August 2020 - Boost-users - lists.preview.boost.org

[Lockfree] spsc example query
by Sean Farrow 25 Aug '20

25 Aug '20

Hi, I'm just about to start using the spsc queue in a project and whilst reading the example at: https://www.boost.org/doc/libs/1_74_0/doc/html/lockfree/examples.html#lockf… looking at the consumer function, there appears to be two while loops retrieving items from the queue inside the outer loop that checks whether the done variable is true. Can somebody please explain why this is, or, is it a bug in the example? I would have thought one inner while loop would be enough. Thanks, Sean.

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JSON library scheduled for review
by Mateusz Loskot 25 Aug '20

25 Aug '20

Hi, A brief announcement from the review wizards: Formal review of the JSON library has been scheduled to take place between October 9 and October 18, 2020. Details and links available here https://www.boost.org/community/review_schedule.html We'd like to thank Pranam Lashkari (Boost.GIL, [Boost.]Astronomy, Boost GSoC) for volunteering to act as the review manager. Best regards, -- Mateusz Loskot, http://mateusz.loskot.net

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[Log] Formatting multiple attributes in a single formatter
by Michael Jones 24 Aug '20

24 Aug '20

TL;DR; I'm looking for help/advice/code on creating a custom formatter, or formatter factory, to format more than one attribute at a time into a single "placeholder" in the format expression. I do not yet have code, working or otherwise, to try to solve this, because I am still trying to understand the design space of Boost.Log's API. I have been using Boost.Log successfully in my project for about 1-2 years now, with many hundreds of log statements of the style MY_LOG_MACRO(Normal) << "My Log Message" << std::endl << "- Attribute 1:" << myAttributeVar1 << std::endl << "- Attribute 2:" << myAttributeVar2; I now want to convert these hand-formatted attributes that are directly inserted into the message into attributes in the Boost.Log meaning. However, these attributes are "free form", in the sense that they are not pre-registered with Boost.Log before use (which is obvious given they are manually formatted into the message string), and they do not conform to any specific naming conventions or value-type scheme. I estimate that there are a few hundred different attributes that are printed to my logs in various places as "one off" attributes that are not really meaningful outside of the specific log statement they are used in. My current plan is to provide my developers with a way to incrementally move their logging toward using Boost.Log's attributes without requiring a flag-day where every log statement is changed in a single merge. I want to do this by allowing these attributes to be added to the log message using boost::log::add_value() from boost/log/utility/manipulators/add_value.hpp, such as: MY_LOG_MACRO(Normal) << "My Log Message" << boost::log::add_value("Attribute 1", myAttributeVar1) << boost::log::add_value("Attribute 2", myAttributeVar2); However, I want to provide the same formatting semantics that they are used to, to make the transition easier. At this stage, it is not clear whether I'll ever be able to get a set of standardized attributes that can be pre-registered with the Boost.Log library. A surface level estimate shows me that I have, at the minimum, many dozens of different attributes with unique names / value-types. This would make addressing each such attribute name individually prohibitively expensive to add to the format expression Currently, using boost::log::import_from_stream(), I use a format expression like: Format="%Process%/%Channel% - [%Severity%] %Message%" And I would like to change that to the following, or something similar: Format="%Process%/%Channel% - [%Severity%] %Message%\n%Attributes%" The intention of %Attributes% here would be a placeholder in the format expression that gets resolved to a custom formatter, or formatter factory, which will iterate through the list of attributes in the current record_view and print each attribute name and value with a new line separator. Ideally, this formatter or format factory would be able to recognize that a particular attribute has already been printed, or will be printed, so that it only adds new information to the log. However, I believe this is not something that can be easily done, so I currently plan to have this duplicated information in the log with the intention to resolve it another time. One way I could resolve the duplicate information problem is by adding an option block to the %Attribute% placeholder, e.g. %Attribute[Exclude:Process, Exclude:Channel, Exclude:Severity, Exclude:Message]%. It would be ugly, but it would do the job. So far I've gathered the following information about this subject: How the default formatter factory works by creating a default_formatter, which has special handling for various time types, but otherwise just calls operator<<(stream_type, value); https://github.com/boostorg/log/blob/develop/src/setup/default_formatter_fa… How the format parser works by, well, parsing the format expression to get attribute names and other important information for later use. I don't have a complete understanding of this, but I get the gist of it. https://github.com/boostorg/log/blob/develop/src/format_parser.cpp How sinks_repository from init_from_settings.cpp works by pre-defining multiple different sink-factories with the repository, and then init_from_settings() uses those pre-defined factories to support loading the settings. Notably, I use default_text_file_sink_factory, and default_console_sink_factory in my own code. Both of those sink factories just use boost::log::parse_formatter() to create the formatter from the format string in the settings file. https://github.com/boostorg/log/blob/develop/src/setup/init_from_settings.c… How formatter_repository and works by storing a map between the Attribute name and the Formatter Factory to use. https://github.com/boostorg/log/blob/develop/src/setup/formatter_parser.cpp Record_view, it's read-only nature, and how to access the attributes for the record using the boost::log::record_view::attribute_values() function. https://github.com/boostorg/log/blob/develop/include/boost/log/core/record_… The next thing I am going to try is to define a custom formatter for the %Attributes% attribute name, and pre-define some constant value for it so that it is always defined for each record. Then I'll have a formatter factory like this (psuedo code, not compile tested) template< typename char_type > struct abitrary_attribute_formatter { void operator() ( boost::log::record_view const& rec, boost::log::basic_formatting_ostream< char_type >& strm ) const { for(auto const& attr : rec.attribute_values()) { strm << "- " << attr.name() << ": " << attr.value(); } } }; What I'm currently trying to understand is how I can create a custom formatter that dispatches to other formatters. How is this done? Is there a more straight-forward way to accomplish the goal of providing a formatter for the concept of "All available attributes"? If not, can anyone offer implementation or design advice?

