December 2023 - Boost - lists.boost.org

CMake Announcement from Boost Steering Committee
by Jon Kalb 13 Aug '24

13 Aug '24

The libraries produced by the Boost community have had a greater impact on the way that the C++ community writes code than any other library implementation. The focus of the Boost community will always be on the libraries, but it is undeniable that we are dependent on and often limited by the infrastructure of our trade. Years ago, the move to Git was contentious; yet, it was required to improve development. In a similar vein our build system has become an impediment for many developers and users, existing and prospective. Therefore, we, the Steering Committee, announce to the Boost community our desire and intent to move Boost’s build system to CMake for users and developers alike. We are soliciting comments and proposals from the community to guide the process and the goals. Our desire is that the community can come to consensus by the end of the calendar year with a vision of supporting users and developers. We understand that the actual process of achieving the end goal will be long, time consuming, and not without its share of conflict. We also recognize that the Boost community is comprised of bright and passionate individuals that are eager to get involved and help get work done. The members of the Steering Committee have been encouraged by the discussions and activity surrounding CMake on the mailing lists over the years and know that many people have voiced visions. We hope that each of you rejoins the discussion to support this initiative and contributes to the common goal of improving Boost’s integration into the broader C++ ecosystem. For the Boost Steering Committee, Jon Kalb, Chair jonkalb(a)boost.org

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Boost Governance?
by René Ferdinand Rivera Morell 09 Aug '24

09 Aug '24

A disclaimer: I write this solely as a Boost Library author. And not in any other capacity or representing any other interest. It's now been at least a year since Boost switched from the Software Freedom Conservancy to the Boost Foundation for its governance organization. But have people noticed any difference? Did you know that switch happened? Did you know that they held an election for the Board of Directors and for the Officers? Do you know when that election happened? Do you know who got elected? Do you know how they were selected? Do you know what their responsibilities are? Do you know what their bylaws are? Do you know their financials? More importantly: Did you answer "no" to any of those questions? As a library author, I can say I definitely answered no to many of those questions. This is a problem. As authors, without asking, we should know those answers from the organization that governs our work. We should have input into how that organization governs. In other words, we deserve open and transparent governance; not the closed and opaque status quo. -- -- René Ferdinand Rivera Morell -- Don't Assume Anything -- No Supone Nada -- Robot Dreams - http://robot-dreams.net

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Paul Bristow
by John Maddock 08 Aug '24

08 Aug '24

I've just heard that Paul Bristow who helped so much with the development of Boost.Math and indeed many other libraries, passed away peacefully last week. RIP Paul. Much missed. John.

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boost::any_cast gives build errors with VS2022, boost v1.82 and C++17
by Bhuyan, Jyotideep 01 Jul '24

01 Jul '24

Hi, I was using VS2019, boost v1.78, and C++14 version and the below code was working well. Same code gives build error with VS2022, boost v1.82, and C++17 due to "o << *boost::any_cast<T>(&a); " Instruction. I wanted to know why it behaves differently. Code : class any_out { struct streamer { virtual void print(std::ostream& o, const boost::any& a) = 0; virtual streamer* clone() = 0; virtual ~streamer() {} }; public: any_out() : m_streamer(nullptr) { } ~any_out() { delete m_streamer; } template <typename T> any_out(const T& value) : m_streamer(new streamer_imp<T>), m_object(value) { } any_out(const any_out& a) : m_streamer(a.m_streamer ? a.m_streamer->clone() : nullptr), m_object(a.m_object) { } template <typename T> any_out& operator=(const T& r) { any_out(r).swap(*this); return *this; } any_out& operator=(const any_out& r) { any_out(r).swap(*this); return *this; } any_out& swap(any_out& r) noexcept { std::swap(m_streamer, r.m_streamer); std::swap(m_object, r.m_object); return *this; } friend std::ostream& operator<<(std::ostream& o, const any_out& a) { if(a.m_streamer) { a.m_streamer->print(o, a.m_object); } return o; } boost::any& get() { return m_object; } const boost::any get()const { return m_object; } bool empty() const { return m_object.empty(); } private: template <typename T> struct streamer_imp : public streamer { virtual void print(std::ostream& o, const boost::any& a) { try { if (a.empty()) { o << "<empty>"; } else { o << *boost::any_cast<T>(&a); } } } virtual streamer* clone() { return new streamer_imp<T>(); } }; // Members streamer* m_streamer; boost::any m_object; }; I have migrated to VS2022, boost v1.82, and C++17, and due to a build error, I have changed the code below. virtual void print(std::ostream& o, const boost::any& a) { try { if (a.empty()) { o << "<empty>"; } else { o << boost::any_cast<T*>(a); } } } any_out anyTest; anyTest = 100; std::string actualStr; actualStr = make_string() << anyTest; actualStr prints nullptr (0000000) instated of value 100 for VS2022, boost v1.82 and C++17 Best Regards, Jyotideep (JD) Please be advised that this email may contain confidential information. If you are not the intended recipient, please notify us by email by replying to the sender and delete this message. The sender disclaims that the content of this email constitutes an offer to enter into, or the acceptance of, any agreement; provided that the foregoing does not invalidate the binding effect of any digital or other electronic reproduction of a manual signature that is included in any attachment. Confidential - Company Proprietary

