August 2009 - Boost-users - lists.preview.boost.org

[Boost.Smart_ptr] global_reset (revamp) for shared_ptr
by Robbie Morrison 19 Aug '09

19 Aug '09

Hello Boost-Users CAVEAT The attached code "works" but has a major design flaw regarding pointer conversion safety -- one which can currently either be accepted (at your risk) or rectified by limiting usage to one-hop inheritance. In particular, statement [3] (see below) is processed without complaint in unsafe mode -- when it should ideally provoke a compile-time error. Indeed, providing a general type-safe solution is not without challenge (read: my knowledge of C++ has been exhausted). If interested, member function 'Common::reVamp' would be a good place to start. PROBLEM I recently needed a 'shared_ptr' that could be globally reset -- that is, any one of the owners of the controlled resource could unilaterally replace that resource for the benefit (perhaps) of everyone else. This has been named "global_reset" in previous postings -- but I have adopted the shorter word "revamp". Moreover, it would be useful if this newly conceived "assign_ptr" could mimic the behavior of 'shared_ptr', whilst simultaneously providing the additional functionality. In particular, implicit upcasting should be maintained. This lead me to quickly reject options like: shared_ptr<scoped_ptr<Resource> > ptrPtrResource; And instead try to implement something like: assign_ptr<T> py(new Y()); My environment is single-threaded and issues like CPU cycles and memory consumption are not significant. Of more importance to me was coding at a high level -- in part, because I imagined that any conforming solution could subsequently be refactored to improve on performance and safety. SOLUTION STRATEGY The proposed solution has parallels with the Observer pattern and relies on 'shared_ptr' to do most of the work. Notwithstanding the code is now considerably more involved than I had originally envisaged. The biggest challenge was getting a multi-step inheritance chain to play ball with the semantics and syntax. Consider, if you would: class A; // base class class B : public A; class C : public B; // and perhaps more assign_ptr<B> b(new C()); // C resource held by B pointer [1] assign_ptr<A> a = b; // resource now also held by A pointer a.revamp(new C()); // not so simple to implement [2] The statement marked [2] looks reasonable. But it offers my observer class a challenge -- namely the need to have available the type information present during statement [1] when re-assigning the replacement resource to 'b' during the 'revamp' call. In addition, the following statement should fail in some way -- ideally at compile-time: a.revamp(new A()); // okay for 'a', nonsensical for 'b' [3] The following member functions are offered, noting that the first two are already part of 'shared_ptr:' a.use_count() // number of owners in the pool a.unique() // true if use_count() == 1 a.revamp_count() // number of revamps a.original() // true if revamp_count() == 0 a.external_count() // number of owners outside the pool a.exclusive() // true if external_count() == 0 PRIOR DISCUSSIONS The problem itself is noted in the Boost documentation: Boost Libraries + Smart Pointers + shared_ptr class template + Smart Pointer Programming Techniques + Obtaining a shared_ptr from a raw pointer One significant thread on this topic can be found at: subject = shared_ptr and global reset date = Feb-2009 http://thread.gmane.org/gmane.comp.lib.boost.user/45032/focus=45034 CODE A single *.cc file is attached. This code was developed using: boost : 1.39.0 (built from source) compiler : GCC g++ 4.1.2 (2006) kernel : Ubuntu 6.10 Linux 2.6.17-12-generic hardware : 1.4GHz Intel Celeron M / 512MiB RAM DESIGN SUMMARY This section gives a brief outline of the design thinking I used. Note too that the code is also extensively documented. The point-of-entry is class template 'assign_ptr'. This holds its own controlled resource pointer, just like 'shared_ptr', but in this case, as a 'shared_ptr'. It also holds a 'shared_ptr' to a class 'Common' object (aka "Subject" under the Observer pattern). Class 'Common' manages a list of its 'Client's (aka "Observers") and, in doing so, defines the pool. The derived class template 'ClientImp' duly retains the type of its associated 'assign_ptr'. The various 'assign_ptr' member functions pass requests to the 'Common' object, which responds accordingly. Most of these member functions were declared above. Class 'Common', in turn, contains the management functions 'attach' and 'detach' and the interventionist function 'reVamp'. The remainder of the 'assign_ptr' implementation is aimed at providing a similar interface to 'shared_ptr'. Finally, everything is placed in namespace 'xeona'. LITERATURE Alexandrescu (2001, ch7, pp157-195) devotes a chapter to smart pointers and the underlying design trade-offs -- however "revamping" is not considered: Alexandrescu, Andrei. 2001. Modern C++ design : generic programming and design patterns applied. Addison-Wesley, Boston, USA. ISBN 0-201-70431-5. The notion of "revamping" is never far away in Meyers (1996 item29 pp183-213) reference counting example, but neither is it directly mentioned. Meyers, Scott. 1996. More effective C++ : 35 new ways to improve your programs and design. Addison-Wesley, Boston, USA. ISBN 0-201-63371-X. Some ideas regarding the use of Boost.Any came from the following book (which I found both instructional and nicely written): Karlsson, Bjoern. 2006. Beyond the C++ Standard Library : an introduction to Boost. Addison-Wesley, Upper Saddle River, New Jersey, USA. ISBN 0-321-13354-4. The following web article offered some insights on solutions: Becker, Thomas. 2007. On the tension between object-oriented and generic programming in C++ : and what type erasure can do about it. C++ Source. [http://www.artima.com/cppsource/type_erasure1.html] Finally, Dattatri (2002 pp487-496) provides an implementation of counted pointers and discusses the design issues in some depth. Dattatri, Kayshav. 2002. C++ : effective-object oriented software construction : concepts, principles, industrial strategies and practices -- Second edition. Prentice Hall PTR, Upper Saddle River, New Jersey, USA. ISBN 0-13-086769-1. ABOUT THE AUTHOR I am a PhD student writing a policy-oriented energy system simulation at the Institute for Energy Engineering (IET), Technical University of Berlin (TU-Berlin), Germany. I have never coded professionally, but consider myself reasonably competent in terms of scientific programming. --- I am happy to take comments, suggestions, and patches and will post an update in due course. Please note that I will be checking my email only infrequently over the next four weeks or so. best wishes to all Robbie Morrison --- PhD student -- policy-oriented energy system simulation Technical University of Berlin (TU-Berlin), Germany University email (redirected) : morrison(a)iet.tu-berlin.de Webmail (preferred) : robbie(a)actrix.co.nz [from IMAP client]

