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[histogram][type_traits] strange compile error in boost.histogram for gcc-7.1+ with -std=c++17; possible bug in gcc?
by Hans Dembinski 30 Oct '20

30 Oct '20

Dear John and the other gurus, a strange bug was reported for Boost.Histogram. It is a difficult one for which I would like to ask for help. To me it looks like a bug in gcc. Here is the issue: https://github.com/boostorg/histogram/issues/290 And here is a link to reproduce it: https://godbolt.org/z/oK4866 The following innocent code compiled with gcc-7.1 or newer with -std=c++17 ``` #include <boost/type_traits/rank.hpp> #include <boost/histogram/axis/traits.hpp> int main() { return 0; } ``` produces this error message (excerpt): ``` cc1plus: error: return type specified for deduction guide In file included from <source>:2: /celibs/boost_1_74_0/boost/histogram/axis/traits.hpp:411:11: error: 'decl-specifier' in declaration of deduction guide 411 | constexpr unsigned rank(const Axis& axis) { | ^~~~~~~~ /celibs/boost_1_74_0/boost/histogram/axis/traits.hpp:411:1: error: 'decl-specifier' in declaration of deduction guide 411 | constexpr unsigned rank(const Axis& axis) { | ^~~~~~~~~ /celibs/boost_1_74_0/boost/histogram/axis/traits.hpp:411:11: error: 'unsigned' specified with 'rank<...auto...>' 411 | constexpr unsigned rank(const Axis& axis) { | ^~~~~~~~ /celibs/boost_1_74_0/boost/histogram/axis/traits.hpp:411:20: error: deduction guide for 'boost::rank<T>' must have trailing return type 411 | constexpr unsigned rank(const Axis& axis) { | ^~~~ In file included from <source>:1: /celibs/boost_1_74_0/boost/type_traits/rank.hpp:82:27: note: 'template<class T> struct boost::rank' declared here 82 | template <class T> struct rank : public integral_constant<std::size_t, (::boost::detail::rank_imp<T, 0>::value)>{}; | ^~~~ ``` gcc interprets my function called `rank` as a deduction guide for `boost::rank` in boost/type_traits/rank.hpp. It then continues to complain that this function does not look like a deduction guide. Well, it isn't one, duh. I can see nothing wrong with my code or the code in boost/type_traits, so my guess is a bug in gcc. If you all agree, I would report this. If this is really a bug in gcc, however, it exists since v7.1 all the way to gcc trunk. In either case, it would be great if we could use a workaround in Boost until that problem is fixed. I found a workaround is to replace line 82 in boost/type_traits/rank.hpp with: ``` template <class T> using rank = integral_constant<std::size_t, (::boost::detail::rank_imp<T, 0>::value)>; ``` This avoids the issue because now the struct ``rank`` is gone and therefore gcc does not try to find a deduction guide for it. Is there a better solution? Best regards, Hans

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[gil] Phasing out support C++11 and switching to C++17
by Mateusz Loskot 27 Oct '20

27 Oct '20

Hi, We're discussing [1] to start switching GIL to C++17 in the near future and, following Peter Dimov's policy on phasing out C++XY support [2], we'd like to: 1. Declare C++11 support deprecated in 1.75 (December 2020) 2. Drop C++11 support in 1.77 (?) (August 2021) Could anyone help us validate and confirm that it's a good plan or are we overlooking any policies from Boost's release perspective? [1] https://lists.boost.org/boost-gil/2020/10/0465.php [2] https://pdimov.github.io/articles/phasing_out_cxx03.html Best regards, -- Mateusz Loskot, http://mateusz.loskot.net

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[pfr] Ready for informal mini review
by Antony Polukhin 20 Oct '20

20 Oct '20

Hi, Issues noted during the review period were fixed. Mainly: * global_ops.hpp is removed * boost::pfr::ops::operator* were changed to functions and moved to boost::pfr:: namespace * Flat reflection was purged * Big rewrite of docs I'd appreciate a look at the updated library https://github.com/boostorg/pfr and its docs https://apolukhin.github.io/magic_get/index.html Any comments and feedback is welcomed! -- Best regards, Antony Polukhin

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Re: [boost] [pfr] Ready for informal mini review
by Glen Fernandes 20 Oct '20

