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[Graph] Attempt to resolve open issue
by Anne van de Graaf 10 Mar '22

10 Mar '22

Hello all, I am trying to get an open issue in Boost.Graph resolved. Since I haven't received any response at the corresponding GitHub page nor from the Boost Users mailing list (where I posted the below message last Monday), I am reaching out to you. My aim is to get in touch with the maintainer(s) of Boost.Graph in order to discuss what is the preferred/most efficient way to fix the issue. Since this is my first post to this mailing list, please accept my apologies if you feel this is not the right way to get in touch with the maintainers. In that case, I would appreciate it very much if you could kindly explain what I should have done instead. Please find below a copy of my message that I sent last Monday to the Boost Users mailing list. Thank you very much for your attention and any replies. Original message: When I remove a vertex from a graph, I occasionally experience a crash (on a system running Red Hat Enterprise Linux 8.5). I have been able to replicate the behavior with the following test program: #include <vector> #include <boost/graph/adjacency_list.hpp> int main() { using MyGraph = boost::adjacency_list<boost::setS, boost::vecS, boost::undirectedS>; using MyVertexDescriptor = boost::graph_traits<MyGraph>::vertex_descriptor; using MyVertexDescriptorVec = std::vector<MyVertexDescriptor>; MyGraph g; MyVertexDescriptorVec vertices{ boost::add_vertex( g ), boost::add_vertex( g ), boost::add_vertex( g ) }; boost::add_edge( vertices[0], vertices[1], g ); boost::add_edge( vertices[1], vertices[2], g ); boost::clear_vertex( vertices[0], g ); boost::remove_vertex( vertices[0], g ); return 0; } Also on a Windows platform, I observe similar behavior. When I build the above code with Microsoft Visual Studio 2019 (MSVC 14.27.29110) and run it with a debugger, a debug assertion is triggered ("cannot increment value-initialized map/set iterator"). In both cases (on Linux and Windows platforms), the debugger reveals that the problem is at line 2071 of boost/graph/detail/adjacency_list.hpp. For clarity, I have copied the involved template function below: template < class EdgeList, class vertex_descriptor > inline void reindex_edge_list( EdgeList& el, vertex_descriptor u, boost::disallow_parallel_edge_tag) { for (typename EdgeList::iterator ei = el.begin(); ei != el.end(); ++ei) // line 2071: increment operator fails { if (ei->get_target() > u) { typename EdgeList::value_type ce = *ei; el.erase(ce); --ce.get_target(); el.insert(ce); } } } While I was searching for a proper place to post my question, I found the following issue at GitHub (which was opened on July 1 of this year): https://github.com/boostorg/graph/issues/268. This appears to be the exact same issue. So last week, I added a comment saying that I am facing the exact same problem. Unfortunately, so far no one has responded yet. To me, it seems that the problem is well-understood. Also, some have proposed a bug fix to resolve the issue (including a pull request). Nevertheless, no progress has been made over the past months. So I was just wondering what is the best way to proceed if we want to fix the issue in Boost itself? Is there anything that I can do (apart from starting this thread) to help in resolving the issue? Any feedback is greatly appreciated. Kind regards, Anne van de Graaf

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Windows Build Issue for Boost Wave
by Tom Kent 09 Mar '22

