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Basic boost::mpl::max_element question.
by Yves Dolce 02 Sep '11

02 Sep '11

Hi everyone, Once past the basic understanding of C++ metaprogramming (factorial, vector/typeslist, .), I must confess I have a hard time using max_element in my particular scenario. And I have not found a lot of samples on the net. Consider this "repro" first: template <unsigned char L> struct Level { static const unsigned char value = L; }; template <class LEVEL> struct CTA { typedef LEVEL Level_t; }; typedef boost::fusion::vector<CTA1, CTA2, CTA3, CTA4> CTAS; Now, using max_element, I'd like to pick the one CTAn for which CTn::Level_t::value is the biggest. Can you enlighten me? Thanks! An MPL/FUSION beginner. J

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[TypeTraits] A patch for clang's intrinsics (was: [type_traits] is_enum on scoped enums doesn't works as expected)
by Michel MORIN 01 Sep '11

01 Sep '11

John Maddock wrote: >> Boost.TypeTraits does not use intrinsics for clang yet. >> (c.f. https://svn.boost.org/trac/boost/changeset/72725) >> >> >> this explains why it is not working now. Is someone working on this issue? > > Not me, we need a Clang expert to look into it - or at least someone with > access to Clang to take care of the testing - someone did submit a patch to > enable intrinsic support for Clang, but it broke more things than it fixed > so it got reverted :-( Let's proceed step by step. As a first step, I made a patch to use clang's intrinsics except problematic intrinsics (i.e. `is_pod` and `is_empty`). All the tests run successfully on gcc-4.4, 4.5, 4.6, clang (trunk) and Mac OS X Lion's clang (Apple clang version 2.1). Regards, Michel

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Re: [boost] [any] new version
by Martin Bidlingmaier 01 Sep '11

01 Sep '11

> Mathias Gaunard wrote: >> You could make next_id a function with weak extern linkage, which should >> be able to make it work. Thanks for the replies, I hope I fixed the linkage problems now, by making next_id() inline. There were other linker related issues, which are hopefully fixed now as well. The modified any.hpp header and a simple linker test are attached -- NEU: FreePhone - 0ct/min Handyspartarif mit Geld-zurück-Garantie! Jetzt informieren: http://www.gmx.net/de/go/freephone

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[shared_ptr] Implicit 2 phase construction with enable_shared_from_this
by Sebastian Karlsson 01 Sep '11

01 Sep '11

There's been some discussion about solving the problem with shared_from_this() not being available in the constructor. This is fairly natural as shared_ptr ownership doesn't happen until after the constructor leaves, and we're not even guaranteed that the object enters shared_ptr ownership at all. What I'm suggesting is the addition of a private void enable_shared_from_this<T>::accepted_shared_ptr() {}. Then inside enable_shared_from_this<T>::_internal_accept_owner we add: const_cast< T* >( static_cast< const T* >( this ) )->accepted_shared_ptr(); Just beneath: weak_this_ = shared_ptr<T>( *ppx, py ); This allows clients of enable_shared_from_this to add their own accepted_shared_ptr() which will correctly be called while I believe not breaking any old code. I just added the const_cast because I'm not familiar with the rationale behind _internal_accept_owner being const. This change nets us: 1 ) Generic code dealing with construction of shared_ptrs ( in any of its forms ) does not need to bother themselves with 2 phase construction. 2 ) The distinction between construction & transfer of ownership to shared_ptr makes it easier to cope with both the case where T is not managed by a shared_ptr and the case where it is. One gotcha I've found is that for example: struct foo : enable_shared_from_this< foo > { foo() { boost::shared_ptr< foo > tmp( this, null_deleter() ); } // We might be allocated on the stack, so use a null_deleter for now void accepted_shared_ptr() { /* here we can patch it up with shared_from_this(), which will use our assigned deletion strategy */ } } Does not behave as one would want to. Basically in this case I'd want to use a shared_ptr<> which makes no assumptions about whether we're managed by a shared_ptr or not, we want a shared_ptr which acts as a plain pointer. The problem however is that the shared_ptr which we construct with the null_deleter will grab the enable_shared_from_this before even construction is complete. This can be solved by the following, as seen in boost::make_shared: ... boost::shared_ptr< foo > empty_with_deleter( static_cast< foo* >( 0 ), null_deleter() ); boost::shared_ptr< foo > tmp( empty_with_deleter, this ); // This one is safe to use ... Which doesn't enter boost::detail::sp_enable_shared_from_this. Would love some input from someone a bit more knowledgeable about shared_ptr. Kind regards, Sebastian Karlsson

