August 2012 - Boost - lists.preview.boost.org

Equivalent of "bootstrap.sh --without-libraries" for Modularized and CMake-ified boost
by Julien Woillez 24 Aug '12

24 Aug '12

Dear boost(a)lists.boost.org, I'm trying to use CMake to build boost, using its Modularized and CMake-ified version at: https://github.com/ryppl/boost-zero I need to do the equivalent of --without-libraries=mpi with bootstrap.sh. Any idea if this is possible, and if so how to do it? Regards, -Julien

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[type_erasure]bug:any(, binding<Concept>) can store wrong type
by Larry Evans 23 Aug '12

23 Aug '12

The attached runs OK without any compile or runtime errors. It seems to create a type_uns<1> from a type_uns<0> although there's no type_uns<1> CTOR for that. This sounds like another instance of the bug reported here: http://article.gmane.org/gmane.comp.lib.boost.devel/233101 Is it? -regards, Larry

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[exception] is static only?
by Neal Becker 23 Aug '12

23 Aug '12

Is it necessary that exception builds only static libs? On many architectures, this would limit it's usefulness.

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Boost 1.51.0 released!
by Marshall Clow 22 Aug '12

22 Aug '12

Release 1.51.0 of the Boost C++ Libraries is now available. These open-source libraries work well with the C++ Standard Library, and are usable across a broad spectrum of applications. The Boost license encourages both commercial and non-commercial use. This release contains 1 new library (Boost.Context, by Oliver Kowalke) and numerous enhancements and bug fixes for existing libraries. For details, including download links, see http://www.boost.org/users/news/version_1_51_0 You can also download directly from SourceForge: http://sourceforge.net/projects/boost/files/boost/1.51.0/ To install this release on your system, see http://www.boost.org/doc/libs/release/more/getting_started/index.html Thanks, --The Boost release team Beman Dawes Daniel James Eric Niebler Marshall Clow Rene Rivera Vladimir Prus

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[1.51] PDF documentation build available
by John Maddock 22 Aug '12

22 Aug '12

The PDF build of the docs for 1.51 is now available from: https://sourceforge.net/projects/boost/files/boost-docs/1.51.0/ John.

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[type_erasure] Review report -- ACCEPTED
by Lorenzo Caminiti 21 Aug '12

