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[gsoc16] Attn: We need 2016 mentors and project ideas for Boost!
by Niall Douglas 21 Jan '16

21 Jan '16

Dear Boost community, We are now within one month of the beginning of Google Summer of Code 2016! Last year's GSoC was a particularly momentus one as we witnessed the approval by the community for Boost.Hana to enter Boost - congratulations to Louis and his mentor Joel for bringing to Boost a very exciting new C++ library which pushes C++ compilers to their limits. We have also seen greatly improved retention of GSoC students in the community thanks to the improved student selection procedures introduced last year, and the fresh blood brought into Boost has been very evident on the mailing list this year. As part of filling in our application for 2016, we must supply to Google a list of potential GSoC mentors and potential GSoC projects for summer 2016 in order to be approved as an organisation this year. Therefore a lengthy page of awesome project ideas at https://svn.boost.org/trac/boost/wiki/SoC2016 before *February 8th* would be an enormous help to getting us approved again this year and bringing to Boost another US$35,000 of Google funding. If you think yourself able to mentor a student doing some work on Boost this summer, *please* consider adding a description of the proposed work item and your name to the list. Last year we had more slots awarded to us by Google than we had good combinations of student and mentor, so we had to return slots which I believe has not happened here at Boost GSoC in at least a decade. That means that your project ideas have a better chance than ever of receiving funding if you can find a good student, and that is best done by getting project ideas floated *early*. If you want to know more about mentoring a Google Summer of Code funded student work before you nominate yourself, please feel free to ask on the main Boost developers mailing list boost(a)lists.boost.org or via gsoc-admin(a)boost.org. Thank you in advance for your time. --- Boost C++ Libraries Google Summer of Code 2016 admin https://svn.boost.org/trac/boost/wiki/SoC2016

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please add safe numerics to the review queue
by Robert Ramey 21 Jan '16

21 Jan '16

Any chance I could get safe numerics added to the Boost Review queue? THe link to the incubator is here: http://rrsd.com/blincubator.com/bi_library/safe-numerics/?gform_post_id=426 documention here: http://htmlpreview.github.io/?https://github.com/robertramey/safe_numerics/… and github repo here: https://github.com/robertramey/safe_numerics Robert Ramey

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[gsoc16] Can I quickly check if the below really is the best approach?
by Niall Douglas 19 Jan '16

19 Jan '16

Dear Boost, In the prototype https://svn.boost.org/trac/boost/wiki/SoC2016 at https://goo.gl/1CQAuQ I claim "Even with all of Boost's facilities [1] and using C++ 14, this program represents the currently best available method of implementing a static constant associative map of keys to values" and you can find either the code at the link or pasted below. Can I quickly check here if that claim is true? Is there a better way than my example program? BTW by static constant associative map of keys to values I mean the thing me and Tony van Eerd were speaking about here last year and the year before, and indeed frequently at BlackBerry! Niall [1]: I believe Boost.Hana could implement a static constan associative map quite easily, but I'd be fairly sure Hana will likely be beyond most students. --- cut --- #include <initializer_list> #include <experimental/string_view> #include <map> #include <unordered_map> #include <iostream> using std::experimental::string_view; enum class weekday { sunday, monday, tuesday, wednesday, thursday, friday, saturday }; // initializer_list, pair and string_view are constexpr, so this list can be constexpr // (i.e. lives in the mind of the compiler only and has zero representation in generated code) #define STRING_VIEW(str) { str, sizeof(str)-1 } constexpr std::initializer_list<std::pair<const string_view, weekday>> string_to_weekday { { STRING_VIEW("sunday"), weekday::sunday }, { STRING_VIEW("monday"), weekday::monday }, { STRING_VIEW("tuesday"), weekday::tuesday }, { STRING_VIEW("wednesday"), weekday::wednesday }, { STRING_VIEW("thursday"), weekday::thursday }, { STRING_VIEW("friday"), weekday::friday }, { STRING_VIEW("saturday"), weekday::saturday } }; int main(void) { { // Calls malloc() at least 7 times static const std::map<string_view, weekday> to_weekday1 = string_to_weekday; std::cout << "'monday' maps to " << static_cast<int>(to_weekday1.at("monday")) << std::endl; std::cout << "'friday' maps to " << static_cast<int>(to_weekday1.at("friday")) << std::endl; // Calls free() at least 7 times } { // Calls malloc() at least 8 times static const std::unordered_map<string_view, weekday> to_weekday2 = string_to_weekday; std::cout << "'monday' maps to " << static_cast<int>(to_weekday2.at("monday")) << std::endl; std::cout << "'friday' maps to " << static_cast<int>(to_weekday2.at("friday")) << std::endl; // Calls free() at least 8 times } return 0; } -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/

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whatever happened to the constrained value library?
by Robert Ramey 18 Jan '16

