).size() | 10 | 12" Notes - If the class can't be serialized, the byte-data is logged instead. - It writes the values to the expression string in order to be compliant with any assert implementations Easy to read implementation available here: https://docs.google.com/file/d/0B69BCPzak52DcnF5dmhRRno3V2M/edit?usp=sharing Modified version of BOOST_ASSERT available here: https://docs.google.com/file/d/0B69BCPzak52DZW1oX1AtREM4eVU/edit?usp=sharing /Viktor Sehr

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Documenting and checking template parameters
by Paul A. Bristow 27 May '13

27 May '13

I am just refactoring the venerable circular_buffer documentation. The docs describe the Template parameter T and specify it thus: The type of the elements stored in the circular_buffer. The T has to be SGIAssignable (SGI STL defined combination of Assignable and CopyConstructible). Moreover T has to be DefaultConstructible if supplied as a default parameter when invoking some of the circular_buffer's methods e.g. insert(iterator pos, const value_type& item = value_type()). And EqualityComparable and/or LessThanComparable if the circular_buffer will be compared with another container. This looks a bit out of date? Questions: How should a parameter like this *best* be described in documentation and enforced in software? http://www.boost.org/doc/libs/1_53_0/doc/html/Assignable.html ? http://www.boost.org/doc/libs/1_53_0/libs/concept_check/concept_check.htm ? Should these concepts be checked using BOOST_STATIC_ASSERT? Should *All* these be checked, even though not required for all uses? Suggestions welcome. Thanks Paul --- Paul A. Bristow, Prizet Farmhouse, Kendal LA8 8AB UK +44 1539 561830 07714330204 pbristow(a)hetp.u-net.com

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