21 Aug '12

Hello all, Steven Watanabe's library Boost.TypeErasure is ACCEPTED into Boost. SUMMARY The library was reviewed from July 18 to August 3, 2012. There were 11 votes to accept the library and 2 votes to accept the library under the condition of some modifications (at the end, these 2 votes were essentially considered as "no" because there was no agreement on implementing all the required modifications). It was noted how the library offers non-intrusive and powerful programming techniques that can be used in place of usual Object Oriented (pure) virtual function interfaces (and with support for some from of virtual "template" functions). The library can also be seen as a generalization of existing Boost libraries like Any, Function, and AnyIterator. The library is accepted "as is", however most reviewers suggested to: 1) Dramatically ease the learning curve of the documentation by providing a motivation section, step-by-step tutorial(s) to introduce the basic features of the library, advanced topics section(s) to introduced the additional features, an example section with non-trivial use cases for the library, and additional examples to the reference section. 2) Support down-casting. While these changes are not a condition for the library acceptance, it would be best if they were made part of the very first release of the library. A few reviewers found the name Type Erasure confusing, however others did not. Alternative name were: Staged Typing, Deferred Typing, Type Interface, Interface, Static Interface, Value Interface, Structural Typing, Duck Typing, and Run-Time Concepts. There was some discussion on the meaning of Type Erasure in Java and on its definition on wiki (these should probably be both referenced in the documentation). The current library name is considered acceptable and the final decision on the library name is left entirely up to Steven. DETAILS "Yes" reviews: [1] http://lists.boost.org/Archives/boost/2012/07/195288.php [2] http://lists.boost.org/Archives/boost/2012/07/195297.php [3] http://lists.boost.org/Archives/boost/2012/07/195299.php [4] http://lists.boost.org/Archives/boost/2012/07/195308.php [5] http://lists.boost.org/Archives/boost/2012/07/195323.php [6] http://www.mentby.com/hossein-haeri/boost-typeerasure-review-ends-today-jul… [7] http://lists.boost.org/Archives/boost/2012/07/195327.php [8] http://lists.boost.org/Archives/boost/2012/07/195329.php [9] http://lists.boost.org/Archives/boost/2012/07/195334.php [10] http://lists.boost.org/Archives/boost/2012/07/195364.php [11] http://lists.boost.org/Archives/boost/2012/08/195420.php "Conditional-yes" reviews (essentially considered as "no"): [12] http://lists.boost.org/Archives/boost/2012/08/195455.php [13] http://lists.boost.org/Archives/boost/2012/08/195561.php Notable comments and discussions (not reviews): (1) http://lists.boost.org/Archives/boost/2012/07/195162.php (2) http://lists.boost.org/Archives/boost/2012/07/195130.php (3) http://lists.boost.org/Archives/boost/2012/07/195149.php (4) http://lists.boost.org/Archives/boost/2012/07/195167.php (5) http://lists.boost.org/Archives/boost/2012/07/195199.php (6) http://lists.boost.org/Archives/boost/2012/07/195200.php (7) http://lists.boost.org/Archives/boost/2012/07/195232.php (8) http://lists.boost.org/Archives/boost/2012/07/195234.php (9) http://lists.boost.org/Archives/boost/2012/07/195408.php (10) http://permalink.gmane.org/gmane.comp.lib.boost.user/75081 (11) http://lists.boost.org/Archives/boost/2012/08/195590.php (12) http://lists.boost.org/Archives/boost/2012/08/195663.php (13) http://lists.boost.org/Archives/boost/2012/07/195342.php (14) http://lists.boost.org/Archives/boost/2012/08/195543.php Note that Steven might have already implemented some of the changes listed below. Features: * Support down-casting. See [1], [10], and (4). * Add macros for defining concepts. These macros could be added in later revisions (they just provide a different syntax, no additional functionality). ** From [10]: BOOST_CREATE_CONCEPT(container_concept, typename T, (void (push_back, T), void (size)) ) ** From Steven: BOOST_TYPE_ERASURE_MEMBERS((push_back)(size)...) template<class V, class T = _self> struct container_concept : mpl::vector< has_push_back<void(const V&), T>, has_size<std::size_t(), const T>, ...> {}; * Add empty() to check if any holds a value or not. See [3] and [11]. * Add a built-in concept for unary plus +x (just pick a name for it). See (11). Implementation: * Can assertion enforce that same bindings are required for the placeholders? See [13] and (3). * Add assertion to check that in functions with multiple arguments the underlining any types must match. See (7). * Add in-code comments to the implementation (at least to normalize.hpp). See [8] and [13]. * Can static assertions be used to improve error messages (see [8])? Concept mismatch spew errors with internal implementation details, see [10]. * Fix bugs: ** http://lists.boost.org/Archives/boost/2012/07/195367.php ** http://lists.boost.org/Archives/boost/2012/07/195396.php ** http://lists.boost.org/Archives/boost/2012/08/195662.php Documentation: * A few typos listed in [9]. * A draft on a proposed way to restructure the docs was offered in [8]. * Sean Parent's BoostCon 2012 "Value Semantics" could be used to write a motivation section. See [9]. * Offer a comparison with usual OO (pure) virtual function interfaces. See [1], [7], [8], and [9]. * Make sure all important elements document in the reference section are also documented in a tutorial section (e.g., how the any constructor binds placeholders, conversions, BOOST_TYPE_ERASURE_MEMBER, concept_of, etc). See (1), (4), (6), and [1]. * Clarify that references can be assigned but they cannot be rebound to a different object. See (2). * Clarify the rationale for using _a, _b, ... instead of _1, _2, ... as soon as _a, _b, ... are first used (most users miss the "named vs. positional parameter" rationale from the design decision section and _1, _2, ... are such common constructs that many users will naturally question the _a, _b, ... choice). See (5). * Add an example section with non-trivial use cases of the library. Consider to add the following examples: ** How to implement visitation, see (8). ** Model view controller, see [1]. ** Run-time expression system (adding Concept = void specialization), see [3]. ** Multi-function, see [4]. ** Printing, see (11) (note that Steven needed to answer too many questions for Paul to be able to program this, ideally the docs would have been clear enough for Paul to program this example without Steven's help). ** Reference related work Adobe::Poly, see (13). * Document all built-in concepts (possibly with a usage example for each) and indicate which ones can be specialized. See [5], and [9]. * Add an example at the bottom of each reference page. See [11]. * Document that any(any<Concpet2, T2>, binding<Concept>) allows to convert the vtable once and re-use it for all anys reducing run-time cost. See [12]. * Consider a number of docs improvements as suggested in [13]. * Document the undefined behaviour pointed out in (12). * Document limitations that some algorithms might encounter if they need to access references and provide rationale of why references cannot be handled differently (i.e., constructor flag and/or overhead of managing vtable in dispatcher layer). See [12], and (14). Many thanks to everyone that participated in the review!! I believe Boost users will find this library useful. The review manager, --Lorenzo On Sat, Aug 4, 2012 at 12:32 PM, Lorenzo Caminiti <lorcaminiti(a)gmail.com> wrote: > Hello all, > > The review of Boost.TypeErasure ended on Aug 3, 2012. Stay tuned for > my decision on the library. > > Thank you very much to Steven, everyone that submitted a review, and > everyone that participated in the discussions. > > The review manager, > --Lorenzo > > On Fri, Aug 3, 2012 at 11:00 AM, Lorenzo Caminiti <lorcaminiti(a)gmail.com> wrote: >> Hello all, >> >> *** The review of Boost.TypeErasure ends today Aug 3, 2012. If you are >> planning to submit a review, please do so by the end of today. *** >> >> Thank you to all that have participated to the review so far. >> >> The review manager, >> --Lorenzo >> >> On Jul 27, 2012 1:41 PM, "Lorenzo Caminiti" <lorcaminiti(a)gmail.com> wrote: >>> >>> Hello all, >>> >>> After consulting the review wizards and Steven, I am extending >>> Boost.TypeErasure review of one week, until August 3, 2012. >>> >>> Thank you to all that have submitted a review already and I am looking >>> forward to receiving additional submissions. >>> >>> The review manager, >>> --Lorenzo >>> >>> On Jul 27, 2012 3:22 AM, "Lorenzo Caminiti" <lorcaminiti(a)gmail.com> wrote: >>>> >>>> Hello all, >>>> >>>> On Mon, Jul 23, 2012 at 9:09 AM, Lorenzo Caminiti <lorcaminiti(a)gmail.com> >>>> wrote: >>>> > On Wed, Jul 18, 2012 at 1:13 AM, Lorenzo Caminiti >>>> > <lorcaminiti(a)gmail.com> wrote: >>>> >> *** The review of Steven Watanabe's proposed Boost.TypeErasure library >>>> >> begins on July 18, 2012 and ends on July 27, 2012. *** >>>> > >>>> > *** Boost.TypeErasure review ends in 5 days. Please submit your reviews >>>> > :D *** >>>> >>>> *** Boost.TypeErasure review ends today July 27, 2012. If you are >>>> planning to submit a review, please do so as soon as possible. *** >>>> >>>> Thank you to everyone who has commented on the library and submitted a >>>> review so far! >>>> >>>> The review manager, >>>> --Lorenzo >>>> P.S. I am considering extending the review until end of day Sunday >>>> July 29, 2012... I will keep you posted. >>>> >>>> > There have been interesting discussions on the library on the ML but I >>>> > have not received any official review yet :( Especially if you are a >>>> > user of Boost Any, Function, and Any Iterator, you definitely want to >>>> > take a look at Type Erasure as it generalizes solutions provided by >>>> > those other libraries. >>>> > >>>> > Thank you. >>>> > >>>> > The review manager. >>>> > --Lorenzo >>>> > >>>> >> THE LIBRARY >>>> >> >>>> >> C++ provides runtime polymorphism through virtual functions. They are >>>> >> a very useful feature, but they do have some limitations. >>>> >> * They are intrusive. In generic programming, we can design an >>>> >> interface which allows third-party types to be adapted to it. >>>> >> * They require dynamic memory management. Of course, most of the >>>> >> problems can be avoided by using an appropriate smart pointer type. >>>> >> Even so, it still acts like a pointer rather than a value. >>>> >> * Virtual functions' ability to apply multiple independent concepts to >>>> >> a single object is limited. >>>> >> The Boost.TypeErasure library solves these problems allowing us to >>>> >> mirror static generic programming at runtime. >>>> >> >>>> >> Library source: >>>> >> http://svn.boost.org/svn/boost/sandbox/type_erasure/ >>>> >> >>>> >> Pre-built documentation: >>>> >> >>>> >> http://steven_watanabe.users.sourceforge.net/type_erasure/libs/type_erasure/ >>>> >> >>>> >> You can also download archives with pre-built documentation from: >>>> >> http://sourceforge.net/projects/steven-watanabe.u/files/ >>>> >> >>>> >> YOUR REVIEW >>>> >> >>>> >> Please submit a review to the mailing-list by replying to this email >>>> >> ("[boost] [type_erasure] Review ..." should be in the subject). >>>> >> >>>> >> Please state clearly whether you think this library should be accepted >>>> >> as a Boost library. >>>> >> >>>> >> Other questions you may want to consider: >>>> >> 1. What is your evaluation of the design? >>>> >> 2. What is your evaluation of the implementation? >>>> >> 3. What is your evaluation of the documentation? >>>> >> 4. What is your evaluation of the potential usefulness of the library? >>>> >> 5. Did you try to use the library? With what compiler? Did you have >>>> >> any problems? >>>> >> 6. How much effort did you put into your evaluation? A glance? A >>>> >> quick reading? In-depth study? >>>> >> 7. Are you knowledgeable about the problem domain? >>>> >> >>>> >> Thanks in advance to all who participate in the review discussion -- >>>> >> I'm looking forward to it!