09 Mar '22

In preparation for the lockdown today, I did a dry run of the windows build based on the latest snapshot. During this build I encountered errors with wave on the msvc-12.0 compiler (both 32 & 64 bit). Roughly the following is repeated several times during the build: compile-c-c++ D:\RB\bin.v2\boost\bin.v2\libs\wave\build\msvc-12.0\debug\address-model-32\threading-multi\token_ids.obj token_ids.cpp D:\RB\boost_1_79_0\boost/wave/token_ids.hpp(335) : error C2146: syntax error : missing ';' before identifier 'token_id' D:\RB\boost_1_79_0\boost/wave/token_ids.hpp(335) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int D:\RB\boost_1_79_0\boost/wave/token_ids.hpp(341) : error C2146: syntax error : missing ';' before identifier 'token_id' D:\RB\boost_1_79_0\boost/wave/token_ids.hpp(341) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int D:\RB\boost_1_79_0\boost/wave/token_ids.hpp(341) : error C2086: 'int boost::wave::constexpr' : redefinition D:\RB\boost_1_79_0\boost/wave/token_ids.hpp(335) : see declaration of 'boost::wave::constexpr' D:\RB\boost_1_79_0\boost/wave/token_ids.hpp(347) : error C2144: syntax error : 'bool' should be preceded by ';' D:\RB\boost_1_79_0\boost/wave/token_ids.hpp(347) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int D:\RB\boost_1_79_0\boost/wave/token_ids.hpp(347) : error C2086: 'int boost::wave::constexpr' : redefinition D:\RB\boost_1_79_0\boost/wave/token_ids.hpp(335) : see declaration of 'boost::wave::constexpr' call "D:\RB\bin.v2\boost\bin.v2\standalone\msvc\msvc-12.0\address-model-32\architecture-x86\msvc-setup.bat" x86 >nul cl /Zm800 -nologo "libs\wave\src\token_ids.cpp" -c -Fo"D:\RB\bin.v2\boost\bin.v2\libs\wave\build\msvc-12.0\debug\address-model-32\threading-multi\token_ids.obj" -TP /wd4675 /EHs /GR /Z7 /Od /Ob0 /W3 /MDd /Zc:forScope /Zc:wchar_t /Zc:inline -DBOOST_ALL_DYN_LINK=1 -DBOOST_ALL_NO_LIB=1 -D_CRT_SECURE_NO_DEPRECATE -D_SCL_SECURE_NO_DEPRECATE "-I." ...failed compile-c-c++ D:\RB\bin.v2\boost\bin.v2\libs\wave\build\msvc-12.0\debug\address-model-32\threading-multi\token_ids.obj... Hope this helps, if not fixed it will be blocking the beta rc when it gets rolled. Tom

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Boost 1.79.0 closed for breaking changes and new libraries today
by Glen Fernandes 02 Mar '22

02 Mar '22

We are closed for new libraries and breaking changes today. We will close for all major changes this Friday (2022-03-04). And will close for the beta the following Wednesday (2022-03-09). The Release Managers

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std::numeric_limits<__float128>::signaling_NaN + __float128-Type
by Gero Peterhoff 26 Feb '22

26 Feb '22

Hello, please see https://godbolt.org/z/Ksshxvhaj 1) std::numeric_limits<__float128>::signaling_NaN always returns 0 but gcc-__builtin_nansq doesn't. I think __builtin_nansq is correct as it is also distinct from std::numeric_limits<__float128>::quiet_NaN (__builtin_nanq). 2) The types __float128, __extension__ __float128 (gcc) and _Quad (intel) are identical, but there are different definitions in boost/config/detail/suffix.hpp and boost/math/cstdfloat/cstdfloat_types.hpp. I think it would make sense to have just one definition or is there a reason why it was done that way? If yes, which one is it? You could always use __float128 directly and/or adjust boost/config/detail/suffix.hpp so that it also works with clang/intel. Which way would be better? I would like to help consolidate this but without having to deal with different typedef's or long #define's. thx Gero

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Boost.URL Proposal
by Alan de Freitas 21 Feb '22

21 Feb '22

Hi! My name is Alan Freitas. Vinnie Falco and I are looking for a review manager for the library that we hope will become Boost.URL. I've taken over the project and in addition to introducing myself I would like to update you guys as to where we are on this in terms of progress: - Path segments can be represented as views of the underlying path - Query parameters can be represented as views of the underlying query - Percent-decoding methods return lazy constant strings with small buffer optimization and shared ownership - The library implements the specification as in RFC3986 <https://datatracker.ietf.org/doc/html/rfc3986> + its errata <https://www.rfc-editor.org/errata/rfc3986> - Non-ASCII characters are not considered valid URLs - Other specifications such as RFC3492 <https://datatracker.ietf.org/doc/html/rfc3492>, WhatWG <https://url.spec.whatwg.org/>, or any other form of relaxed parsing are out of the scope of the library Both Vinnie and I are available to answer questions about the library on the list, on the GitHub repository page <https://github.com/CPPAlliance/url>, via email <alandefreitas(a)gmail.com>, or on Slack (in #boost-url). -- Alan Freitas https://github.com/alandefreitas