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boost::enable_if, did I get it wrong ?
by topoden 19 Aug '09

19 Aug '09

Hello, I'm trying to use boost::enable_if to customize function declaration for a particular set of template arguments. But instead of using SFINAE to remove incorrect declaration it produces error. See the example below to see better what I mean: #include <boost/utility.hpp> #include <boost/type_traits.hpp> template<class A, class B> struct BaseListener { typename boost::disable_if<boost::is_void< B >, void>::type void virtual process(A *, B *) = 0; typename boost::enable_if<boost::is_void< B >, void>::type void virtual process(A *) = 0; }; struct Request { }; struct Response { }; typedef BaseListener<Response, void> BaseResponseListener; typedef BaseListener<Request, BaseResponseListener> BaseRequestListener; struct RequestListener : public BaseRequestListener { void process(Request *, BaseResponseListener *) { } }; struct ResponseListener : public BaseResponseListener { void process(Request *) { } }; int main(int argc, char * argv[]) { RequestListener reqListener; ResponseListener resListener; return 0; } error message: error C2039: 'type' : is not a member of 'boost::enable_if<Cond,T>' 1> with 1> [ 1> Cond=boost::is_void<BaseResponseListener>, 1> T=void 1> ] 1> c:\!projects\temp\consoleeventhandler\consoleeventhandler.cpp(134) : see reference to class template instantiation 'BaseListener<A,B>' being compiled 1> with 1> [ 1> A=Request, 1> B=BaseResponseListener 1> ] -- View this message in context: http://www.nabble.com/boost%3A%3Aenable_if%2C-did-I-get-it-wrong---tp250279… Sent from the Boost - Users mailing list archive at Nabble.com.