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[fusion] move-enabled containers?
by Gabriel Redner 21 Aug '12

21 Aug '12

Hi folks, I have been starting to ease into C++11, and am currently looking to add move-awareness to some classes which have members of boost::fusion container types. However, it seems that fusion containers are not move-aware. For a minimal example, take the code below: ========== #include <boost/fusion/include/vector.hpp> #include <iostream> class noisy { public: noisy() { std::cout << "noisy()\n"; } noisy(const noisy&) { std::cout << "noisy(const noisy&)\n"; } noisy(noisy&&) { std::cout << "noisy(noisy&&)\n"; } }; int main() { typedef boost::fusion::vector<noisy, noisy> noisyVector; noisyVector n; std::cout << '\n'; noisyVector n2(n); std::cout << '\n'; noisyVector n3(std::move(n)); } ========== I was hoping that the output would be: ========== noisy() noisy() noisy(const noisy&) noisy(const noisy&) noisy(noisy&&) noisy(noisy&&) ========== But instead I get: ========== noisy() noisy() noisy(const noisy&) noisy(const noisy&) noisy(const noisy&) noisy(const noisy&) ========== It is a simple matter to add move-awareness to fusion containers, or is there some technical reason why it cannot been done? Or (perhaps more likely), am I misunderstanding something about the situation? Thanks, -Gabe

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Some library dependency and auto liker issues
by Vicente J. Botet Escriba 20 Aug '12

20 Aug '12

Hi, there is an issue https://svn.boost.org/trac/boost/ticket/7045 that I don't know how to fix. The problem is related to library dependencies and auto-linker. * 1st issue: Boost.Threads depends on Boost.DateTime but only for the header only part. Anyway, it includes a DateTime file that includes the boost/config/auto_link.hpp, and this makes the tools using the auto linker to create a dependency on Boost.DateTime. As the Boost.Threads build Jamfile doesn't link with Boost.DateTime, the command bjam toolset=msvc-11.0 --build-type=complete --with-thread doesn't build the library Boost.DateTime and so when a tool such as Visual studio uses the auto linker information, it tries to link with Boost.DateTime and not finds it, as not build (even if it is not needed). What is the best way to fix this? Should Boost.Thread link with Boost.DateTime, even if it don't needs it? Or just document that the user relaying on auto-linker needs to build explicitly the boost_date_time library with the command: bjam toolset=msvc-11.0 --build-type=complete --with-thread --with-date_time * 2nd issue: Boost.Threads depends on Boost.Chrono and the library is linked with boost_chrono. When a user includes for example <boost/thread/tss.hpp> the autolinker doesn't detects the chrono dependency as this file don't depends on any file in boost/chrono. But as boost_thread depends on boost_chrono, the linker find some unresolved symbols. What is the best way to fix this? Should Boost.Thread include boost/chrono.hpp in all their public files even if not needed other than by auto linker tools? Or just document that the user relaying on auto-linker needs to include only the top level file <boost/thread.hpp> so that all the dependencies are covered? Any suggestions are welcome, Vicente

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Re: [boost] [type_erasure] Review started (July 18-27, 2012)
by Steven Watanabe 20 Aug '12

20 Aug '12

AMDG On 08/02/2012 12:29 PM, Larry Evans wrote: > On 07/18/12 00:13, Lorenzo Caminiti wrote: >> Hello all, >> >> *** The review of Steven Watanabe's proposed Boost.TypeErasure library >> begins on July 18, 2012 and ends on July 27, 2012. *** > [snip] > Should type_erasure be accepted into Boost? > > Answer: > No, not yet. > Why(in order of importance): > > 1) any<C,T>::any(U&, static_binding<Map>&) bug > > The bug reported here: > http://article.gmane.org/gmane.comp.lib.boost.devel/233101 > should be resolved. > > The bug is just too dangerous for the resolution to be delayed. > I added a static assert. > <snip> > > My review: > > 1. What is your evaluation of the design? > > Good, except for: > > [need variadic static_binding ctor]: > > The any CTOR: > > template<class U, class Map> > any(U & arg, const static_binding< Map > & binding); > > documented here: > > > http://steven_watanabe.users.sourceforge.net/type_erasure/libs/type_erasure… > > allows only a single argument to the constructor to the > underlying type. I'd suggest adding another CTOR with > interface: > > template<class Map, class... U> > explicit any(const static_binding< Map > & binding, U &&... arg); > > which is similar to: > > explicit any(const binding< Concept > & binding, U &&... arg); > > Then any constructor of the underlying could be used. > I think something like this is reasonable. I have to consider whether this is best approach, though. I don't consider this high priority, however, since the copy constructor should work most of the time. The STL got by just fine for years without emplace. > [too many any ctors]: > > According to `grep -e ' any('`, there are more than 45 any > constructors. That many makes it hard for the novice to know > which one to use. The Design notes: > The solution is simple. Don't use the source. Use the documentation page for any, which only shows 9 distinct constructors. > > http://steven_watanabe.users.sourceforge.net/type_erasure/libs/type_erasure… > > are some help, but those notes also mention ambiguities and > user rules, which sound like indications of usability problems > to me. > > Why wouldn't the following ctors be sufficent to do all the > current any constructors do (except for maybe requiring more > typing and maybe more constructor calls)? > > template > < class... U > > > any > ( static_binding<Map>const& > , U&&... > );//kind=binding constructor > > template > < class... U > > > any > ( binding<Concept>const& > , U&&... > );//kind=dispatching constructor > > template > < class Concept2 > , class Tag2 > > > any > ( any<Concept2,Tag2>U&& > , static_binding<Map>const& > );//kind=converting constructor > > template > < class Concept2 > , class Tag2 > > > any > ( any<Concept2,Tag2>U&& u > , binding<Concept2>const&b=binding_of(u) > );//kind=converting constructor > > Providing just these 4 constructors would improve the usabilty. Why? > Of course, I'm probably missing something, and if so, > explaining why the above would not work in the Design notes > would be helpful. > I think it should be obvious what's missing: a) any(const any&); The copy constructor is not a template. b) template<class U> any(const U&); This is the most commonly used constructor besides the copy constructor. It would be very inconvenient if it didn't exist. c) any(); The default constructor creates an empty any. d) A default argument can't refer to a prior argument. Your fourth constructor has to be two separate constructors. > 2. What is your evaluation of the implementation? > > [in-source comments]: > Very few in-source comments, as for example, the in the code: > > > http://steven_watanabe.users.sourceforge.net/type_erasure/boost/type_erasur… > > there are no in-source comments. This lack of in-source comments > made it very hard to understand the implementation. There is also > no tests in the directory: > > > > http://steven_watanabe.users.sourceforge.net/type_erasure/libs/type_erasure… > > which contain any string with 'normalize' in it; hence, I assume > there are no unit tests for any class or function with normalize > as part of its name. Such unit tests, hopefully, would have > provided more information of the purpose of the code in > */detail/normalize.hpp; hence, I'd recommend providing such tests > in addition to more in-source comments. > The tests only cover the public interface. I'm not going to add specific tests of the implementation details. > > 3. What is your evaluation of the documentation? > > Mostly good, but some documentation needs improvement. > For example: > > [vague:rebinding_any.html]: > > This doc: > > > http://steven_watanabe.users.sourceforge.net/type_erasure/libs/type_erasure… > > Claims rebind_any<Any,T>::type: > > returns any type corresponding to a placeholder. > > However, it doesn't say what the Any in rebind_any<Any,T> may be, > and if one assumes Any is a type_erasure::any<C,S> for some > concept, C, and placeholder, S, then that description makes no > sense because C does not contain any information about which > actual type is matched to which placeholder. Huh? As the documentation you quoted says, rebind_any returns a specialization of /any/. rebind_any<any<C, S>, T>::type is any<C, T>. > <snip> > > from Steven, in the reply: > > http://article.gmane.org/gmane.comp.lib.boost.devel/233067 > > did I finally conclude: > > The placeholders in concepts do *not* stand for the bound types > but instead for the any's containing those bound types. > That depends on the context. boost::type_erasure::call "unwraps" these any's. > Which is similar to the conclusion which I posted here: > > http://article.gmane.org/gmane.comp.lib.boost.devel/233078 > > Since the 233078 post got no replies, I assume that conclusion is > right. If so, then that should be made clear in the > documentation (including tutorials). In addition, the following > example code (and especially the comments), should reduce future > confusion on this topic: > > /* {***any_ctor_kinds.cpp*** */ > <snip> > I find this code almost completely unreadable. > If that conclusion is wrong, then the docs need to be modified > (I've no idea how) to clearly define "matching" as used in the > any.html#id2445089-bb reference. > > [incomplete:tuple.html]: > > The doc: > > > http://steven_watanabe.users.sourceforge.net/type_erasure/libs/type_erasure… > > makes no mention of how the binding produced by this call are > generated. > Already fixed. In Christ, Steven Watanabe