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[scope] Some random thoughts
by Andrzej Krzemienski 10 Jan '24

10 Jan '24

Hi All, Two observations from reading the docs. Why do I need a condition in scope_exit when I have a scope_fail? I recommend putting warning somewhere in the docs that while using scope guards you may inadvertently read an object after move: std::string checked(std::string name) { scope::scope_exit g{[&]{ std::clog << "checked " << name <<std::endl; }}; validate(name); return name; // name becomes moved-from } Regards, &rzej;

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Forthcoming Boost.Charconv review January 15th - 25th, 2024
by Christopher Kormanyos 09 Jan '24

09 Jan '24

Dear all, The review of Boost.Charconv by Matt Borland is scheduledfrom Monday, January 15th to January 25th, 2024. ----------------------------------------------------- About the library: Boost.Charconv provides a highly portable and efficient, C++11 implementation of <charconv>. The API containsfunctions compatible with <charconv>. Boost.Charconv can be highly useful for portability and efficiency. The standard library's <charconv> requires C++17 or higher. Currently only quite modern compilers such as GCC 11+ and MSVC 19.24+ support both integer and floating-point conversions in their implementation of <charconv>. If you are using either of those compilers, Boost.Charconv is at least as performant as <charconv>, and can be up to several times faster. If you are using an older compiler or a language standard lower than C++17, then there is no <charconv> available. In such cases, Boost.Charconv provides a highly-viable, portable and efficient header-only implementation. Boost.Charconv is a compiled library and does require a few other Boost libraries to run. The docs providedirections for how to consume with b2, vcpkg and Conanto facilitate this. Boost.Charconv fits into Boost stylistically and through itshigh quality, its Boost-like testing and its documentation. Boost.Charconv has been well-received and endorsed on the Boost-lists and is already being used for prototyping in several existing Boost libraries. Tool vendors have expressed interest in Boost.Charconv particularly due to its high performance and clarity of implementation details. docs: https://develop.charconv.cpp.al git: https://github.com/cppalliance/charconv --------------------------------------------------- Naturally, you need not wait until the review formally opens to start reviewing the Charconv library. Please always state in your review whether you think the library should be accepted as a Boost library! Please consider giving feedback on the following general topics: - 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 what compiler? 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? Christopher Kormanyos Boost.Charconv Review Manager ------------------------------

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Google Summar of Code 2024 - Collecting Projects & Mentors! 📢
by Inbal Levi 05 Jan '24

05 Jan '24

Hello all! We would like to collect projects & mentors for the GSOC 2024 :) If you would like to volunteer as a mentor and/or have a proposal for a Boost project which you like to collect volunteers for, please fill in the proper tab: https://docs.google.com/spreadsheets/d/1_eUWRsXnhi_JwKZbHZsGQf3WNUESsAPE4lV… Once we have a list of proposed projects, we will upload them to GitHub. Thanks! Best regards, Inbal Levi ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ *Lead Software Engineer @ Millenium* Isocpp, Boost &"Hamakor" Non-Profits Board Member ISO C++ LEWG Chair & Israeli NB Chair C++Now Program Chair & CoreC++ Conference Organizer ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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Interest in new parsing library?
by Zach Laine 30 Dec '23