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[Proto] Building a mpl::vector from an expression
by christophe henry 19 Aug '09

19 Aug '09

Hi, I am trying to build a mpl::vector from an expression like (string(),ClassA(),ClassB()) To get a mpl::vector<string,ClassA,ClassB> With string, ClassA and ClassB being not enough under control to protoify them with a proto::extends. I naively tried the following grammar struct BuildFlags : proto::or_< proto::when < proto::terminal<proto::_>, boost::mpl::vector<proto::_child>() >, proto::when < proto::comma<BuildFlags,BuildFlags >, boost::mpl::push_back< BuildFlags(proto::_left),proto::_right>() > > {}; Or some typeof variants of it, with no more success and I start wondering if I got on a rocky way leading me to unavoidable doom. Is there a way to do this or I should give up the idea? Thanks, Christophe _________________________________________________________________ Share your memories online with anyone you want. http://www.microsoft.com/middleeast/windows/windowslive/products/photos-sha…

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[bind] How do I....?
by Robert Jones 19 Aug '09

19 Aug '09

Use bind to call a free function on a vector of shared_ptrs? struct A { }; void f(const A&); std::vector<boost::shared_ptr<A> > v; for_each(v.begin(), v.end(), boost::bind(f, ???? ) ); what goes where ???? is? Thanks, Rob.

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[GRAPH]write_graphviz and setS edges
by Matthieu BOUSSARD 18 Aug '09

18 Aug '09

Hi, I'd like to use the simple write_graphviz function write_graphviz(std::cout,g); but since my graph doesn't allow multiple edges between two vertices, the graph type is defined by : typedef adjacency_list <listS,setS, directedS,...>Graph; but then I can't use the write_graphviz function anymore (compilation error). If I declare my graph with typedef adjacency_list <listS,vecS, directedS,...>Graph; it is fine, but have multiple edges... So my question is : is there a way to use write_graphviz with the setS edges representation ? Thank you, Matthieu

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[function] equivalent of std::binary_function
by alfC 18 Aug '09

18 Aug '09

Does Boost have the technology to define the equivalent of std::binary_function<double, double, double> eg something like boost::nary_function<double(double, double)> Note that I am not interested in storing any function pointer, so using boost::function<double(double, double)> would be a waste. But I want to use the nice syntax "<double(double,double)>" that boost::function uses, instead of "<double, double, double>". The objective is to inherit from this class and instead of class op : std::binary_function<double, double, double>{ ... } to have class op: nary_function<double(double, double)>{ ... } if possible. Thank you, Alfredo

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Linker error
by vikas chandra 18 Aug '09

18 Aug '09

Hi,I was using boost version 1.33.1 earlier. Recently I have downloaded the src file of boost_1.39. Built the regx library and replaced the old libs and header files in my project. But while linking it gives the error: "fatal error LNK1104: cannot open file 'libboost_regex-vc80-mt-sgd-1_33_1.lib'". I dont understand why it is still looking for old 1_33_1.lib. I tried to rebuild the project also but doesnt work. Can anyone help me out? -- Thank You. Regards, Vikas Chandra

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boost 1.40 beta, serialization via base pointer crashes
by Avi Bahra 18 Aug '09