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[multiprecision] Some rvalue reference experiments (performance inhancements #1)
by John Maddock 20 Aug '12

20 Aug '12

Folks, I've started to work through some of the points that came up in the multiprecision review, starting with further rvalue reference support. The idea (due to Marc Glisse) was to add operator overloads that look something like: Number&& operator+(Number&&a, const Number& b) { return static_cast<Number&&>(a += b); } Using my Polynomial Horner evaluation scheme as an example the number of temporaries dropped as follows: Type # temporaries No expression templates, no rvalue refs 24 No expression templates, rvalue refs 1 Expression template 0 Which is pretty impressive, but not necessarily typical usage, so by way of "real world" tests, I compared the times taken to evaluate Boost.Math's Bessel function test cases to 50 decimal places: Library Time/s # Allocations MPFR/mpreal (no ET's or rvalue refs) 9.4 13226029 MPFR/mpfr_class (ET's) 6.3 3948316 MPFR/mp_number(ET's) 6.0 2683646 GMP/mp_number(no ET's) 5.2 4186977 GMP/mp_number(ET's) 5.0 2594441 As you can see the number of allocations required when using mp_number with ET's turned off is almost double that without, I guess this is because many statements are similar to: a = b op c; Where a temporary is still required when relying on rvalue refs, but not for expression templates. Having said that the times are getting much closer now.... John.

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Is there any interest in getting pseudo random number generation using preprocessor at compile time into the Preprocessor library?
by Alexander Stoyan 20 Aug '12

20 Aug '12

The complete code, approach and efficiency proofs I described in my blog: http://alexander-stoyan.blogspot.com/2012/07/getting-pseudo-random-numbers-a t.html Need your opinion regarding the relevance and the approach correctness of the feature. Thank you in advance.

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[1.51.0] release candidates available
by Marshall Clow 19 Aug '12

19 Aug '12

Release candidate files for 1.51.0 are available at http://boost.cowic.de/rc/ At this point, the release branch is closed for changes without explicit permission from the release managers. As always, the release managers would appreciate it if you download the candidate of your choice and give building it a try. Please report both success and failure, and anything else that is noteworthy. This helps ensure the candidates build OK before we push them out to SourceForge. The files (and associated md5s) are: 6a1f32d902203ac70fbec78af95b3cf8 *boost_1_51_0_rc1.tar.gz 4b6bd483b692fd138aef84ed2c8eb679 *boost_1_51_0_rc1.tar.bz2 ee8112e48088b05c248d68329cd5d908 *boost_1_51_0_rc1.zip e87d3747966ab7e0c9487884326d7d17 *boost_1_51_0_rc1.7z Thanks, --The release managers

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Modularized Boost: How the files in libs/libXX must be included?
by Vicente J. Botet Escriba 19 Aug '12

19 Aug '12

Hi, the following ticket https://svn.boost.org/trac/boost/ticket/7200 report an issue while building Boost.Thread using the modularized cmake. What is the correct way to include the files in lib/libXXX? Best, Vicente

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[type_traits][parameter] Inconsistent boost::is_convertible between gcc and clang
by John Bytheway 19 Aug '12

19 Aug '12

I have been tracking down an issue with some of my code which compiles fine with gcc but not clang. It manifested as a failure in Boost.Parameter but I eventually pinned it down to an inconsistency in the behaviour of boost::is_convertible. The following program demonstrates the issue: ============== #include <boost/type_traits/is_convertible.hpp> #include <boost/static_assert.hpp> class A {}; int main() { BOOST_STATIC_ASSERT((boost::is_convertible<A, A&>::value)); } ============== This compiles fine under g++ (4.7.1), but the assertion fires under clang++ (r160940). This is not too surprising because is_convertible is implemented in completely different ways on the two compilers: - on clang it is in terms of the __is_convertible intrinsic, which behaves like std::is_convertible and fails because rvalue references don't convert to lvalue references. - on gcc it has a complex custom implementation which at some point performs add_reference<> on the first argument and thus determines that the conversion is OK. Reading the docs for boost::is_convertible I think the clang interpretation is closer to the intention, in which case the gcc implementation is wrong, and so is Boost.Parameter (which relies on this behaviour). On the other hand, I suspect there's probably other code out there that depends on this in a similar way to Boost.Parameter, so perhaps it would be safer to tweak the clang implementation instead, and deviate from std::is_convertible. Any thoughts? John Bytheway

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[odeint] Iterator semantics
by Karsten Ahnert 19 Aug '12

19 Aug '12

Hi, we have posted a review request for odeint (a library for solving ordinary differential equations) on this list some days ago. An ODE is dx/dt = f(x(t)) and one is looking for the solution x(t) which is a function of t. Solving ODEs is usually an iterative task, so one starts with x(0) and iteratively applies a solver to obtain a discretized (and approximated) representation of the solution: x(0) -> x(dt) -> x(2*dt) -> x(3*dt) -> ... I played around with an iterator which is doing exactly this iterative procedure. For example, it can be used via odeint::runge_kutta4< state_type > stepper; // ode solver std::array< double , 3 > x = {{ 10.0 , 10.0 , 10.0 }}; // initial state double res = std::accumulate( // parameters : solver , ode , state , time , time_step make_const_step_iterator_begin(stepper, lorenz(), x, 0.0, 0.01) , make_const_step_iterator_end( stepper, lorenz(), x, 1.0, 0.01) , 0.0 , []( double sum, const state_type &x) { return sum + x[0]; } ); The first iterator increments the time until the time of the second iterator is rearched t=0.0 -> t=0.01 -> t=0.02 -> .. t=1.0 and the first component of the solution is accumulated. In principle, this approach works quite well, but there are some problems at least semantically. Maybe someone here has some comments or ideas on this: The defintion of this iterator is: template< class Stepper , class System , class StepperTag > class const_step_iterator : public boost::iterator_facade < const_step_iterator< Stepper , System , StepperTag > , typename Stepper::state_type const , boost::single_pass_traversal_tag > { ... }; It is a single pass iterator, so it can only check for equality of two iterators. This is problematic since equality here means that the time (t) of the begin iterator is smaller then the time of the end iterator. To ensure commutativity of the != or == operation I needed to flag the begin and end iterator explicitly. This is the reason for the two factory functions make_const_step_iterator_begin and make_const_step_iterator_end, which are doing this flagging. The second semantically problem is that the end iterator in principle does not need to know the stepper as well as the system (lorenz() in the above example). But all algorithms from the standard library and Boost.Range assume that the begin and end iterator are of same type. Therefore you have to put the stepper and the system into the end iterator too. Of course, there are techniques like type erasure which might solve this problem, but they introduce some run-time overhead which we want to avoid here. Any ideas or comments about this? P.S. The code can be found in the iterator branch in odeint: boost/numeric/odeint/iterator/const_step_iterator.hpp and some examples libs/numeric/odeint/examples/const_step_iterator.cpp git clone -b iterator git@github.com:headmyshoulder/odeint-v2.git

5 19

Request for a typedef for the larger integer type that supports atomic loads and stores.
by Vicente J. Botet Escriba 18 Aug '12

18 Aug '12

Hi, I'm locking for a portable integer type that is the larger integer type that supports atomic loads and stores. The sig_atomic_t type defined in <csignal> supports atomic loads and stores but I'm not sure is the larger one. Does boost define such a typedef? If not, could such typedef be included in boost and where? Best, Vicente