20 Oct '20

Antony Polukhin wrote: > I'd appreciate a look at the updated library > > https://github.com/boostorg/pfr and its docs > https://apolukhin.github.io/magic_get/index.html > > Any comments and feedback is welcomed! Note that the last day for new libraries for Boost 1.75 is tomorrow (2020-10-21). PFR has a develop submodule, but no master submodule yet. I was waiting until you believe it is ready, and have ironed out all the issues on develop. If you want it included in the 1.75 release, and there are no objections from any reviewers, I can add the master submodule today. (It was already added to the in-progress release notes for 1.75, but that can be removed if you don't want to release it until 1.76). Glen

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[FPR] Review of FPR starts Today: Sept 28 - Oct 7
by Benedek Thaler 16 Oct '20

16 Oct '20

The Boost formal review of the FPR (Flat Precise Reflection, ex Magic Get, ex PODs Flat Reflection) starts Today, taking place from September 28, 2020 to October 7, 2020. The library is authored by Antony Polukhin, author of Boost.DLL, Stacktrace, Type Index libraries. Documentation: http://apolukhin.github.io/magic_get/ Source: https://github.com/apolukhin/magic_get The library is meant for accessing structure elements by index and providing other std::tuple like methods for user defined types without any macro or boilerplate code. The FPR documentation summarizes the out-of-the-box added functionality for aggregate initializable structures: - comparison operators - heterogeneous comparators - hash - stream operators - access to members by index - member reflections - methods for cooperation with std::tuple - methods to visit each field of the structure If the description isn't immediately obvious for you, here's a motivating example: // requires: C++14 #include <iostream> #include <string> #include "boost/pfr/precise.hpp" struct some_person { std::string name; unsigned birth_year; }; int main() { some_person val{"Edgar Allan Poe", 1809}; std::cout << boost::pfr::get<0>(val) // No macro! << " was born in " << boost::pfr::get<1>(val); // Works with any aggregate initializables! } Please provide in your review information you think is valuable to understand your choice to ACCEPT or REJECT including FPR as a Boost library. Please be explicit about your decision (ACCEPT or REJECT). Some other questions you might want to consider answering: - What is your evaluation of the design? - What is your evaluation of the implementation? - What is your evaluation of the documentation? - What is your evaluation of the potential usefulness of the library? - Did you try to use the library? With which compiler(s)? Did you have any problems? - How much effort did you put into your evaluation? A glance? A quick reading? In-depth study? - Are you knowledgeable about the problem domain? More information about the Boost Formal Review Process can be found at: http://www.boost.org/community/reviews.html Thank you for your effort in the Boost community. Benedek - review manager of the FPR library

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Latest boost documentation
by Raghu Huchchannavar 15 Oct '20

15 Oct '20

Hi, Can anyone help me in getting the latest boost documentation either a link or the pdf one. -- Regards Raghavendra Huchchannavar.

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boost::asio::deadline_timer segmentation in version 1.71
by Jupiter 14 Oct '20

14 Oct '20

Hi, My application is running one thread under boost::asio::io_service, it is an embedded system running on an ARM processor with Linux, so no gdb installed. The application is also using a boost::asio::deadline_timer to call a function every second, but I got following segmentation stack trace: _ZN5boost4asio6detail9scheduler10do_run_oneERNS1_27conditionally_enabled_mutex11scoped_lockERNS1_21scheduler_thread_infoERKNS_6system10error_codeE+0xdd _ZN5boost4asio6detail9scheduler3runERNS_6system10error_codeE+0x83 I thought I could run c++filt to demangling the library name, not sure why it didn't demangle the name today. I used the same boost::asio::deadline_timer version 1.69 for another application running on Ubuntu which was working fine, what I could be missing? Thank you. Kind regards, Jupiter

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[PFR][review] Result
by Benedek Thaler 10 Oct '20

10 Oct '20

Thanks to all of you who participated in the formal review of PFR, henceforth known as Boost.PFR, and Antony Polukhin for submitting this magical library. PFR is ACCEPTed into Boost, with the following results: ACCEPT: 5 votes CONDITIONAL ACCEPT: 1 vote REJECT: 0 votes Reviewers highlighted that the library is "very useful", "magical" and "provides a form of reflection in a simpler manner than other solutions thus far". During the review, several points of improvements were identified, and Antony committed to improve the library accordingly. The main concerns were: - Definition of the type concept the library can work with - Clarification of the flat / precise use-cases - The presence of global operators - The way output formatting is defined - Dropping "Reflection" from the name (this is already reflected in the docs) Antony plans to: - Remove Flat reflection - Remove global_ops - Add more examples to the docs, introduce terms in a cleaner manner - Improve the reference section - Take another look at the pfr::ops and macro for operators After all these changes have been implemented, Antony would really like to have a mini-review, regardless that the library is already ACCEPTed, further confirming his high standards. Good luck Antony, keep up the good work! Benedek review wizard