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Understanding the code base.
by Rahul Sharma 21 Feb '22

21 Feb '22

So far I have been reading multiple documentations and reading the projects from past year GSoC and exploring some modules given on the GitHub and I came to understand now that in order to contribute to this organization the knowledge of only C++ and data structures is not enough. I have some questions which are halting my progress so far, it would be very generous of you if someone could address these issues I am facing…… 1. I am unable to understand the code base in the repositories on GitHub. 2. In order to contribute or to understand things in a better way do I need to learn how Boost libraries work. 3. What are some packages I need to install. It would be very kind of you if you could provide some useful resources for the same and also for the things you think could be of use to understand things clearly and for making some PRs. When I read the code base of some repositories and some documentations I got excited as I think there are many things and skills I need to learn, the skills, it takes to be a decent developer. Sent from [1]Mail for Windows References 1. https://go.microsoft.com/fwlink/?LinkId=550986

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[PATCH] futex: fix build on 32-bit architectures using 64-bit time_t
by Khem Raj 21 Feb '22

21 Feb '22

Fix the following build failure on 32-bit architectures using 64-bit time_t (e.g. riscv32): | ./boost/fiber/detail/futex.hpp:31:23: error: use of undeclared identifier 'SYS_futex' | return ::syscall( SYS_futex, addr, op, x, nullptr, nullptr, 0); | ^ | 1 error generated. Signed-off-by: Khem Raj <raj.khem(a)gmail.com> --- boost/fiber/detail/futex.hpp | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/boost/fiber/detail/futex.hpp b/boost/fiber/detail/futex.hpp index e64bd5990..d0353e9e5 100644 --- a/boost/fiber/detail/futex.hpp +++ b/boost/fiber/detail/futex.hpp @@ -12,6 +12,10 @@ #include <boost/fiber/detail/config.hpp> +#ifndef SYS_futex +#define SYS_futex SYS_futex_time64 +#endif + #if BOOST_OS_LINUX extern "C" { #include <linux/futex.h> -- 2.35.1

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unordered_map implementation
by Peter Dimov 19 Feb '22

19 Feb '22

I've been helping Christian Mazakas maintain Unordered, and we've been looking into trying to improve its performance on the synthetic benchmarks we have for it (https://github.com/boostorg/unordered/tree/develop/benchmark) One of the things that are hurting us is that we need to do a double indirection in order to find an element, because Unordered uses a singly-linked list for all its nodes, with the bucket array pointing at the element before the first one (in order to be able to support erasing.) There's also the option of using a doubly-linked list, which however increases the memory footprint. I'm looking at the iterator invalidation rules for std::unordered_map, and iterators are invalidated on every rehash. This makes me wonder why is it necessary to have all the elements in a single list, as every implementation seems to do. It seems to me that it's possible to just have singly- linked lists for each bucket, and then have a "fat" iterator that contains the bucket index and the pointer to the element. On increment, when the end of the bucket list is reached, the iterator can just go to the next non-empty bucket. Is there something I'm missing in the requirements that renders this implementation non-conforming?

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std::launder and its usage in Boost
by Mikhail Kremniov 18 Feb '22