2 1

What to do with platform defining 'null' as preprocessor symbol?
by Jan Hudec 17 Aug '12

17 Aug '12

Hello Folks, At work we develop, among others, for Bada. The system library provided is unfortunately somewhat flaky. A tell-tale sign is, that it's native API headers define lowercase 'null' as preprocessor symbol expanding to 0. Unfortunately boost uses identifier 'null'. So far we only hit the two cases in boost/test/utils/basic_cstring/basic_cstring.hpp, where I fixed it by renaming the symbols in our local copy of that header, but I noticed there are many more instances in other modules. Would it make sense to rename those identifiers and avoid that name like boost avoids 'min' and 'max'? Or perhaps I could try simply undefining it in config/platform somewhere and see whether Bada headers would break? Unfortunately I don't think gcc has ability to redefine macros temporarily to reinstate the original definition at the end of boost. Generally speaking, Bada provides it's own native API, which is different from anything else. It's object-oriented and looks nice at first sight, but sometimes is inflexible in surprising places. It looks like they wrote whatever they needed to implement applications that ship with the phones and stopped there. The platform also provides ANSI C library and some parts of POSIX, but many functions don't really work. Fortunately the platform is going to be merged with Titzen, so it should at least get working POSIX kernel (Linux) and standard libraries. That makes supporting this obscure platform even less worth big trouble. -- Jan 'Bulb' Hudec <bulb(a)ucw.cz>

1 0

[general] Endianity detection for ARM
by Jan Hudec 17 Aug '12

17 Aug '12

Hello All, Something in Boost.Test started using detail/endian.hpp internally between 1.43 and 1.50. Since at work I build Boost.Test-based tests for several mobile platforms, all of which use ARM, I needed to fix it. It is easy for all platforms built with either gcc or clang (Android, iOS and Bada). Gcc defines __ARMEB__ for big-endian ARM and __ARMEL__ for little-endian ARM. Windows CE is however another issue. Microsoft usually defines rather characteristic _M_<arch> macros, but _M_ARM is *not* defined in Windows CE, though it appears to be defined for Windows 8 ARM (I don't work with those yet). The WinCE projects define ARM and _ARM_, but those may appear on big-endian ARM platforms too. However since there are no big-endian Win32 platforms and they are unlikely to be created, I suggest just hardcoding little endian for anything that defines _WIN32 (the _M_* macros could then possibly be removed, but they don't cause any problems there). I therefore propose this patch: --- a/boost/detail/endian.hpp +++ b/boost/detail/endian.hpp @@ -57,7 +57,7 @@ || defined(_POWER) || defined(__powerpc__) \ || defined(__ppc__) || defined(__hpux) || defined(__hppa) \ || defined(_MIPSEB) || defined(_POWER) \ - || defined(__s390__) + || defined(__s390__) || defined(__ARMEB__) # define BOOST_BIG_ENDIAN # define BOOST_BYTE_ORDER 4321 #elif defined(__i386__) || defined(__alpha__) \ @@ -66,7 +66,9 @@ || defined(_M_ALPHA) || defined(__amd64) \ || defined(__amd64__) || defined(_M_AMD64) \ || defined(__x86_64) || defined(__x86_64__) \ - || defined(_M_X64) || defined(__bfin__) + || defined(_M_X64) || defined(__bfin__) \ + || defined(__ARMEL__) \ + || defined(_WIN32) // ARM Windows CE don't define __ARMEL__ nor _M_ARM, but there are no big-endian Windows versions # define BOOST_LITTLE_ENDIAN # define BOOST_BYTE_ORDER 1234 -- Jan 'Bulb' Hudec <bulb(a)ucw.cz>

1 0

Boost.Units does not honor setw width?
by Paul A. Bristow 17 Aug '12

17 Aug '12

> -----Original Message----- > From: Paul A. Bristow [mailto:pbristow@hetp.u-net.com] > Sent: Thursday, August 09, 2012 4:44 PM > To: 'boost(a)lists.boost.org' > Subject: RE: [boost] [type_erasure] Review started (July 18-27, 2012) Flushed with success using Steven Watanabe's type_erase for printing columns, I have enjoyed extending it to use a fixed column width. But I have stumbled on a feature of Boost.Units output where Boost.Units does not honor the iostream width fully. This can be seen in this minimal example: std::cout << std::setw(20) << 'm' << std::endl; outputs 19 spaces and 'm', total 20 as expected. quantity<length> ql = 2345.6 * meters; std::cout << std::setw(20) << ql << std::endl; outputs 2345.6 m but the m does not line up with the previous 'm'. There are 14 leading spaces and 5 digits, 1 period and a space and letter m for the unit, a total of 22. The reason is that the first item value() is output using the ios width setting, but the second unit() 'm' (and separating space) is not, as in this line in io.hpp . os << q.value() << ' ' << Unit() I have 'cured' this 'bug' (I suspect it is a feature really) thus in boost/units/io.hpp template<class Char, class Traits, class Unit, class T> inline std::basic_ostream<Char, Traits>& operator<<(std::basic_ostream<Char, Traits>& os, const quantity<Unit, T>& q) { if (units::get_autoprefix(os) == autoprefix_none) { std::streamsize w = os.width(); if (w > 0) { // Need to build a single string with the width specified // (or outputting severally would not honour the width). std::ostringstream oss; oss << q.value() << ' ' << Unit(); os << oss.str(); } else { // No os.width set (so save the time needed to construct an ostringstream?) os << q.value() << ' ' << Unit(); } } This works for no_prefix, giving the expected number of leading spaces. But not for autoprefix - when a similar trick looks to get much more complicated (and when I start to get that 'angels fear to tread' feeling ;-) Of course the jaggedness of the layout becomes clearer both as the units get longer and get auto-prefixes (eg engineering). And there is a similar problem using a User defined Type like the measurement.hpp example in std::ostream& operator<< os << val.value() << "(+/-" << val.uncertainty() << ")"; where the second item is additional to the width, not included in it. 1 But I am not sure if this is wise. Is this a 'feature'? 2 Is this the best way of solving it? 3 Can/how should this be applied to autoprefix? 4 Should submit a ticket for this? Paul --- Paul A. Bristow, Prizet Farmhouse, Kendal LA8 8AB UK +44 1539 561830 07714330204 pbristow(a)hetp.u-net.com