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[PFR][review] Review of PFR ends Today
by Benedek Thaler 07 Oct '20

07 Oct '20

The review of the PFR: Precise and Flat tuple Representation (ex Precise and Flat Reflection, ex Magic Get, ex PODs Flat Reflection) ends Today. Do not hesitate to send late reviews, there's still time. Documentation: http://apolukhin.github.io/magic_get/ Source: https://github.com/apolukhin/magic_get I'll announce the review results soon. Benedek review manager

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[PFR][review] Andrzej's review
by Andrzej Krzemienski 07 Oct '20

07 Oct '20

Hi Everyone, This is my partial review of PFR library. I have read the docs, watched Antony's talk, played with some toy examples, and had a brief look at the implementation of pfr::tuple_size. I spent like 14 hours in total on this. I have studied the subject of tuple-like access to class objects a while back, which resulted in this proposal: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3326.pdf. I like the library, I think it has the potential to address a small but a well understood demand. While I would like to see it included in Boost, I believe it should not be accepted just yet, in its current shape. I disagree with parts of the design (injecting the interface of user types) as explained below. Missing documentation parts (I mean the concept "Baseless Aggregate") is also an issue for me. First, it might be an indication of a flaw in the design. Second, it might cover some bugs in the implementation. Third, making another review with updated docs might attract more reviews and may make people appreciate the library more. Alternatively, I could have called it a conditional acceptance with everything listed below as conditions. Since reviewers are not casting votes but give recommendations instead, the Review Manager may want to take this into account. I think that this type of the library is mostly about the documentation and design (like, the scope). The implementation will necessarily be a number of hacks to provide the magic. Since I question parts of the design, I didn't find it necessary to review the implementation in detail. The following list may appear long, and look like a long list of complaints, so I need to make this clear up front. I really like the library, and I intend to use it next time I have a need for the tuple-like access to PODs, even download it from GitHub. I recommend the rejection because I see the second review as an opportunity for the library to become even better. DOCUMENTATION Documentation does explain how to use the library, and after a while it becomes easy to use every function, at least the functions that I tried. But it is missing some important parts, therefore it confuses the potential users as to what its scope is, and gives an (false) impression that it is defective. (Some of the below points may not hold anymore, as I have noticed the documentation has changed during the review period.) 1. The library name is misleading. It uses the name "reflection" but it doesn't really implement reflection. A reflection would be a way of querying the system for different properties of declarations in the program. This is not the case here. 2. The intro page misses the opportunity to present the scope of the library, its power, and the minimal example of how to use it and how it makes the life of the programmer easier. It is a very good library, and it would be a great loss if it was overlooked due to not being advertised effectively. I volunteer to provide the content of the initial page that I believe would meet the criteria. 3. The library buds on a basic concept of "Base-less aggregate". It must document it up front. And if possible try to statically detect if the types satisfy it to the extent that this is doable. After reading the docs, the user has to have a clear picture which types will and which won't work with the library. 4. It should contain a couple of motivating use cases, both for flat and precise representation. It is not enough to show how they work. The user must also see why they would be useful for anyone. Some users immediately see that this library addresses their use, others are confused, and cannot see any purpose in having it. 5. One use case for flat reflection is when a couple of aggregates have a common subset: ``` struct Person { std::string firstName; std::string lastName; int age; }; struct Employee { Person person; double salary; }; struct Prisoner { Person person int cellNumber; }; std::set<Person, boost::pfr::flat_less<>>; std::set<Employee , boost::pfr::flat_less<>>; std::set<Prisoner , boost::pfr::flat_less<>>; ``` Just saying, "use flat variants when you want to flatten your aggregate" is not enough to make the user see the capabilities of this library. I think that this use case should be documented. 