November 2006 - Boost - lists.preview.boost.org

Proposed addition to the iterator library for iterating over all tuples.
by Alex Mendes da Costa 19 Jun '09

19 Jun '09

Hi folks: I quite often have to write code to do the following: Given N sequences, generate all the N-tuples s.t. the i'th element in each tuple is drawn from the i'th sequence. For example, if sequence S1 is { 1, 2, 3 } and sequence S2 is { 10, 20 }, I need to generate these tuples: (1, 10), (2, 10), (1, 20), (2, 20). I think this problem could be solved elegantly with a iterator similar to zip_iterator. I have some questions: (1) Is there already a way to do this in boost? (2) If not, would this be a worthwhile addition to the iterator library? (3) If so, could I contribute code for this, or must the existing library owners write it? (4) If so, how would I go about getting my code added? Most of the docs seem to be about submitting entire libraries, rather than extensions to existing libraries. Thanks, Alex

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[fusion] list/detail/at_impl.hpp, Suggest recrusive elem_at_c instead of recursive apply
by Larry Evans 24 Oct '07

24 Oct '07

In boost/fusion/.../list/detail/at_impl.hpp the apply is recursive; however, the only reason for the recursion is to "find the right element type". The other reason for apply is the same as that of the apply in.../vector/detail/at_impl.hpp. "find the right element type" in vector's at_impl is done with mpl::at. The similarity between list and vector at_impl would be more apparent if: template <typename Sequence, int N> struct elem_at_c { typedef typename mpl::eval_if< mpl::bool_<N == 0> , mpl::identity<typename Sequence::car_type> , elem_at_c<typename Sequence::cdr_type, N-1> >::type type; }; were used to find the right element type. Then the frist part of list::at_impl::apply would be: template <typename Sequence, typename N> struct apply { typedef elem_at_c<Sequence, N::value> element; typedef typename mpl::eval_if< is_const<Sequence> , detail::cref_result<element> , detail::ref_result<element> >::type type; which is very similar to the counterpart in vector: template <typename Sequence, typename N> struct apply { typedef mpl::at<typename Sequence::types, N> element; typedef typename mpl::eval_if< is_const<Sequence> , detail::cref_result<element> , detail::ref_result<element> >::type type;

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[Ann] Variadic Templates
by Doug Gregor 20 Aug '07

20 Aug '07

Hello Fellow Boosters, I have just completed implementing support for "Variadic Templates" in GCC. Variadic templates are C++ templates that can accept any number of "extra" template arguments, just like a function that uses C-style varargs (with "...") can accept any number of extra arguments. Unlike C varags, however, variadic templates work both for class templates and function templates, and are completely type-safe. Why do variadic templates matter for Boost? For one thing, they will allow us to eliminate the use of preprocessor metaprogramming in many, many places throughout the library. This can simplify our library implementations drastically, and eliminate those annoying "caps" on the number of arguments one can use (ever tried to use bind () with 11 arguments?). Beyond that, variadic templates are just plain fun, and there are undoubtedly a great number of metaprogramming applications. Here are a few teasers to illustrate what variadic templates can do: We can implement an MPL-like type vector with one line of code: template<typename... Types> struct vector { }; The "ellipsis" means that we can pack any number of arguments into the "template parameter pack" called Types. Of course, we can do the same for MPL's vector_c: template<typename T, T... Values> struct vector_c { }; Unimpressed? Here's an implementation of tuple that accepts any number of element types: template<typename... Values> class tuple; template<> class tuple<> { }; template<typename Head, typename... Tail> class tuple<Head, Tail...> : private tuple<Tail...> { protected: Head m_head; }; We're using recursion to build up the tuple. The ellipsis operator works with partial specializations, too, packing together "extra" arguments at the end. Of course, all of this would be useless if we couldn't write some of the other tuple operations, like operator==: inline bool operator==(const tuple<>&, const tuple<>&) { return true; } template<typename T, typename... TTail, typename U, typename... UTail> bool operator==(const tuple<T, TTail...>& t, const tuple<U, UTail...>& u) { return t.head() == u.head() && t.tail() == u.tail(); } Variadic function templates can accept any number of function arguments, so we can now (finally!) implement a type-safe printf() without mangling the tried-and-true printf() syntax or restricting ourselves to POD types: void printf(const char* s) { while (*s) { if (*s == '%' && *++s != '%') throw std::runtime_error("invalid format string: missing arguments"); std::cout << *s++; } } template<typename T, typename... Args> void printf(const char* s, const T& value, const Args&... args) { while (*s) { if (*s == '%' && *++s != '%') { std::cout << value; return printf(++s, args...); } std::cout << *s++; throw std::runtime_error("extra arguments provided to printf"); } I've also built prototypes of of Function and Bind, among other things. The Bind example is especially interesting, and worth a read (see below) Interested? There is more information and a complete implementation in GCC available at: http://www.generic-programming.org/~dgregor/cpp/variadic- templates.html I've written two documents describing variadic templates in more detail: http://www.generic-programming.org/~dgregor/cpp/brief-intro.pdf : A brief introduction to variadic templates, that walks through the printf() implementation. http://www.generic-programming.org/~dgregor/cpp/variadic- templates.pdf : A complete proposal to introduce variadic templates into C++0x. This contains annotated versions of the tuple, function, and bind prototypes along with a detailed description of the capabilities of variadic templates. The GCC implementation is complete, supporting all of the features described in the proposal. I've used it for proof-of-concept implementations of several libraries. In the (near) future, I hope to introduce variadic templates support into the "official" GCC, but I need your help! Are variadic templates worthwhile? Will they help us build better, cleaner Boost libraries in the future? Should we bring variadic templates to the C++ committee, to enable better implementations of TR1 components and eliminate the need for much of the preprocessor metaprogramming we do today? Feedback is greatly appreciated. Cheers Doug Gregor Open Systems Lab @ Indiana University