1 0

Review Wizard Status Report for August 2012
by Ronald Garcia 16 Aug '12

16 Aug '12

============================================== Review Wizard Status Report for August 2012 ============================================== News ==== 1. Multiprecision Arithmetic Reviewed 2. Type Erasure Reviewed 3. Boost 1.50.0 Released. New Libraries: 1) Algorithm 2) Functional/OverloadedFunction 3) LocalFunction 4) Utility/IdentityType Open Issues =========== The following libraries have been reviewed and await reports from their review managers: * Predef - reviewed February 2012; review manager: Joel Falcou. * Multiprecision - reviewed June 2012; review manager: Jeffrey Hellrung The following libraries have been accepted to Boost, but have not yet been submitted to SVN: * Constrained Value - accepted September 2010; author: Robert Kawulak. * GIL.IO - accepted January 2011; author: Christian Henning. The following libraries have been accepted and submitted to SVN, but have not yet appeared in a release: * Atomic - accepted July 2011; author Helge Bahmann. * Type Traits Introspection - accepted August 2011; author: Edward Diener. * Lockfree - accepted August 2011; author: Tim Blechmann. * Context - accepted January 2012; author: Oliver Kowalke. The following libraries have been accepted provisionally to Boost, but have not been submitted for mini-review and full acceptance: * Endian - accepted provisionally November 2011; author: Beman Dawes. * Log - accepted provisionally March 2010; author: Andrey Semashev. General Announcements ===================== As always, we need experienced review managers. Please take a look at the list of libraries in need of managers and check out their descriptions. In general review managers are active boost participants, including library contributors, infrastructure contributors, and other mailing list participants with a substantial track record of constructive participation. If you can serve as review manager for any of them, email Ron Garcia or John Phillips, "rxg at cs dot cmu dot edu" and "phillips at pacific dot mps dot ohio-state dot edu" respectively. We are also suffering from a lack of reviewers. While we all understand time pressures and the need to complete paying work, the strength of Boost is based on the detailed and informed reviews submitted by you. If you are interested in reviewing a library but won't have time during the review period, you can always prepare your review ahead of time. No rule says you can only work on a review during the review period. A link to this report will be posted to www.boost.org. If you would like us to make any modifications or additions to this report, please email Ron or John. The review schedule is an unordered list of the libraries awaiting review. As such, any library on the schedule can be reviewed once the developer is ready, a review manager has been secured, and the manager, developer, and wizards agree on a date to schedule the review. Review Schedule =============== * Join (M) * Pimpl (M) * Sorting (M) * Quaternions, Vectors, Matrices (M) * Variadic Macro Data (M) * Block Pointer (M) * Singularity (M) * Extended Complex Numbers (M) * Metaparse (M) * Polygon: Voronoi Extensions * Coroutine * Range Extensions * Contract * Nowide (M) * ODEint * Array (M) * TypeIndex (M) ``(M)`` marks libraries that need review managers. -------------------- Join ---- :Author: Yigong Liu :Review Manager: Needed :Download: http://channel.sourceforge.net/ :Description: Join is an asynchronous, message based C++ concurrency library based on join calculus. It is applicable both to multi-threaded applications and to the orchestration of asynchronous, event-based applications. It follows Comega's design and implementation and builds with Boost facilities. It provides a high level concurrency API with asynchronous methods, synchronous methods, and chords which are "join-patterns" defining the synchronization, asynchrony, and concurrency. Pimpl ----- :Author: Vladimir Batov :Review Manager: Needed :Download: https://github.com/boost-vault/Miscellaneous/blob/master/Pimpl.zip :Description: The Pimpl idiom is a simple yet robust technique to minimize coupling via the separation of interface and implementation and then implementation hiding. This library provides a convenient yet flexible and generic deployment technique for the Pimpl idiom. It's seemingly complete and broadly applicable, yet minimal, simple and pleasant to use. Sorting ------- :Author: Steven Ross :Review Manager: Needed :Download: https://github.com/boost-vault/Sorting :Description: A grouping of 3 templated hybrid radix/comparison-based sorting algorithms that provide superior worst-case and average-case performance to std::sort: integer_sort, which sorts fixed-size data types that support a rightshift (default of >>) and a comparison (default of <) operator. float_sort, which sorts standard floating-point numbers by safely casting them to integers. string_sort, which sorts variable-length data types, and is optimized for 8-bit character strings. All 3 algorithms have O(n(k/s + s)) runtime where k is the number of bits in the data type and s is a constant, and limited memory overhead (in the kB for realistic inputs). In testing, integer_sort varies from 35% faster to 2X as fast as std::sort, depending on processor, compiler optimizations, and data distribution. float_sort is roughly 70% faster than std::sort. string_sort is roughly 2X as fast as std::sort. Quaternions, Vectors, Matrices ------------------------------ :Author: Emil Dotchevski :Review Manager: Needed :Download: http://www.revergestudios.com/boost-qvm/ :Description: QVM defines a set of generic functions and operator overloads for working with quaternions, vectors and matrices of static size. The library also defines vector and matrix data types, however it allows users to introduce their own types by specializing the q_traits, v_traits and m_traits templates. Variadic Macro Data ------------------- :Author: Edward Diener :Review Manager: Needed :Download: `Boost Sandbox <http://svn.boost.org/svn/boost/sandbox/variadic_macro_data/>`__ :Description: This library adds support and functionality for variadic macros to Boost as well as integrating variadic macros with the Boost PP library without changing the latter library in any way. Block Pointer ------------- :Author: Phil Bouchard :Review Manager: Needed :Download: https://svn.boost.org/svn/boost/sandbox/block_ptr/ :Description: Deterministic memory manager of constant complexity capable of handling cyclic collections. Singularity ----------- :Author: Ben Robinson :Review Manager: Needed :Download: https://github.com/cppmaven/Singularity :Description: The Singularity Design Pattern allows you to restrict any class to a single instance. Unlike the infamous Singleton, Singularity gives you direct control over the lifetime of the object, does not require you to grant global access to the object, nor does it limit you to the default constructor for that object. Extended Complex Numbers ------------------------ :Author: Matthieu Schaller :Review Manager: Needed :Download: http://code.google.com/p/cpp-imaginary-numbers/ :Description: The library is an extension of the std::complex class addressing two issues: 1. The standard does not guaranty the behaviour of the complex class if instantiated with types other than float/double/long double. 2. Some calculation where pure imaginary numbers (i.e. multiples of sqrt(-1)) appear are unnecessarily slowed down due to the lack of support for these numbers. The code I submit contains two interleaved classes boost::complex and boost::imaginary which can be instantiated with any type T provided T overloads the usual arithmetic operators and some basic (real) mathematical functions depending on which complex function will be used. It is thus an extended version of Thorsten Ottosen's n1869 proposal (http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1869.html) Metaparse --------- :Author: Abel Sinkovics :Review Manager: Needed :Download: http://abel.web.elte.hu/metaparse/metaparse.zip :Description: Metaparse is a library for constructing parsers parsing at compile-time based on template metaprogramming. The parsers built with the library take boost::mpl::strings as input and can produce - types - objects (types with public static members) - callable C++ functions (types with public static method) - template metafunction classes as output (based on the input being parsed). On compilers supporting constexpr the library provides the following syntactic sugar for writing the input of the parsers: BOOST_STRING("this is a string") The library can be used for implementing DSLs in C++, including DSLs making C++ template metaprogramming easier (see examples). Polygon: Voronoi Extensions --------------------------- :Author: Andril Sydorchuk :Review Manager: Lucanus Simonson :Download: http://svn.boost.org/svn/boost/sandbox/gtl/ :Description: This library extends Boost.Polygon with the following features: - Robust and efficient implementation of the sweepline algorithm that allows to construct Voronoi diagram, Delaunay triangulation and medial axis of a set of points and line segments. - Coordinates of the output geometries are computed within the 64 machine epsilon relative error (6 mantissa bits). - Voronoi diagram data structure that allows efficient traversal and data association with the output Voronoi graph. - No 3rd party dependencies (e.g. GMP, MPFR), all the required multiple precision types are implemented as part of the library. - The input and output coordinate type domains are configurable via coordinate type traits, thus allowing to compute coordinates of the Voronoi vertices within any required precision. - The full construction of the Voronoi diagram of 100 000 points takes only 0.27 seconds (see benchmarks). Coroutine --------- :Author: Oliver Kowalke :Review Manager: Hartmut Kaiser :Download: http://gitorious.org/boost-dev/boost-dev/archive-tarball/coroutine :Description: This library is an implementation of coroutines and generators using Boost.Context. Range Extensions ---------------- :Author: Akira Takahashi :Review Manager: Neil Groves :Download: https://github.com/faithandbrave/OvenToBoost :Description: This project adds some features of the Oven Range Library to Boost.Range. Features: - Additional Range Adaptors (taken, taken_while, dropped, dropped_while, elements, elements_key, memoized, outdirected) - Extensions for using Lambda (regular function, regular operator) - Infinite Range (iteration function) - and additional range utilities. Contract -------- :Author: Lorenzo Caminiti :Review Manager: Dave Abrahams :Download: http://sourceforge.net/projects/contractpp/files/latest/download :Description: This library implements Contract Programming for the C++ programming language (see the end of this email). In addition, the library implements virtual specifiers (final, override, and new, see C++11), concept checking, and named parameters. This library is implemented for the C++03 standard and it does not require C++11. Nowide ------ :Author: Artyom Beilis :Review Manager: Needed :Download: http://cppcms.com/files/nowide/ :Description: This library makes cross platform Unicode aware programming easier. It provides an implementation of standard C and C++ library functions, such that their inputs are UTF-8 aware on Windows without requiring to use Wide API. ODEint ------ :Author: Karsten Ahnert and Mario Mulansky :Review Manager: Steven Watanabe :Download: https://github.com/headmyshoulder/odeint-v2 :Description: odeint provides routines for numerically solving Ordinary Differential Equations, a task heavily required when approaching scientific problems numerically. In odeint we have implemented the most common algorithms in a modern, generic way which provides several advantages over existing implementations. The main focus of odeint is to separate algorithms from the underlying basic computation unit. This allows for example to easily use the power of modern GPUs via CUDA or OpenCL to speed up computational heavy simulations. Array ----- :Author: Brian Smith :Review Manager: Needed :Download: https://github.com/BrianJSmith/Array :Description: The array class is a C++11 compatible implementation of static multidimensional arrays. TypeIndex --------- :Author: Antony Polukhin :Review Manager: Needed :Download: https://github.com/apolukhin/type_index :Description: TypeIndex is an extended C++11 type_index library, that * works with disabled RTTI * can store const-volatile-reference info about types (if user requested it) * has all the functionality of std::type_index * has portable across compilers and platforms functionality for getting demangled type names * works across modules/shared libraries * does not require C++11 to work Libraries under development =========================== See http://svn.boost.org/trac/boost/wiki/LibrariesUnderConstruction for a current listing of libraries under development.