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[review] Conversion - About specialization, overloading and ODR
by Vicente J. Botet Escriba 01 Sep '11

01 Sep '11

Hi, during the Boost.Conversion review and pre-review it has been argued that as the user must provide specializations of the conversion function for other UDT's, it could promotr ODR violations as two independent libraries can provide the same specialization making them incompatible for an end user. This issue is present for any library for which the user could provide an overload or could specialize a class. The standard library protects itself from this issue saying that is undefined behavior any specializations of standard classes (except maybe numeric_traits that has a single parameters and it is intendeed to be specialized by the library providing the UDT). My question is: Should Boost (all the Boost libraries) document explicitly that any class specialization or overload of free fuctions could result in undefined behavior? Is this already implicitly acknowledged? Best, Vicente

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[review] Conversion review ends today
by Gordon Woodhull 01 Sep '11

01 Sep '11

Folks, The review of Vicente's Conversion library ends today. Unfortunately there have only been four reviews, which is hardly enough to provide proper feedback. If you've considered writing a review or if you have any thoughts about the topic, please speak up now. Or if you'd like to submit a late review, just let me know. Thanks, Gordon

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[boost/property_tree/json_parser] - Consuming a Json file returning from rest web service
by Demetrio Car 31 Aug '11

31 Aug '11

> > Could someone send me an example of how to use json_parser to read a Json > file? I will need to consume a Rest web service developed in Rails. This web > service will respond to my code using json, in other words, from my C++ code > running over linux I will access an url which will return to me a file in > Json format. The main point is: how do I access the url > http://my_gienah:3000/dominios/dominiosAll.json<http://gienah:3000/dominios/dominiosAll.json> or > the url > http://my_gienah:3000/Tipoestruturas/ObtemTipoEstrutura_IdDominio/1.json<http://gienah:3000/Tipoestruturas/ObtemTipoEstrutura_IdDominio/1.json> .In > the last url, I must set parameters. The return is something like this: > > [{"DscDominio":"Johnson Argentina"},{"DscDominio":"Johnson Brasil"},{"DscDominio":"Farmoqu\u00edmica"},{"DscDominio":"Dom\u00ednio Inativo"},{"DscDominio":"NIKKHO"},{"DscDominio":"teste"},{"DscDominio":"Dominio"},{"DscDominio":"teste"},{"DscDominio":"teste"},{"DscDominio":"Teste"},{"DscDominio":"Teste"},{"DscDominio":"Dom"},{"DscDominio":"teste"},{"DscDominio":"teste"},{"DscDominio":"Teste"},{"DscDominio":"descricao"},{"DscDominio":"DesccricaoTeste"},{"DscDominio":"teste"},{"DscDominio":"teste p"},{"DscDominio":"teste p 2"},{"DscDominio":"teste p 3"},{"DscDominio":"Teste Dominio Produto x"},{"DscDominio":"Teste Dominio Produto x"},{"DscDominio":"Teste Dominio Produto x"},{"DscDominio":"Dominio Produto 123"},{"DscDominio":"subdom2"},{"DscDominio":"teste"},{"DscDominio":"2"},{"DscDominio":"3d"},{"DscDominio":"X"}] > >

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[review] conversion - Vladimir's comments
by Gordon Woodhull 31 Aug '11

31 Aug '11

Hi Vicente, all, I had hoped that Vladimir Batov, author of Boost.Convert reviewed earlier this year, would contribute an "informal review". Instead I hope he doesn't mind if I quote some correspondence he sent off-list, and perhaps get him to join us. (I normally would never quote off-list communication on-list, but I don't see how a review manager can use off-list contributions without quoting them, if there's no request for anonymity. I hope Vladimir agrees that I've been judicious. ;) On Aug 1, 2011, at 8:47 PM, Vladimir Batov wrote: > On the conceptual level it is far from clear to me if such a framework is needed to begin with. In the Scope section Vicente explicitly states that his library "is not particularly concerned with cases of...". Those "excluded" cases happen to be the most tricky/used/needed cases. With those string-to-type, etc. cases taken out of the picture from the set-go, the usefulness of the remainder was many times questioned during Convert discussions (well before the Convert review) ... I never had a need for such a generic type-to-type conversion framework. Obviously, it's my own limited experience and it can be vastly different for others. > Secondly, on the implementation level I raised a few concerns in our conversations with Vicente about his library ... I do not feel Vicente's implementation is sufficiently generic. In particular, I am under strong impression that having Target and Source together as in > > template< class Target, class Source > > Target explicit_convert_to( Source const & u ); > > does not really work on a bigger scale. That does indeed work for *concrete* Target and Source types. However, when one tries to generalize (partially specialize) for a range of similar types, then I feel the framework easily falters. Namely, if I decide to specialize the above for a range of related Target types (say, string-related types) *and* separately specialize for a range of related Source types. Then compilation will fail if these two separate specializations happen to overlap for some Target-Source pair. That is not an academic case but concrete issues I faced with Convert. I'd expect it to be as relevant with Vicente's library. I agree that it will take some careful shepherding to define a set of conversions without ambiguous overloads, which might be tricky to avoid if you're working with converters written by other people. Vicente, do you have any solutions for this? Cheers, Gordon