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[Boost.Accumulator] retrieve percentage corresponding to a value
by Timo Sachsenberg 24 Aug '20

24 Aug '20

Hi, Boost offers convenient methods in the accumulator library to get e.g. a value (let's say 1234) corresponding to a certain quantile (let's say 0.995). For my specific use-case I wonder if the opposite (value input=1234, output=0.995) is also possible. Is there an elegant way to calculate/estimate the percentile of value in an online fashion (=while accumulating) and without storing all values? One thing I came up with is using extended_p_square_quantile with many probabilities and then do a binary search over the values output by the quantile() function. Is there an easier way to directly get the percentile from the value that I am missing? Best, Timo

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limited accuracy for tanh-sinh integrator
by Anirban Pal 23 Aug '20

23 Aug '20

Hello, I’m using the tanh-sinh integrator to integrate a simple function f(x) = 0.26*x from 3.0 to 4.0. The exact result is 0.91. With the integrator I’m getting a result accurate to only 10^-18 with cpp_bin_float_100 multiprecision. typedef boost::multiprecision::cpp_bin_float_100 Real; int main(int argc, char **argv) { using namespace boost::math::quadrature; using namespace std::placeholders; tanh_sinh<Real> integrator; auto f2 = [](Real x) { Real a = 0.26; return x*a; }; std::cout << std::setprecision(std::numeric_limits<Real>::max_digits10) << "Comp.: " << integrator.integrate(f2, (Real) 3.0, (Real) 4.0) << std::endl; return 0; } I'm getting an output of Comp.: 0.9100000000000000310862446895043831318616867065429687499999999999999999999999999999999999999999999999429 which I'm a bit curious about. I was wondering if I can improve the result. Thank you. -- Anirban Pal

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[Beast] streaming a large compressed file
by Sean Farrow 22 Aug '20

22 Aug '20

Hi, I'm trying to download a compressed .7z file with boost.beast. The file is about 9GB in size, so what I'd like to do is stream the response in to boost.iostreams to allow decompression. Is there any examples of interfacing boost.beast with boost.iostreams in this sort of streaming context? Any help appreciated. Thanks, Sean.