18 Jan '16

I just stumbled upon the following announcement. http://lists.boost.org/boost-announce/2010/09/0265.php But I don't see any such library as part of boost. What's up with this? Robert Ramey

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Re: [boost] [interprocess] boost::hash - different values for the same input?
by Daniel James 17 Jan '16

17 Jan '16

I altered the subject to include interprocess, since this is relevant there. On 9 December 2015 at 20:13, trafdev <trafdev(a)mail.ru> wrote: > > As per hash_range docs > (http://www.boost.org/doc/libs/1_59_0/doc/html/hash/reference.html#boost.has…) > > "This hash function is not intended for general use, and isn't guaranteed to > be equal during separate runs of a program - so please don't use it for any > persistent storage or communication." > > As per code > (http://www.boost.org/doc/libs/1_59_0/boost/functional/hash/hash.hpp) it > uses hash_combine_impl which doesn't use any random, process_id etc values > which might change during new process launches. > > At the same time, in an example describing container usage in the shared > memory > (http://www.boost.org/doc/libs/1_59_0/doc/html/interprocess/allocators_conta…) > boost::hash is being used: > > typedef boost::unordered_map > < KeyType , MappedType > , boost::hash<KeyType> ,std::equal_to<KeyType> > , ShmemAllocator> > MyHashMap; > > My question is: is it safe to use boost::hash in a shared (persistent) > storage or it's safe only for a particular (non-pointer) types? > > In this case, boost::hash docs should be changed to mention this exceptions, > because otherwise no one could use boost::hash in a shared memory > containers. Sorry about the very slow reply. I'm afraid the answer is that it depends. The reason why I originally wrote that note was because it can generate different hash values when compiled for different platforms or architectures, e.g. a 32-bit executable might generate a different hash value to a 64-bit executable. Also, I occasionally change the algorithm in new versions of boost (and probably will in the next version). For interprocess this currently won't be an issue if all the processes are using the same executable, or using executables built with same version of boost and the same architecture, but could be a problem if that isn't the case. So maybe the Interprocess documentation needs to explain this. But there's another problem coming up. The last proposal I saw for a standard generic hash function will support randomized hash functions (such as siphash) which will only work with interprocess if they are somehow seeded identically. I'll want to support whatever is accepted to the standard (although maybe as a separate library). I'll need to catch up with how that is progressing, as I haven't kept up to date.

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Any-Range containing transformed range fails when compiled with optimizations
by Auer, Jens 17 Jan '16

17 Jan '16

Hi, I have a program which produces different results based on the compiler options. The program uses a boost::any_range containing a transformed range where the transformation function just returns a new object of a simple struct: #include <vector> #include <boost/range.hpp> #include <boost/range/any_range.hpp> #include <boost/range/adaptor/transformed.hpp> #include <iostream> template<typename T1, typename T2> void ASSERT_EQ(T1 x,T2 y) {assert(x == y);}; struct SourceEvent { std::uint32_t event; void* ptr; }; int main() { using boost::adaptors::transformed; using R = boost::any_range<SourceEvent, boost::forward_traversal_tag, SourceEvent, std::ptrdiff_t>; std::vector<std::int32_t> v(100); R r = v | transformed( [](auto const&) {return SourceEvent{0u, nullptr};} ); auto t = v | transformed( [](auto const&) {return SourceEvent{0u, nullptr};} ); ASSERT_EQ( 100u, boost::size(r) ); ASSERT_EQ( 100u, boost::size(t) ); for (auto const& i: t) { ASSERT_EQ(0u, i.event ); ASSERT_EQ(nullptr, i.ptr); } for (auto const& i: r) { ASSERT_EQ(0u, i.event ); ASSERT_EQ(nullptr, i.ptr); } std::cout << "OK" << std::endl; return 0; } When compiled without optimization, it just prints ok. The same source compiled with optimizations asserts, and prints some interesting warnings: In file included from /usr/local/boost-1.60.0/include/boost/range/detail/any_iterator.hpp:22:0, from /usr/local/boost-1.60.0/include/boost/range/any_range.hpp:17, from prog.cc:3: /usr/local/boost-1.60.0/include/boost/range/detail/any_iterator_wrapper.hpp: In member function 'boost::range_detail::any_forward_iterator_wrapper<WrappedIterator, Reference, Buffer>::reference boost::range_detail::any_forward_iterator_wrapper<WrappedIterator, Reference, Buffer>::dereference() const [with WrappedIterator = boost::iterators::transform_iterator<boost::range_detail::default_constructible_unary_fn_wrapper<main()::<lambda(const auto:1&)>, SourceEvent>, __gnu_cxx::__normal_iterator<int*, std::vector<int> >, boost::iterators::use_default, boost::iterators::use_default>; Reference = SourceEvent; Buffer = boost::any_iterator_buffer<64ul>; boost::range_detail::any_forward_iterator_wrapper<WrappedIterator, Reference, Buffer>::reference = SourceEvent&]': /usr/local/boost-1.60.0/include/boost/range/detail/any_iterator_wrapper.hpp:302:57: warning: function may return address of local variable [-Wreturn-local-addr] return dereference_cast<reference>(*m_it); ^ /usr/local/boost-1.60.0/include/boost/range/detail/any_iterator_wrapper.hpp:302:52: note: declared here return dereference_cast<reference>(*m_it); ^ prog.exe: prog.cc:7: void ASSERT_EQ(T1, T2) [with T1 = unsigned int; T2 = unsigned int]: Assertion `x == y' failed. Aborted The example is compiled with gcc 5.2 and boost 1.60 (http://melpon.org/wandbox/permlink/QF1qyw5bWHKwyKJ8) Is there an error in my code? I would guess that transforming a range into a range of new objects of another type is quite common, so it should work. I am also curious why the warning is only emitted when compiling with optimizations. Best wishes, Jens -- Jens Auer | CGI | Software-Engineer CGI (Germany) GmbH & Co. KG Rheinstraße 95 | 64295 Darmstadt | Germany T: +49 6151 36860 154 jens.auer(a)cgi.com<mailto:jens.auer@cgi.com> Unsere Pflichtangaben gemäß § 35a GmbHG / §§ 161, 125a HGB finden Sie unter de.cgi.com/pflichtangaben<http://de.cgi.com/pflichtangaben>. CONFIDENTIALITY NOTICE: Proprietary/Confidential information belonging to CGI Group Inc. and its affiliates may be contained in this message. If you are not a recipient indicated or intended in this message (or responsible for delivery of this message to such person), or you think for any reason that this message may have been addressed to you in error, you may not use or copy or deliver this message to anyone else. In such case, you should destroy this message and are asked to notify the sender by reply e-mail.