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interprocess locks and move semantics compile problem [SEC=UNCLASSIFIED]
by Levy, Gabriel (Contractor) 31 Aug '11

31 Aug '11

UNCLASSIFIED Hi all, I'm having a problem getting this trivial couple of statements compile on VC9 SP1 (VS2008), specifically the "move" emulation. The sanitised snippet looks like this: using namespace boost::interprocess; interprocess_upgradable_mutex mMtx; sharable_lock<interprocess_upgradable_mutex> sherLock(mMtx); upgradable_lock<interprocess_upgradable_mutex> upgLock(move(sherLock)); The error message I'm getting is (nothing more, nothing less): 1>d:\experimental\notificationcentre.cpp(632) : error C2664: 'boost::interprocess::upgradable_lock<UpgradableMutex>::upgradable_lock( boost::interprocess::scoped_lock<Mutex> &)' : cannot convert parameter 1 from 'boost::interprocess::rv<T>' to 'boost::interprocess::scoped_lock<Mutex> &' 1> with 1> [ 1> UpgradableMutex=boost::interprocess::interprocess_upgradable_mutex, 1> Mutex=boost::interprocess::interprocess_upgradable_mutex 1> ] 1> and 1> [ 1> T=boost::interprocess::sharable_lock<boost::interprocess::interprocess_u pgradable_mutex> 1> ] 1> and 1> [ 1> Mutex=boost::interprocess::interprocess_upgradable_mutex 1> ] Strangely the compiler thinks that the best fit for the constructor overload (with the "move") is the one with "scoped_lock" parameter type instead of the "sharable_lock" version of it. The same is the problem combining lock's move across the other types (up or down grading the type of lock) I must be missing something since I can't see anyone else experiencing this problem. Could anyone advise and shad some light as to why the "move" doesn't work for me on the original interprocess locks? Thanks in advance for any help. Gabe Gabe Levy Senior Software Engineer, HiQ Systems Pty Ltd IMPORTANT: This email remains the property of the Department of Defence and is subject to the jurisdiction of section 70 of the Crimes Act 1914. If you have received this email in error, you are requested to contact the sender and delete the email.

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[Conversion] Review
by Antony Polukhin 30 Aug '11

30 Aug '11

First of all, there is already a Boost.Conversion library (released in 1.20.0). Users will be confused, because two libraries have same names. * What is your evaluation of the design? It is simple and looks good. But consider the following situation: lexical_cast<char>(1) and numeric_cast<char>(1) will produce output. It must be documented, which cast will your library use in which cases. There must be some tags, to allow user to choose, which conversion is required. There are some dummy::type_tag tags in source code, but looks like they are not used, and not documented! * What is your evaluation of the implementation? Did not look very carefully trough the sources. * What is your evaluation of the documentation? There is no documentation about which conversion when used. convert_to<char>(1) will call numeric_cast or lexical_cast? * What is your evaluation of the potential usefulness of the library? Usually user just want to use lexical_cast or numeric_cast. Situations, when generic conversion is required are less often. * Did you try to use the library? With what compiler? Did you have any problems? I did not used it. * How much effort did you put into your evaluation? A glance? A quick reading? In-depth study? A quick reading. * Are you knowledgeable about the problem domain? I am knowledgeable about lexical conversions domain. Generic conversions are close enough to that domain. * Do you think the library should be accepted as a Boost library? Yes, it should. But only after the documentation will be updated and tags added. Until then, it is not clear which cast is when used and library is not as generic as possible. But such big changes to library design will require some new short review.

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