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Integrate function with arguments using quadrature
by Anirban Pal 21 Aug '20

21 Aug '20

Hello, I'm trying to integrate functions with BOOST quadrature routines. So far they have been extremely impressive accuracy-wise, particularly with multiprecision features. I wish to integrate a function and pass some arguments to it. These arguments can be scalars, matrices, structs or objects. I was wondering if there is an example that explores this. Would I need to use a boost.function? Or would a function pointer work? I currently have an implementation using GSL_functions which explicitly allow the function and parameters to be passed as pointers. I am curious if I can do something similar with boost. Thank you. -- Anirban Pal

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PFR (Precise and Flat Reflection) by Antony Polukhin scheduled for review
by Mateusz Loskot 18 Aug '20

18 Aug '20

Hi, A brief announcement from the review wizards: Formal review of Antony's library has been scheduled to take place between September 28 and October 7, 2020. Details and links available here https://www.boost.org/community/review_schedule.html Best regards, -- Mateusz Loskot, http://mateusz.loskot.net

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help with serialization error
by Lorenzo L 14 Aug '20

14 Aug '20

I'm trying to compile a piece of software called Bagel (a quantum chemistry program) written in C++ and which uses Boost. I get a compilation error, and as I don't know C++ and I've never used boost I'm essentially lost. The author of the program says the problem I'm experiences depends on my boost installation and not on the program itself. I installed boost 1.73.0 (Linux Mint 20, g++ 9.3.0) doing: sudo ./bootstrap.sh --prefix=/usr/local sudo ./b2 install When compiling the program I get the following error: /usr/local/include/boost/serialization/access.hpp:119:11: error: 'class std::unordered_set' has no member named 'serialize' 119 | t.serialize(ar, file_version); | ~~^~~~~~~~~ /usr/local/include/boost/serialization/access.hpp: In instantiation of 'static void boost::serialization::access::serialize(Archive&, T&, unsigned int) [with Archive = boost::archive::binary_oarchive; T = std::unordered_set]': /usr/local/include/boost/serialization/serialization.hpp:69:22: required from 'void boost::serialization::serialize(Archive&, T&, unsigned int) [with Archive = boost::archive::binary_oarchive; T = std::unordered_set]' /usr/local/include/boost/serialization/serialization.hpp:128:18: required from 'void boost::serialization::serialize_adl(Archive&, T&, unsigned int) [with Archive = boost::archive::binary_oarchive; T = std::unordered_set]' /usr/local/include/boost/archive/detail/oserializer.hpp:148:40: required from 'void boost::archive::detail::oserializer::save_object_data(boost::archive::detail::basic_oarchive&, const void*) const [with Archive = boost::archive::binary_oarchive; T = std::unordered_set]' /usr/local/include/boost/archive/detail/oserializer.hpp:101:1: required from 'class boost::archive::detail::oserializer' /usr/local/include/boost/archive/detail/oserializer.hpp:253:13: required from 'static void boost::archive::detail::save_non_pointer_type::save_standard::invoke(Archive&, const T&) [with T = std::unordered_set; Archive = boost::archive::binary_oarchive]' /usr/local/include/boost/archive/detail/oserializer.hpp:308:22: [ skipping 21 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ] ... The following post https://lists.boost.org/boost-users//2014/03/81583.php [1] report on a similar error, but I couldn't understand what I should to solve it. Any advice would be appreciated! Con Tiscali Mobile Smart 30 4G hai minuti illimitati, 100 SMS e 30 Giga in 4G a soli 8,99€ al mese. http://tisca.li/smart30

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[release] Boost 1.74.0
by Marshall Clow 14 Aug '20

14 Aug '20

Release 1.74.0 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 and numerous enhancements and bug fixes for existing libraries. For details, including download links, see < http://www.boost.org/users/news/version_1.74.0> You can also download directly from: < https://dl.bintray.com/boostorg/release/1.74.0/> To install this release on your system, see < http://www.boost.org/doc/libs/release/more/getting_started/index.html> Thanks to everyone who participated in this release. Thanks, -- The Boost Release Team Marshall Clow, Michael Caisse, Glen Fernandes

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