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[config] RFC PR 82
by Domagoj Saric 01 Aug '16

01 Aug '16

Hi everyone, I am finally getting back to (trying to) contributing some of my personal projects to Boost. For two of those, Err (https://github.com/psiha/err) and especially Functonoid (a C++11 generalization and rewrite of my previous Boost.Function related work) I need some lower level codegen and/or optimiser control functionality (i.e. portable macros wrapping toolset specific attributes and pragmas) that I've added to my personal fork of Boost.Config and which I've now submitted in the subject PR (https://github.com/boostorg/config/pull/82) I don't expect this PR to be accepted as is/'just like that' so I'm opening this thread where we can discuss which of those changes/macros are welcome, which need more work and which, for some reason, should not be part of Boost.Config (and which, in turn then, I have to move to some 'internal implementation headers' in libraries that will need them). To avoid 'spamming' (and save time;) I'll skip the explanation of the individual macros as I expect them to mostly be self-explanatory (if not from their name then from the minimal Boost.Config documentation additions that are part of th PR). ps. I'll be on the (off) road for the next three weeks so I don't know when I'll be able to respond until I get back... -- "What Huxley teaches is that in the age of advanced technology, spiritual devastation is more likely to come from an enemy with a smiling face than from one whose countenance exudes suspicion and hate." Neil Postman

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constexpr, cmath functions and other functions and distributions
by Paul A. Bristow 01 Aug '16

01 Aug '16

After much head scratching (and some head banging) - against a string of "Computer Says No" inscrutable diagnostic messages. I believe that I have now produced a (partial) version of Boost.Math normal_distribution that can be a constexpr using GCC6.1.1. (diagnosis has got to improve if constexprizing existing code is to become popular? ) constexpr normal_distribution<double> n01(0., 1.); producing this code 960 .loc 1 47 0 961 000d 660FEFC0 pxor %xmm0, %xmm0 # tmp89 962 0011 F20F1145 movsd %xmm0, -32(%rbp) # tmp89, n01.m_mean 962 E0 963 0016 F20F1005 movsd .LC1(%rip), %xmm0 #, tmp90 963 80080000 964 001e F20F1145 movsd %xmm0, -24(%rbp) # tmp90, n01.m_sd 964 E8 and more interestingly, a PDF can be calculated at compile-time constexpr double p = pdf(n01, 0.5); 965 .loc 1 56 0 966 0023 F20F1005 movsd .LC2(%rip), %xmm0 #, tmp91 966 88080000 967 002b F20F1145 movsd %xmm0, -8(%rbp) # tmp91, p 967 F8 Perhaps the assembler whizzes can confirm that this looks like a compile-time computation? This relies on lots of C++14 and 17 constexpr features, and Most crucially it relies on being able to use several cmath functions as constexpr. For a couple of trivially simple examples: abs(x) and isfinite(x). constexpr double ax42 = abs(-42.); // Unspecified version of abs. constexpr double ax42g = ::abs(-42.); // C double version? constexpr double ax42b = __builtin_abs(-42.); // builtin GCC only constexpr double ax42s = std::abs(-42.); // std:: version Without most cmath functions it seems to me impossible to write any reasonable (so-called special or not) mathy function or statistical distribution that can be constexpr. At present, only GCC 6.1.1 authors have foreseen this need and provided these so-called built_in_* cmath function versions that are constexpr, see https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html. (Aside on other compilers: · Clang 3.8 has some problem deep inside type_traits that I haven't yet investigated. · VS2015 update 3 still doesn't compile many essential C++17 features from my experiments so far, and doesn't provide any constexpr cmath functions. ) The many potential benefits of compiler-time computations are quite enticing to contemplate :-) * No run-time where possible * Errors caught at compile-time * Improved code readability * Reduce executable footprint and potentially most important * Avoid global initialization order issues * Avoid initialization thread races See · Scott Schurr, C++ Developer, Ripple Labs https://cppnow2015.sched.org/event/2oLK/constexpr-c-at-compile-time · Dietmar Khul Constant Fun https://www.youtube.com/watch?list=PL9hrFapz4dsM1B9bI8VmEE4JJlR0m-dvo <https://www.youtube.com/watch?list=PL9hrFapz4dsM1B9bI8VmEE4JJlR0m-dvo&v=T37…> &v=T37ElKPICvo&app=desktop Is there any support for making std::abs and friends constexpr where possible in future versions of the C++ Standard? Paul PS I can't conceive of any UDT using or providing floating-point without some function like frexp to get and set exponent and significand. I also note that making constexpr frexp and ldexp and other functions passing a pointer to a variable to be updated may be more difficult/impossible, and is very likely to require at least initialization of the to-be-updated variable before calling frexp. I've investigated overcoming this by writing a naïve new constexpr version of frexp returning a std::pair (significand and exponent) but, of course, it can never be a 'drop-in' replacement because it has a different signature, so lots of code changes are inevitable. --- Paul A. Bristow Prizet Farmhouse Kendal UK LA8 8AB +44 (0) 1539 561830