18 Feb '22

Hi, Initially, I posted this question to the boost-users list, but then I was advised to repost it here. I wonder, what is the Boost community's opinion of std::launder? In particular, of its necessity when accessing an object that was created via placement-new in an aligned_storage. As I can see, it's used by Boost.Beast in its implementation of the variant type, but not in other parts of Boost. The reason I'm asking this is that I'm working on a C++14 code-base that uses Boost.Variant and Boost.Optional extensively (as well as other parts of Boost that use Variant and Optional internally). Now we're trying to switch to C++17 at least, and I worry whether it could potentially break things. To put things into context, C++ standard states in the [basic.life] section that if you had an object and then created a new one in the same location, you can use the pointer to the old object to refer to the new one only if (among other things) "the new object is of the same type as the original object (ignoring the top-level cv-qualifiers)". This means, as I understand it, that it's technically illegal to reinterpret_cast the pointer to aligned_storage to the pointer to the actual type of the object created via placement-new, because placement-new has already ended the lifetime of the aligned_storage object. So, implementations of Boost.Optional and Boost.Variant are illegal then (?). But it's been like that since C++03 and Boost worked fine all that time, so it looks like this UB existed only "on paper". But C++17 then added "Note: If these conditions are not met, a pointer to the new object can be obtained from a pointer that represents the address of its storage by calling std::launder". So now the language has the ability to deal with that UB, and a question arises, is it possible that compilers could start to use the UB to perform additional optimizations and make it a real UB? Also, I've seen a couple of times on stackoverflow.com people saying that it's actually fine to reinterpret_cast the storage in C++14, but in C++17 it's not (they didn't explain why though). So, can switching from -std=c++14 to -std=c++17 be a breaking change when using Boost? The fact that Boost.Variant and Boost.Optional don't use std::launder - is it an oversight or a conscious decision? Regards, Mikhail.

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Diagnostic improvements in 1.78/1.79
by Peter Dimov 15 Feb '22

15 Feb '22

This is a quick summary of some 1.78 and 1.79 changes in Assert, ThrowException, and System related to error reporting. In Boost 1.73, Assert acquired a new component, boost::source_location. This is a class encapsulating a source location (a collection of source file name, line number, column, and function name) that is modeled after C++20's std::source_location, except of course it doesn't require C++20. In 1.79, the macro that creates the current source location, BOOST_CURRENT_LOCATION, is now usable in a function default argument, similarly to std::source_location::current(). This enables functions to be declared like this: T& my_vector::at( size_t i, boost::source_location const& loc = BOOST_CURRENT_LOCATION ) { if( i >= size() ) { boost::throw_exception( std::out_of_range( "at" ), loc ); } return data_[ i ]; } Since functions such as `at` are called from many places in the program, it's more useful for the source location in the thrown exception to point at the call site, rather than inside of `at` (which doesn't provide much information.) See for example https://godbolt.org/z/6o9e187a1. Note that the location returned by diagnostic_information(x) points to line 35, which is the call to `v.at(5);` inside `main`. Recommendation one: add source_location default arguments to your throwing functions so that the information logged at the catch site points to the call that threw. In case you find boost::throw_exception too heavyweight, as it adds a boost::exception base class and support for boost::exception_ptr, and this would prevent you from applying the technique above, there is now an alternative function, boost::throw_with_location, in <boost/throw_exception.hpp>. It doesn't integrate with Boost.Exception or boost::exception_ptr, but does add a source location to the exception that can later be retrieved with boost::get_throw_location(x). The above example using boost::throw_with_location can be seen at https://godbolt.org/z/Pcj1xad3v. In 1.78, boost::system::error_code had also acquired support for source locations, although it's not as automatic as the above technique where the default argument automatically adds a location without the user needing to do anything. Since there's no space in error_code for the entire boost::source_location struct, it can only store a pointer to it. This requires the following incantation BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION; ec.assign( ENOENT, boost::system::generic_category(), &loc ); which can be wrapped in a macro. See the documentation of Boost.System: https://www.boost.org/doc/libs/1_78_0/libs/system/doc/html/system.html#usag… Unlike the case with exceptions, when working with error codes, the situation is reversed and we want to store the source location immediately after discovering the error, rather than the one of the user call that failed. That's both because this location gives the most information as to the source of the error (ENOENT on its own doesn't say much), and because error codes are generally checked close to the user call, so the call location isn't as useful. Recommendation two: add source locations to your error_codes if using Boost.System. Finally, when providing dual APIs, that is, APIs that can signal an error using both error_code or by throwing an exception, you can combine the above two approaches and store both the "deep" source location in the error_code and the "shallow" (user call) location in the system_error exception. I hope you find those useful. Further reading: https://www.boost.org/doc/libs/develop/libs/assert/doc/html/assert.html#sou… https://www.boost.org/doc/libs/develop/libs/throw_exception/doc/html/throw_… https://www.boost.org/doc/libs/develop/libs/system/doc/html/system.html

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