2 2

dramatically improved template error messages (was: C++11 decltype/SFINAE puzzler)
by Eric Niebler 16 Aug '12

16 Aug '12

(Andrew, a specific question for you below...) (Moderators: can you see why this msg from Andrew never reached the list (which he cc'ed))? On 8/14/2012 8:39 AM, Andrew Sutton wrote: >> GCC-4.7 gives a similarly terse error message. This, I think, is what >> I've been looking for. > > That's neat. Bravo! I've refined this idea. It now avoids cascading errors by detecting when a sfinae_error is passed as an argument to another function. (For the Haskell-ers out there, that makes it like the Either monad.) You can also get the full backtrace by uncommenting one line of code. IMO, this is now approaching something of real value. My personal experience using this with Proto-11 is that it dramatically improves error messages. I can also easily imagine how this can be used to implement a very nice concept-checking library for C++11 that approximates auto-concepts. Andrew, any thoughts on this? You've done far more work in this space than I have. I'd also like to know *why* this works, and indeed if it's guaranteed to. In particular, is it guaranteed that the virtual what() member of the sfinae_error class is instantiated at the outermost call site and not elsewhere (thereby eliminating the deep template back-trace)? I really just stumbled on this technique by accident, and I don't know how thin the ice is here. Both clang-trunk and gcc-4.7 think it's ok. (I'd also like to apologize to Paul Fultz for spelling his name wrong in an earlier message.) -- Eric Niebler BoostPro Computing http://www.boostpro.com

3 8

boost::asio and resolve_async
by david.hagood＠gmail.com 16 Aug '12

16 Aug '12

I was looking at needing to do a large number of reverse DNS lookups for a log analysis program I am writing, and wanted to use some form of async lookup to speed the program up (vs. doing each look up in series, at a couple of seconds per lookup). boost::asio and the async_resolve functions looked to be just what I wanted, BUT: I can find no good examples of a simple lookup. By "simple" I mean "given an address, give me the host name(s) associated with that address". All the examples were for a lookup by host name/transport type/port, e.g. host name/tcp/80. In my case I neither need a port # nor even have one - I just want the RDNS for the address. It seems to me the documentation in the boost::asio is a bit light on the various types for resolver queries, endpoint types, and such - and seems to be more focused on TCP and UDP than generic IP. Are there any better examples out there of RDNS using boost::asio?