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[type_erasure] Do we have a maintainer (minor fix needed)?
by John Maddock 31 Jul '16

31 Jul '16

This trivial PR is open for type_erasure and fixes an issue introduced by changes to Boost.Config (enabling constexpr support for VC14up3). https://github.com/boostorg/type_erasure/pull/11 BTW the failure is not caused by buggy constexpr support, there's some other issue going on once that code path is enabled. Anyhow, since we really should be merging Config to master soon, I'd like to see this collateral-damage fixed if possible. Thanks, John.

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[contract] Template types
by Lorenzo Caminiti 29 Jul '16

29 Jul '16

Hello all, I would like to discuss the extra type requirements that contracts can impose of generic types and how Boost.Contract handles that. I think I mentioned some of this already in different emails to this list, but I wanted to summarize this topic in one email. For example, note that the following code will not compile when the old value template parameter T is not copyable: template<typename T> void swap(T& a, T& b) [[ensures: a == old(b)]] ... In Boost.Contract (approach 1): template<typename T> void swap(T& a, T& b) { boost::contract::old_ptr<T> old_b = BOOST_CONTRACT_OLDOF(b); boost::contract::guard c = boost::contract::function() .postcondition([&] { BOOST_CONTRACT_ASSERT(a == *old_b); }) ; ... } In some cases, that might be what the contract writers want: the generic old value type T is not copyable so postconditions cannot be checked and the program should not compile. However, in some other cases the contract writes might not want to add the extra copyable requirement on T and check the related postconditions only when callers specify a template parameter T that happens to be copyable. That can be done allowing some sort of static-if in contracts: template<typename T> void swap(T& a, T& b) [[ensures: static if(is_copyable<T>::value) a == old(b)]] ... In Boost.Contract (approach 2): template<typename T> void swap(T& a, T& b) { boost::contract::old_ptr_if_copyable<T> old_b = BOOST_CONTRACT_OLDOF(b); boost::contract::guard c = boost::contract::function() .postcondition([&] { if(old_b) BOOST_CONTRACT_ASSERT(a == *old_b); }) ; ... } Both approach 1 and 2 above are valid and should be supported by the framework. The decision between which approach to use for a specific contract should be left up to the programmers. Like old values can introduce the extra copyable type requirement, any contract assertion can in general introduce an number of extra requirements on generic types, namely the type requirements necessary to support the operations used to program the contract assertions. For example, the following code will not compile if T is not equality comparable (which is not a type requirement vector normally imposes on T): template<typename T> class vector { void push_back(T const& value) [[ensures: back() == value]] ... }; In Boost.Contract (approach 1): template<typename T> class vector { void push_back(T const& value) { boost::contract::guard c = boost::contract::public_function(this) .postcondition([&] { BOOST_CONTRACT_ASSERT(back() == value); }) ; ... } }; Again, that might be what contract writes want in some case but in other cases they might want to just skip the assertion when T does not meet all the type requirements needed by the operations used in the assertion: template<typename T> class vector { void push_back(T const& value) [[ensures: static if(has_equal_to<T>::value) back() == value]] ... }; In Boost.Contract (approach 2): template<typename T> class vector { void push_back(T const& value) { boost::contract::guard c = boost::contract::public_function(this) .postcondition([&] { BOOST_CONTRACT_ASSERT( boost::contract::check_if<has_equal_to<T>::value >( boost::bind(equal_to<T>, cref(back()), cref(value)) ) ); }) ; ... } }; --Lorenzo

