August 2016 - Boost - lists.boost.org

[contract] noexcept and throwing handlers
by Lorenzo Caminiti 10 Aug '16

10 Aug '16

Hello all, I would like to discuss contracts and exception specifications (e.g., noexcept) specifically with respect to what Boost.Contract does. On one hand, it seems silly to allow a contract failure handler to throw an exception to only translate such an exception into a call to terminate() for noexcept functions. However, noexcept can be considered part of the function contract (namely, the contract that says the function shall not throw) and contract failure handlers should not be allowed to break such contract, not even when they are allowed to throw. For example consider a noexcept function fclose() that is called from a destructor without a try-catch statement (correctly so because fclose is declared noexcept). If fclose() is now allowed to throw when its preconditions fail, that will cause the destructor ~x() to throw as well?! void fclose(file& f) noexcept [[requires: f.is_open()]] ... class x { ~x() noexcept { fclose(f_); // f_ might not be open because of some bug... ... } file f_; }; (A point similar to the above is also made in N4160, in contrast with what argued in N4110.) Functions should honor their exception specifications (noexcept, etc.) even when their contracts fail and contract failure handlers are configured to throw. That is what Boost.Contract does (given that contract checking code appears within the function definition so it is subject to the exception specifications that appear in the enclosing function declaration): void fclose(file& f) noexcept { // This will call terminate()... boost::contract::guard c = boost::contract::function() .precondition([&] { BOOST_CONTRACT_ASSERT(f.is_open()); // ...even if this throws. } ; ... } int main() { boost::contract::set_precondition_failure( [] (boost::contract::from where) { // Re-throw exceptions (assertion_failure, user-defined, etc.). if(where != boost::contract::from_destructor) throw; std::terminate(); // But destructors never throw. } ); ... } Also note that a contract framework that allows to install failure handlers that throw exceptions should also pass some sort of parameter to the handler function to indicate if the contract failure happened in a destructor or not. That is so the failure handlers can choose to never throw for destructors, not even if class invariants (or postconditions) fails when checked by destructors. --Lorenzo

3 4

[release] Continuous release archives.
by Rene Rivera 09 Aug '16

09 Aug '16

Starting with this release the archives for the prospective beta and release candidates are continuously built from the master branch and uploaded to SourceForge [1] (and to Bintray [2]). The release team would really appreciate if authors would download the archives and check the contents. In particular that the archives contain all the content you expect of your library, or libraries. And if you can also try building in your system that would stupendous. The more eyes we have on the final release products the smoother the process will be. And result in more time that the master branch is not blocked. Please report any problems sooner rather than later. So that we can try and address them before it's took late. Thank you, The Release Team [1] <https://sourceforge.net/projects/boost/files/boost/snapshots/master/> [2] <https://bintray.com/boostorg/master/snapshot#files> -- -- Rene Rivera -- Grafik - Don't Assume Anything -- Robot Dreams - http://robot-dreams.net -- rrivera/acm.org (msn) - grafikrobot/aim,yahoo,skype,efnet,gmail