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Review Request: Boost Exception
by Emil Dotchevski 07 Jul '07

07 Jul '07

I believe the proposed Boost Exception library is now mature enough for formal review. Please read the documentation at: http://www.revergestudios.com/boost-exception/boost-exception.htm Here is a direct link to the source code, boost build jamfiles and tests. Tested with MSVC 7.1, MSVC 8.0, GCC 3.4.4, and Comeau C++ 4.3.8: http://www.revergestudios.com/boost-exception/boost-exception.zip Regards, Emil Dotchevski

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[BGL] Adapter from bidirectional to undirected graph
by Benoit 05 Jul '07

05 Jul '07

Hello everyone, I have been using the BGL for quite some time with both directed and bidirectional graphs (with adjacency_list). Sometimes i need to use these graphs in algorithms as if they were undirected... AFAICT, this is currently not possible because out_edges() exists in all cases, but don't do the same... The obvious solution consists in writing a dedicated adapter used for translating a bidirectional graph into a directed one at least for out_edges(), and optionally for other stuff (internal properties, vertex and edge descriptors?) to make this subtle change as transparent as possible. Can you think of another way ? Is there already such an adapter somewhere (i have unsuccessfully tried to find one...) ? In case it didn't exist, would anyone else be interested to have one ? Benoît PS : I'd like to thank the authors of the BGL, it's a pleasure to work with it everyday !

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[defer] - comments?
by simon meiklejohn 14 Mar '07

14 Mar '07

Anyone had a play with the defer library in the vault and want to comment on its usefulness? It clearly shares much in common with the asio Dispatcher concept but with the freedom to define more general Dispatchers. (e.g. 'Create thread per callback' and 'Dispatch Callback via Message Pump', both of which violate the specific guarantees of the asio Dispatcher concept, that the callback occur in a thread that has called asio::demuxer::run().) One potential application for defer would be in conjunction with asio, creating the thread(s) which call demuxer::run() in a succinct form. Cheers Simon

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Re: [boost] Optimized portable isnan, isfinite, fpclassify etc
by Boris Gubenko 16 Jan '07

16 Jan '07

Boris Gubenko wrote: > I'm checking with the OS folks and will report back here, > for both HP-UX/ia64 and Tru64 Alpha. And the answer is: > You are correct that both platforms are capable of executing in > either endianness. However, the operating systems operate in only > one. > HP-UX is *always* big endian > Tru64 is *always* little endian > [...] > Summary: > A compile time check is adequate. > So, with current <endian.hpp>, I think, that we are all set for these platforms. Thanks, Boris ----- Original Message ----- From: "Boris Gubenko" <Boris.Gubenko(a)hp.com> To: <boost(a)lists.boost.org> Cc: "Boris Gubenko" <Boris.Gubenko(a)hp.com> Sent: Tuesday, October 17, 2006 9:12 AM Subject: Re: [boost] Optimized portable isnan, isfinite, fpclassify etc >> Now endian.hpp defines BOOST_BIG_ENDIAN on HP-UX. >> This was suggested by Boris Gubenko. >> >> If you are right John, then this seems to be incorrect. >> Shouldn't endian.hpp then define something like >> BOOST_UNKNOWN_ENDIAN on HP-UX? >> >> Boris, do you have any comments on this? > > I'm checking with the OS folks and will report back here, > for both HP-UX/ia64 and Tru64 Alpha. > > Thanks, > Boris > > ----- Original Message ----- > From: "Johan Råde" <rade(a)maths.lth.se> > To: <boost(a)lists.boost.org> > Sent: Tuesday, October 17, 2006 9:00 AM > Subject: Re: [boost] Optimized portable isnan, isfinite, fpclassify etc > > >> John Maddock wrote: >>> I'm not completely certain, but I believe HP-UX falls into this >>> category: >>> the idea is that you can pass binary data from one platform to another, >>> and >>> just switch the endianness bit to access it natively (think distributed >>> applications). >> >> Now endian.hpp defines BOOST_BIG_ENDIAN on HP-UX. >> This was suggested by Boris Gubenko. >> >> If you are right John, then this seems to be incorrect. >> Shouldn't endian.hpp then define something like BOOST_UNKNOWN_ENDIAN on >> HP-UX? >> >> Boris, do you have any comments on this? >> >> Then my fpclassify could use a fast implementation when BOOST_BIG_ENDIAN >> or BOOST_LITTLE_ENDIAN is defined and a slower implementation when >> BOOST_UNKNOWN_ENDIAN is defined. >> >> --Johan >> >> >> >> >> >> >> >> _______________________________________________ >> Unsubscribe & other changes: >> http://lists.boost.org/mailman/listinfo.cgi/boost >> >> >