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Gmane going away
by Tom Kent 29 Jul '16

29 Jul '16

So it looks like Gmane is under attack and the guy who runs it is thinking about just turning it off (that would be sad). https://lars.ingebrigtsen.no/2016/07/28/the-end-of-gmane/ Anyway, I know a lot of boost users use is as the gnu mailman archive pages are really hard to use. I would like to propose that we (specifically the list owner(s)) mirror the boost mailing lists to the mail-archive.com site. It has good search features and decent thread browse-ability. Thoughts? Tom

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Boost Social Event at CppCon 2016
by Jon Kalb 28 Jul '16

28 Jul '16

Boost Developers, Because a lot of Boosters are going to be attending CppCon, we¹d like to arrange a social event for Boosters/C++Now attendees. http://cppcon.org/ If you are planning to attend CppCon or even If you are just going to be in or around Bellevue, Washington between September 17th and 23rd and you¹d like to join us for dinner, please let us know so we can get a count. Please send mail to: social(a)boost.org Once we settle on a date, etc, we¹ll let everyone who responded know when and where, so please let us know, even if you are a ³maybe.² If you are attending, please make your hotel reservations soon. We are likely to sell out of our room block. http://cppcon.org/lodging/ I hope to see you at CppCon. Jon

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Approximate matching to Boost.Algorithm
by Зайцев Александр 27 Jul '16

27 Jul '16

Hello. I want to discuss about some proposals. I am working on Levenshtein distance and Damerau-Levenshtein distance algortihms. I hope, that these implementations will be pulled to the Boost String library. But i think that we can add to the Boost String Library some more algorithms for fuzzy searhing. You can find this list here: https://fuzzystring.codeplex.com/ List: 1)Hamming Distance 2)Jaccard Distance 3)Jaro Distance 4)Jaro-Winkler Distance 5)Levenshtein Distance 6)Longest Common Subsequence 7)Longest Common Substring 8)Overlap Coefficient 9)Ratcliff-Obershelp Similarity 10)Sorensen-Dice Distance 11)Tanimoto Coefficient Maybe something else... What do you think about this idea? If you think that it can be implemented to the Boost String Algo, i will start work on this list immediately. Best regards, Zaitsev Alexander.

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I am working on Aho-Corasik
by Зайцев Александр 27 Jul '16

27 Jul '16

Hello. FYI: Now i am working on Aho-Corasik algorithm. Best regards, Alexander Zaitsev.

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Fast nth-elemnt on the segment
by Зайцев Александр 26 Jul '16

26 Jul '16

Hello. What do you think about algorithm of fast searching nth-elemnt on the segment with O(logN), where N is count of elements. I would implement it to Boost.Algorithm. Best regards, Alexander Zaitsev

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[type_erasure] Conflict with private member called data
by Nigel Pattinson 22 Jul '16

22 Jul '16

Hi I'm running into a problem with the type erasure library when trying to declare a member called 'data', e.g. BOOST_TYPE_ERASURE_MEMBER((has_data), data) using SomeClass = any< vector< copy_constructible<>, typeid_<>, relaxed, has_data<void*()> > >; This conflicts with the private member variable called 'data' in the any class. SomeClass does compile, but attempting to call the data() does not compile due to the conflict, with the error message being 'cannot access private member ... ' . I'm not sure if this is a limitation in the library or a compiler bug - I'm using Visual Studio 2015 Update 1. Using boost 1.57.0 at present, I could use a more recent version if that would help. Does anyone know of a possible workaround ? Thanks Nigel Pattinson

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