18 Aug '09

The following example works on Linux/gcc but crashes on latest AIX v10.1 compiler #include <fstream> #include <assert.h> #include <boost/archive/text_iarchive.hpp> #include <boost/archive/text_oarchive.hpp> #include <boost/serialization/base_object.hpp> #include <boost/serialization/string.hpp> // no need to include <string> #include <boost/serialization/export.hpp> // explicit code for exports (place last) class RepeatBase { public: RepeatBase() {} virtual ~RepeatBase() {} virtual const std::string& name() const = 0; private: friend class boost::serialization::access; template<class Archive> void serialize(Archive & ar, const unsigned int version) {} }; class RepeatDerived : public RepeatBase { public: RepeatDerived( const std::string& name) : name_(name) {} RepeatDerived() {} virtual const std::string& name() const { return name_;} private: std::string name_; friend class boost::serialization::access; template<class Archive> void serialize(Archive & ar, const unsigned int version) { // save/load base class information ar & boost::serialization::base_object<RepeatBase>(*this); ar & name_; } }; class Defs { public: Defs(const std::string& f) : fileName_(f), repeat_(new RepeatDerived(f)) {} Defs() : repeat_(NULL) {} ~Defs() { delete repeat_;} bool operator==(const Defs& rhs) const { return fileName_ == rhs.fileName_;} const std::string& fileName() const { return fileName_;} private: std::string fileName_; RepeatBase* repeat_; friend class boost::serialization::access; template<class Archive> void serialize(Archive & ar, const unsigned int /*version*/) { ar & fileName_; ar & repeat_; } }; BOOST_CLASS_EXPORT(RepeatBase); BOOST_CLASS_EXPORT(RepeatDerived); BOOST_SERIALIZATION_ASSUME_ABSTRACT(RepeatBase) int main() { // With this explicit cast we get a bit further, but still crashes on AIX // i.e crash on ar.save_pointer(vp, bpos); // // boost::serialization::void_cast_register<RepeatDerived, RepeatBase>(0,0); const Defs saveDefs("saveFile.txt"); { // Save the defs file std::ofstream ofs( saveDefs.fileName().c_str() ); boost::archive::text_oarchive oa( ofs ); oa << saveDefs; } { // Restore the defs file Defs loadDefs; std::ifstream ifs(saveDefs.fileName().c_str()); boost::archive::text_iarchive ia(ifs); ia >> loadDefs; assert( saveDefs == loadDefs ); } return 0; } //========================================================================================= This bug was present in boost 1.39 and is still broken in boost 1.40. The code above shows that base/derived relationship work on one compiler but is _competely_ broken another. Explicitly registering the base/derived relationship just moves the crash to a later point. template<class T> struct polymorphic { static void save( Archive &ar, T & t ){ ...... // convert pointer to more derived type. if this is thrown // it means that the base/derived relationship hasn't be registered vp = serialization::void_downcast( *true_type, *this_type, static_cast<const void *>(&t) ); if(NULL == vp){ boost::serialization::throw_exception( archive_exception(archive_exception::unregistered_cast) //< crash1 here ); } ..... ar.save_pointer(vp, bpos); //< crashe2 here after explicit base/derived registration I thought this bug had been fixed, so I am not sure if this is a regression ? Is there any way I can help, as this is show stopper bug for us. Best regards, Ta, Avi

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shared_ptr questions
by John Dlugosz 18 Aug '09

18 Aug '09

I'm looking at shared_ptr and weak_ptr as a TR1 standard approach. It is hard to follow the implementation, but I'm wondering about some specific issues. My set of classes (<http://www.dlugosz.com/Repertoire/refman/Classics/Smart%20Pointers%20Overvi…>), published over ten years ago, has similar features. My class "baro" is like your weak_pointer. I recall a threading issue where one thread is dropping the last owned reference at the same time another thread is realizing a weak_pointer. The first decrements the refcount to zero, and the second increments it again. Is the object destructed? I'm using a specific mechanism to support this and related features, and I don't see that in your code. Is the Boost implementation for Windows (32 and 64-bit) robust and lock-free in cases like this? How does it work? --John TradeStation Group, Inc. is a publicly-traded holding company (NASDAQ GS: TRAD) of three operating subsidiaries, TradeStation Securities, Inc. (Member NYSE, FINRA, SIPC and NFA), TradeStation Technologies, Inc., a trading software and subscription company, and TradeStation Europe Limited, a United Kingdom, FSA-authorized introducing brokerage firm. None of these companies provides trading or investment advice, recommendations or endorsements of any kind. The information transmitted is intended only for the person or entity to which it is addressed and may contain confidential and/or privileged material. Any review, retransmission, dissemination or other use of, or taking of any action in reliance upon, this information by persons or entities other than the intended recipient is prohibited. If you received this in error, please contact the sender and delete the material from any computer.

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boost 1.40 beta libs, no longer include compiler version/boost version ?
by Avi Bahra 18 Aug '09

18 Aug '09

Volodya wrote: > The default naming of libraries in Unix-like environment now matches system conventions, and does > not include various decorations. > > Does this answer your question? Ok so it was done deliberately. Why the change, the decorations were useful in denoting the kind of library ? How stable is this decision is likely to change, between now and the release.? -- Best regards, Ta, Avi

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