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Re: [boost] [gsoc16] Can I quickly check if the below really is the best approach?
by Shangtong Zhang 16 Jan '16

16 Jan '16

Hi, > // At global scope, allocates no memory whatsoever > static const constexpr static_map<int, const char *> map { > { 5, "apple" }, > { 8, "pear" }, > { 0, "banana" }, > ... > }; > // Works, constexpr "apple" > constexpr const char *what_is_5 = map[5]; > // Works > assert(map[5] && !strcmp(map[5], "apple"); > // Works > map[5]="orange"; > assert(map[5] && !strcmp(map[5], "orange"); > // Throws exception as there is no key 6 > map[6]; > map.at <http://map.at/>(6); I have difficulty understanding this, As variable map is const and constexpr, how can map[5]=“orange” work? I don’t know how to implement this, where to store the “orange”. Could anyone help me? Thanks, Shangtong Zhang, Senior Student, School of Computer Science, Fudan University, PRC.

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[gsoc16]Interest in static map
by Shangtong Zhang 15 Jan '16

15 Jan '16

Hi Niall Douglas, I’m interested in the project static map and want to contribute to it in gsoc 2016. And I try to start with the programming competency test. Following is what I have done, https://github.com/ShangtongZhang/StaticMap/blob/master/main.cpp <https://github.com/ShangtongZhang/StaticMap/blob/master/main.cpp> Could you please give me some advice? In addition, I’m a little confused with // This should fail elegantly and usefully ... //constexpr std::array<unsigned, 0> string_hashes_fail = hash_strings(5); My understanding is that it will raise a run-time exception, is it right? Thanks, Shangtong Zhang, Senior Student, School of Computer Science, Fudan University, PRC.

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Is there interest in Computable Calculus?
by Damian Vicino 15 Jan '16

15 Jan '16

Hi, I’m interested in developing basic support of “real numbers” for computable calculus applications. In computable calculus, reals have infinite precision (certain restrictions may apply). The usual approach is representing numbers as functions producing digits. An arithmetic operation produces a new function using the functions in the operands and some simple expression manipulations. When a number needs to be evaluated (e.g., for comparisons where expression rules are unknown), enough digits for reaching an answer are generated using the functions. For example, 3.14 < pi requires generating 3 digits to answer “true”. The algorithms for arithmetic operations and evaluation are well known, and multiple implementations were developed (mostly in the 90s). However, I couldn’t find an open source implementation and had to come up with my own for working in a project during the last 2 years. My preferred reference for the required algorithms is the book: Aberth, Oliver. Computable Calculus. Academic Press, 2001. In particular, my initial scope would be the 4 arithmetic operations (+, -, *, /), and the comparison operators (<, =) for reals. Someone is interested in such a library in boost?

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IS THERE A WAY TO FIND IF A CERTAIN TYPE HAS A CERTAIN MEMBER?
by Juan Dent 15 Jan '16

15 Jan '16

> HI, > > > I have read that via enable_if and other constructs, one can declare a member of a type conditionally. > > What I need is a way to be able to test whether a certain type has a certain method or member data, so that I can call it or use it accordingly. > > Does boost have something like this?? > > > Regards, > Juan

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