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Re: [boost] Boost Timer Update
by Stephan Kaiser 08 Jan '07

08 Jan '07

Hello Philippe, Do you also plan to tackle the problem with QueryPerformanceCounter() on multi-core systems? QPC reports problematic/mismatching values on certain multi-core CPUs (e.g. Athlon X2). Cheers, Stephan ----- Original Message ----- From: "Philippe Vaucher" <philippe.vaucher(a)gmail.com> Newsgroups: gmane.comp.lib.boost.devel Sent: Sunday, October 29, 2006 6:16 PM Subject: Boost Timer Update > Hello guys, > > I've been quite away from home in the last months so I wasn't able to work > on the timer, but now I'm back so I'd have some time now. > Today I've implemented Gennaro's device idea for QueryPerformanceCounter() / > timeGetTime(). > > As the vault's down, you can find it at > http://www.unitedsoft.ch/boost/timer_2006_10_29.zip. > > Are you guys still using QuickBook for the documentation ? Before going away > I started using it but it was quite painful. > > Philippe Vaucher > _______________________________________________ > Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost >

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[BGL] make_filtered_graph() should accept const Graph&
by Vladimir Sykora 02 Jan '07

02 Jan '07

Hi, Can anyone correct the make_filtered_graph() function on boost/graph/filtered_graph.hpp to accept a const Graph& ? Thanks, --vjs

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interest in pair projection iterators?
by Lewis Hyatt 22 Dec '06

22 Dec '06

Hi All, I am in the middle of writing a large body of numerical code for data analysis and have found the boost library very useful for a number of things. I do a lot of work with std::map and with containers of std::pair objects, and one thing I've always felt annoyed with is the difficulty of interfacing a value_type of std::pair with the STL algorithms. Frequently, for instance, it is nice to represent data as a std::list< std::pair<double,double> > and then to be able to work with the first and second coordinates independently and pass them to std::accumulate() or whatever. The SGI extensions select1st() and select2nd() are somewhat useful but also non-standard, and they frequently require memory-wasting buffers to store only the first or second elements. I have a relatively simple system for solving this problem and it strikes me as the kind of quick thing that might be worth including in boost. The idea is to define a template class, template<typename Iterator> class first_iterator_t { Iterator pair_iter; public: first_iterator_t(Iterator const&); /*...*/ }; which overloads operator*() and operator->() to access the element named "first" of the object returned by Iterator::operator*(). (With something analagous for second_iterator_t.) A function named first_iterator() provides a convenient creation function a-la std::make_pair(). That's it for the most part, the class only needs to provide the required iterator interface which mostly consists of just calls to Iterator::operator functions. There is also the need to define the appropriate iterator_traits typedefs. There is a small issue of deducing whether the iterator needs to be a const iterator or not, which can be solved with partial specialization of a second template class containing nested typedefs. The end result is that something like this is now conveniently available: map<int, double> m; m[1] = 0.1; m[2]=0.2; m[5]=0.5; //prints: 1 2 5 copy(first_iterator(m.begin()), first_iterator(m.end()), ostream_iterator<int>(cout, "\t")); //prints: 0.1 0.2 0.5 copy(second_iterator(m.begin()), second_iterator(m.end()), ostream_iterator<double>(cout, "\t")); I don't see a way to do this otherwise without resorting to something like select1st(), and that also requires the use of std::transform(), which I find distasteful since the required operation is really just a simple copy. So anyway do you think this is useful enough to possibly include in boost somewhere, or is it too trivial? I have what I believe to be correct code, which could be made boost-ready with little effort. Thanks for your advice! -Lewis

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