2 1

[Interest] Value Dispatch
by *smgreened＠obbligato.org 16 Aug '12

16 Aug '12

Hi all, This might be old hat to verteran boosters but I found it nifty. Maybe it could make a small addition to Boost. Inspired by Sean Parent's C++Now! talk and the various articles on cpp-next, I've been playing around with value semantics and C++11. I found an interesting use of pack expansion at stackoverflow and expanded on it in a local project to convert from a traditional inheritance/virtual function model to a more value-centered approach. It turns out that with pack expansion one can implement "virtual functions" without needing inheritance or pointer/reference semantics. One can consider a vrtual function as "just" a compiler-generated switch statement. With that in mind, here's an example: ----------- #include <iostream> /// This is a collection of node kinds. We use it to categorize /// various kinds of objects and dispatch to the correct routine. template<typename T, typename T::kind ...I> class kind_tuple {}; /// This is the "virtual function" dispatch mechanism. It /// statically generates a lookup table of routines, one for each /// kind. template<typename R, typename D, typename T> class dispatcher {}; /// This is the actual dispatch implementation. template<typename R, typename D, typename O, typename O::kind ...I> class dispatcher<R, D, kind_tuple<O, I...>> { private: // Call the "virtual function override" via the provided class-level // dispatcher. template<typename P, typename O::kind K, typename ...Args> static R dispatch_handler(P payload, Args ...args) { return D::template dispatch<K>(payload, args...); } public: // dispatcher is an object that knows which routine in N to call. template<typename P, typename ...Args> static R dispatch(P payload, typename O::kind kind, Args ...args) { using F = R(P, Args...); F *table[] = { dispatch_handler<P, I, Args...> ... }; return table[kind](payload, args...); } }; class vehicle { public: enum kind { Car, Bus, Boat, Train, Airplane }; typedef kind_tuple<vehicle, kind::Car, kind::Bus, kind::Boat, kind::Train, kind::Airplane> kinds; private: class drive_dispatcher { public: template<kind K> static void dispatch(vehicle v) { v.drive_impl<K>(); } }; class turn_dispatcher { public: template<kind K> static bool dispatch(vehicle v, std::string direction) { return v.turn_impl<K>(direction); } }; template<kind K> void drive_impl(void) {} template<kind K> bool turn_impl(std::string direction) { return false; } kind the_kind; public: vehicle(kind k) : the_kind(k) {} void drive(void) { dispatcher<void, drive_dispatcher, kinds>::dispatch(*this, this->the_kind); } // Return whether the turn was successfully completed. bool turn(std::string direction) { return dispatcher<bool, turn_dispatcher, kinds>:: dispatch(*this, this->the_kind, direction); } }; template<> void vehicle::drive_impl<vehicle::Car>(void) { std::cout << "Driving a car!\n"; } template<> void vehicle::drive_impl<vehicle::Bus>(void) { std::cout << "Driving a bus!\n"; } template<> void vehicle::drive_impl<vehicle::Boat>(void) { std::cout << "Driving a boat!\n"; } template<> void vehicle::drive_impl<vehicle::Train>(void) { std::cout << "Driving a train!\n"; } template<> void vehicle::drive_impl<vehicle::Airplane>(void) { std::cout << "Flying an airplane!\n"; } template<> bool vehicle::turn_impl<vehicle::Car>(std::string direction) { std::cout << "Turning a car " << direction << "!\n"; return true; } template<> bool vehicle::turn_impl<vehicle::Bus>(std::string direction) { std::cout << "Turning a bus " << direction << "!\n"; return true; } template<> bool vehicle::turn_impl<vehicle::Boat>(std::string direction) { std::cout << "Turning a boat " << direction << "!\n"; return true; } template<> bool vehicle::turn_impl<vehicle::Train>(std::string direction) { std::cout << "Turning a train " << direction << "?!?\n"; return false; } template<> bool vehicle::turn_impl<vehicle::Airplane>(std::string direction) { std::cout << "Turning an airplane " << direction << "!\n"; return true; } template<typename Iterator> void drive(Iterator start, Iterator end) { for (auto i = start; i != end; ++i) i->drive(); } template<typename Iterator> void turn(Iterator start, Iterator end, std::string direction) { for (auto i = start; i != end; ++i) if (i->turn(direction)) { std::cout << "Successfully turned " << direction << "\n"; } else { std::cout << "Could not turn " << direction << "\n"; } } int main(void) { vehicle rides[] = { vehicle(vehicle::kind::Car), vehicle(vehicle::kind::Bus), vehicle(vehicle::kind::Boat), vehicle(vehicle::kind::Train), vehicle(vehicle::kind::Airplane) }; drive(rides, rides + 5); std::cout << "\n"; turn(rides, rides + 5, "left"); return 0; } ----------- The central idea is the creating of a "virtual function table" via pack expansion. Not only can we keep value semantics and avoid coupling via inheritance, we also get virtual function-like behavior for template functions. Expanding the idea allows us to implement a completely generic GoF visitor without using inheritance at all: ----------- #include <iostream> #include <tuple> /// This is a collection of node kinds. We use it to categorize /// various kinds of objects and dispatch to the correct routine. template<typename T, typename T::kind ...I> class kind_tuple {}; /// This is the "virtual function" dispatch mechanism. It /// statically generates a lookup table of routines, one for each /// kind. template<typename R, typename D, typename T> class dispatcher {}; /// This is the actual dispatch implementation. template<typename R, typename D, typename O, typename O::kind ...I> class dispatcher<R, D, kind_tuple<O, I...>> { private: // Call the "virtual function override" via the provided class-level // dispatcher. template<typename P, typename O::kind K, typename ...Args> static R dispatch_handler(P payload, Args ...args) { return D::template dispatch<K>(payload, args...); } public: // dispatcher is an object that knows which routine in N to call. template<typename P, typename ...Args> static R dispatch(P payload, typename O::kind kind, Args ...args) { using F = R(P, Args...); F *table[] = { dispatch_handler<P, I, Args...> ... }; return table[kind](payload, args...); } }; class vehicle { public: enum kind { Car, Bus, Boat, Train, Airplane }; typedef kind_tuple<vehicle, kind::Car, kind::Bus, kind::Boat, kind::Train, kind::Airplane> kinds; private: kind the_kind; public: vehicle(kind k) : the_kind(k) {} kind get_kind(void) const { return the_kind; } }; // U is to distinguish among different kinds of visitors for the same // object type. template<typename O, typename U, typename R, typename ...Args> class visitor { private: typedef visitor<O, U, R, Args...> This; // This dispatcher expects a single object so wrap the visitor and // the visited object together. typedef std::tuple<This *, O> payload; class visit_dispatcher { public: template<typename O::kind K> static R dispatch(payload object, Args ...args) { return std::get<0>(object)-> template visit_impl<K>(std::get<1>(object), args...); } }; template<typename O::kind K> R visit_impl(O object, Args ...args) {} public: R visit(O object, Args ...args) { return dispatcher<R, visit_dispatcher, typename O::kinds>:: dispatch(std::make_tuple(this, object), object.get_kind(), args...); } }; // Visitor types class Driving {}; class Turning {}; template<> template<> void visitor<vehicle, Driving, void>::visit_impl<vehicle::Car>(vehicle v) { std::cout << "Driving a car!\n"; } template<> template<> void visitor<vehicle, Driving, void>::visit_impl<vehicle::Bus>(vehicle v) { std::cout << "Driving a bus!\n"; } template<> template<> void visitor<vehicle, Driving, void>::visit_impl<vehicle::Boat>(vehicle v) { std::cout << "Driving a boat!\n"; } template<> template<> void visitor<vehicle, Driving, void>::visit_impl<vehicle::Train>(vehicle v) { std::cout << "Driving a train!\n"; } template<> template<> void visitor<vehicle, Driving, void>::visit_impl<vehicle::Airplane>(vehicle v) { std::cout << "Flying an airplane!\n"; } template<> template<> bool visitor<vehicle, Turning, bool, std::string>:: visit_impl<vehicle::Car>(vehicle v, std::string direction) { std::cout << "Turning a car " << direction << "!\n"; return true; } template<> template<> bool visitor<vehicle, Turning, bool, std::string>:: visit_impl<vehicle::Bus>(vehicle v, std::string direction) { std::cout << "Turning a bus " << direction << "!\n"; return true; } template<> template<> bool visitor<vehicle, Turning, bool, std::string>:: visit_impl<vehicle::Boat>(vehicle v, std::string direction) { std::cout << "Turning a boat " << direction << "!\n"; return true; } template<> template<> bool visitor<vehicle, Turning, bool, std::string>:: visit_impl<vehicle::Train>(vehicle v, std::string direction) { std::cout << "Turning a train " << direction << "?!?\n"; return false; } template<> template<> bool visitor<vehicle, Turning, bool, std::string>:: visit_impl<vehicle::Airplane>(vehicle v, std::string direction) { std::cout << "Turning an airplane " << direction << "!\n"; return true; } template<typename Iterator> void drive(Iterator start, Iterator end) { visitor<vehicle, Driving, void> driver; for (auto i = start; i != end; ++i) driver.visit(*i); } template<typename Iterator> void turn(Iterator start, Iterator end, std::string direction) { visitor<vehicle, Turning, bool, std::string> turer; for (auto i = start; i != end; ++i) if (turer.visit(*i, direction)) { std::cout << "Successfully turned " << direction << "\n"; } else { std::cout << "Could not turn " << direction << "\n"; } } int main(void) { vehicle rides[] = { vehicle(vehicle::kind::Car), vehicle(vehicle::kind::Bus), vehicle(vehicle::kind::Boat), vehicle(vehicle::kind::Train), vehicle(vehicle::kind::Airplane) }; drive(rides, rides + 5); std::cout << "\n"; turn(rides, rides + 5, "left"); return 0; } ----------- Not only is inheritance not required, we only need single dispatch to implement the visitor. Because we don't use inheritance at all for these "virtual functions" and the above visitor we can maintain value semantics throughout any code that uses the above techniques. Of course the code above can be cleaned up and beautified. It's just a quick proof-of-concept. I'm sure someone has already posted about this technique somewhere. Is there any interest in expanding it into a Boost library? -Dave

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[1.51, heap] permission to merge r80039
by Tim Blechmann 16 Aug '12

16 Aug '12

request for release manager: i'd like to merge r80039 to release, as it fixes bug #7233. ok or too late? thanks, tim

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[1.51][hash] Permission to merge.
by Daniel James 15 Aug '12

15 Aug '12

Hello release managers, Please can I merge changeset 80038: https://svn.boost.org/trac/boost/changeset/80038/ The tests have cycled for most major compilers and libraries. I think the only missing one is Visual Age C++ which isn't affected since it doesn't have support for C++11 headers. This fixes a bug when compiled with Intel on windows. thanks, Daniel

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