5 7

Feature suggestions for Boost's program_options
by Eyal Rozenberg 09 Aug '16

09 Aug '16

Greetings Boost developers, [ Disclaimer: I'm new to this list - I joined by Vladimir Prus' suggestion that I raise my feature ideas here on the list. I've read the posting guidelines, and I haven't seen any discussion of program_options (at least not by Vladimir himself) for at least a year back in the list archives, so I think my post is legit. ] I've been using Boost's program_options library for several months now. It mostly does the job (that's a compliment...) - but there are some features whose lack is really bugging me. For some of them (especially Feature Idea 1) I actually work around by taking some options as unparsed remainders, sticking them in a string->string map and parsing them myself. Without further ado, and in decreasing order of importance to me: -------------------------------- Feature Idea 1: Support specifying multiple alternative long option names, using comma-separation. Right now we can specify, for example, "include-path,I" - one short form, one long form. Why not allow specifying "include-path,include-dir,headers-path,I" ? Only the first long option name would be printed as part of the usage information, to not complicate things overmuch, and merely tolerate alternative phrasing. Feature Idea 2: Have a (new?) parser which supports taking "semi-parsed command-line options" as input: A map from string keys to string values, or to optional<string> values to represent options without an argument as well. This is actually very close to environment parsing, except that the environment parser gets its "map" from, well, the environment variables. So the implementation might be simply exposing the existing functionality with a slightly richer interface. Feature Idea 3: A facility for pretty-printing parsed options to an std::ostream, in which the option description starts the line, and the option value (not the name used for parsing) is printed after it. There is some pretty-printing for the usage info to the console, so why not this? (Argument against this idea: Choosing a default is a very touchy subject. Counter-argument: ... but that's also somewhat true of printing usage information; and nobody has to use this default, it would be entirely optional.) Feature Idea 4: Support three rather than just two levels of describing options: name, for the command line; short description, for listing values (see Feature Idea 3; long description for the usage info. Feature Idea 5: Create a variant of update_with which doesn't just set the option value, but creates a link of sorts between the variable which the app would use and the variable_map entry. With this in place, further changes to the variable map automagically affect the app variable. -------------------------------- So, - What do you think about these suggestions, in principle? - If you find some of them relevant, can you help estimate the necessary development effort and possible negative implications/caveats to consider? - Assuming a feature idea is both relevant and feasible, is it customary to request implementation by the maintainer? Commit yourself to implementing it vis-a-vis statement of statement of partiality to accepting a patch? Some other practice? Thank you for getting through the long(ish) email and for your hard work, Eyal Rozenberg DB architecture group CWI Amsterdam

3 4

[release] Boost 1.62 closing for new libraries this week
by Rene Rivera 07 Aug '16

07 Aug '16

Hopefully everyone has had a good Summer and has gotten all the new libraries and breaking changes in.. As we are closing the release window for that this week. Please consult the calendar [1] for the full release schedule. And take this as a reminder to merge your changes that you want in the next release as soon as possible. [1] <http://www.boost.org/development/index.html> -- -- Rene Rivera -- Grafik - Don't Assume Anything -- Robot Dreams - http://robot-dreams.net -- rrivera/acm.org (msn) - grafikrobot/aim,yahoo,skype,efnet,gmail

1 0

[boost-process] Beta & Review
by Klemens Morgenstern 07 Aug '16

07 Aug '16

Hi, boost-process is now in beta, you can check it out here: https://github.com/klemens-morgenstern/boost-process and read the doc here: http://klemens-morgenstern.github.io/process/ It will probably be in review in the beginning of November, Antony Polukhin will be the review manager. I really hope I'll get some feedback before that, so I can work on the weak points and also get more detailed in the documentation. Thanks, Klemens

4 10

[boost-spirit-qi] Support for n-ary operators PR
by Frank Hein 06 Aug '16

06 Aug '16

Hello, some days ago I raised an PR (https://github.com/boostorg/spirit/pull/200) which adds support for a new class of parsers, in particular named n-ary operators. Joel de Guzman reviewed that. The code is ok. Because this addition touches the inner core of qi, the metagrammar, Joel asked me to drum for support in the community. So if you are in qi, may I kindly ask you to review this PR, the code, the discussion so far, and respond, if you want to support this PR or if you think, it's not needed. Thank you very much. I highly appreciate your feedback. The PR description is very detailled, so this mail has not to be. :) Regards, Frank

1 0

Review Schedule and Review Manager for PODs Flat Reflection (ex magic_get)
by Antony Polukhin 05 Aug '16

05 Aug '16

Hello, PODs Flat Reflection (or PFR) is a C++14 library that represents POD structures as tuples and provides tuple-like methods for PODs. Library has a set of predefined operators for PODs and useful functions for everyday use. Works with user-defined types without any macro or boilerplate code. Library: https://github.com/apolukhin/magic_get Docs: http://apolukhin.github.io/magic_get/ Boost Library Incubator: http://blincubator.com/bi_library/pfr-pod-flat-reflection/?gform_post_id=16… Could the PODs Flat Reflection library be put on review schedule? Anyone wishing to become a Review Manager for the library? -- Best regards, Antony Polukhin

6 9

[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

8 31

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

5 6