6. I believe that individual operations are underspecified. For instance: i. pfr::write (http://apolukhin.github.io/magic_get/boost/pfr/write.html) has a section "Description", but it doesn't say what the function is doing. To some extent, we can guess that from the signature but some important questions remain unanswered: What format of the serialized output does the library guarantee? Does it guarantee any format at all? Do you leave yourself a freedom to change it in the future without notice? Can users customize it? If so, where is it specified? (This would be a no-problem if the library didn't provide stream insertion operations.) ii. The library says it can inject relational operators (and others) but it doesn't specify what their semantics are. 7. The means of specifying the semantics of the functions could be improved (i.e., made shorter and more precise) if you specified prf::structure_to_tuple (and pfr::flat_structure_to_tuple) very precisely as std::tie() on the corresponding aggregate elements, and then define the semantics of every other function in terms of prf::structure_to_tuple. Instead, I can see that prf::structure_to_tuple is underspecified ( http://apolukhin.github.io/magic_get/boost/pfr/structure_to_tuple.html) Section "Description" does not describe what the function is doing. DESIGN 1. I am missing the concept "Base-less aggregate" that is a basic building block of this library. I would expect it to appear in one of the front pages of the docs, and static checks in the code that would mention this name, so that I am immediately informed if I fail to satisfy the concept. I realize that defining a type trait for this is impossible, but even if it is a line of code deep inside the implementation that breaks (like, decomposing the aggregate into a structure binding), this line should have a comment saying something like "if compilation breaks on this line, it is likely because your type does not satisfy the requirements of a BaselessAggregate." 2. I appreciate functions that this library defines in namespace boost::pfr, such as pfr::less or pfr::for_each_field or pfr::structure_to_tuple; but I do not appreciate that this library injects functions into global namespace or my namespace or even that it provides operators (like <<, which are naturally part of my classes' interface) for my types in namespace boost::pfr. I really consider this harmful, causing subtle ADL-related bugs. If I want my types logged, I would rather expect to be able to type: ``` // in my namespace inline std::ostream& operator<<(std::ostream& o, MyAggregate const& a) { return boost::fpr::write(o, a); } ``` Same with relational operators: rather than library injecting them (and potentially causing ADL problems or ODR violations, especially in C++20), I would rather use a named PFR function where needed: ``` sort(myAggregates.begin(), myAggregates.end(), boost::fpr::less<>{}); std::set<MyAggregate, boost::fpr::less<>> map; ``` This is a bit more verbose, but: i. Still does not require me to list members anywhere (the primary advantage of this library). ii. There is no possibility of getting any ADL- or ODR-related bug. The ODR-violation bugs are really nasty. You do everything fine in your .cpp file and you have no clue why the program desn't work as you expect. And this is because someone else in another .cpp file has put different declarations. And if you have 100.000 cpp files, you may not be able to find the cause for weeks. Offering these operators encourages this type of bugs. 3. If we eliminate the injected operators from the picture, we get a sound interface that can be reused in the future revisions of C++, where the internals will be able to use native C++26 reflection instead of the current hacks. We would get the beautiful effect that the initial Boost libraries provided: an interface that can work uniformly both with old and modern versions of C++. 4. There is another reason why I do not like the stream insertion operators. They just don't belong conceptually to this library. Stream insertion is another problem domain: i. How do you serialize type `char`: as a numeric value, or as a graphical symbol ii. How do you serialize a pointer? As an address it represents? Do you make an exception for type `const char *`? Do you make an exception for type `char32_t const*`? iii. How do you separate the individual values? What character do you use for grouping them? iv. How do you allow users to control what happens for individual types? v. What if the user is nonetheless disappointed with your design choices? These issues go far beyond providing a tuple-like access to aggregates. They should be addressed by a separate library built on top of PFR. 5. Docs say, "provides other std::tuple like methods for user defined types without any macro or boilerplate code."This is not actually the case. In order to get the operators for aggregates -- at least in one mode -- one has to use a macro. 6. I would also say that this is inconvenient that the library, as documented, allows me to inject all the operators or none. I cannot opt into taking only relational operators. At least this is not documented. But I cannot honestly complain about it, because I do not agree with PFR synthesizing any of these operations at all. FInally, I wanted to thank Antony for writing and sharing this library here. I learned a great lot from it. I didn't realize that such reflection was possible in C++17. I hope that my review does not sound discouraging. Regards, &rzej;

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