30 Dec '23

I'm trying to gauge interest in a parsing library to replace Boost.Spirit 2/Spirit X3. I'm also looking for endorsements. The library is intended to remedy some shortcomings of Boost.Spirit*. I think these are great libraries, but Spirit 2 was written in pre-11 C++ (I think; certainly its dependencies were). Most-to-all of the downsides stem from that -- long compile times, inscrutable compilation failures, etc. (Boost.Parser compile times are quite low.) I'm calling my proposal Boost.Parser, and it follows many of the conventions of Boost.Spirit 2 and X3, such as the operators used for overloading, the names of many parsers and directives, etc. It requires C++17 or later. >From the introduction in the online docs: """ Boost.Parser is a parser combinator library. That is, it consists of a set of low-level primitive parsers, and operations that can be used to combine those parsers into more complicated parsers. There are primitive parsers that parse epsilon (the empty string), chars, ints, floats, etc. There are operations which combine parsers to create new parsers. For instance, the Kleene star operation takes an existing parser p and creates a new parser that matches zero or more occurrences of whatever p matches. Both callable objects and operator overloads are used for the combining operations. For instance, operator*() is used for Kleene star, and you can also write repeat(n)[p] to create a parser for exactly n repetitions of p. Boost.Parser also tries to accommodate the multiple ways that people often want to get a parse result out of their parsing code. Some parsing may best be done by returning an object that represents the result of the parse. Other parsing may best be done by filling in a preexisting data structure. Yet other parsing may best be done by parsing small sections of a large document, and reporting the results of subparsers as they are finished, via callbacks. Boost.Parser accommodates all these ways of working, and even makes it possible to do callback-based or non-callback-based parsing without rewriting any code (except by changing the top-level call from parse() to callback_parse()). All of Boost.Parser's public interfaces are sentinel- and range-friendly, just like the interfaces in std::ranges. Boost.Parser is Unicode-aware through and through. When you parse ranges of char, Boost.Parser does not assume any particular encoding — not Unicode or any other encoding. Parsing of inputs other than plain chars assumes that the input is Unicode. In the Unicode-aware code paths, all parsing is done by matching code points. This means that you can feed UTF-8 strings into Boost.Parser, both as input and within your parser, and the right sort of matching occurs. For instance, if your parser is trying to match repetitions of the char '\xcc' (which is a lead byte from a UTF-8 sequence, and so is malformed UTF-8 if not followed by an appropriate UTF-8 code unit), it will not match the start of "\xcc\x80" (UTF-8 for the code point U+0300). Boost.Parser knows that the matching must be whole-code-point, and so it interprets the char '\xcc' as the code point U+00CC. Error reporting is important to get right, and it is important to make errors easy to understand, especially for end-users. Boost.Parser produces runtime parse error messages that are very similar to the diagnostics that you get when compiling with GCC and Clang (it even supports warnings that don't fail the parse). The exact token associated with a diagnostic can be reported to the user, with the containing line quoted, and with a marker pointing right at the token. Boost.Parser takes care of this for you; your parser does not need to include any special code to make this happen. Of course, you can also replace the error handler entirely, if it doesn't fit your needs. Debugging complex parsers can be a real nightmare. Boost.Parser makes it trivial to get a trace of your entire parse, with easy-to-read (and very verbose) indications of where each part of the trace is within the parse, the state of values produced by the parse, etc. Again, you don't need to write any code to make this happen — you just pass a parameter to parse(). Dependencies are still a nightmare in C++, so Boost.Parser can be used as a purely standalone library, independent of Boost. """ Boost.Parser aims to be a superset of Boost.Spriit* in most ways. Major things missing from the set of features in Spirit 2 + Spirit X3 are: - A separate lexer. - Binary parsers (meaning for parsing bits, not binary numbers written as text; the latter is fully supported). I've been in touch with Joel de Guzman, Hartmut Kaiser, and Michael Caisse, to make sure I was not toe-stomping, for those who are concerned about that. They gave this new library their blessing. One feature comes entirely from them: Boost.Parser is usable in a Boost-free environment -- as a standalone library -- at the user's option. They said that was the #1 request from users, which surprised me a bit. The Github page is here: https://github.com/tzlaine/parser The online docs are here: https://tzlaine.github.io/parser To see an extended example, here's a JSON parser that passes all the published JSON tests, including most of the optional ones, in only about 300 lines of code, go here: https://tzlaine.github.io/parser/doc/html/boost_parser__proposed_/extended_… Finally, for those wanting to know how this lib differs from Boost.Spirit* without digging through the docs, here is the doc page that explains Boost.Parser's relationship to Boost.Spirit*: """ Boost.Spirit is a library that is already in Boost, and it has been around for a long time. However, it does not suit user needs in some ways. Spirit 2 suffers from very long compile times. Spirit 2 has error reporting that requires a lot of user intervention to work. Spirit 2 requires user intervention, including a (long) recompile, to enable parse tracing. Spirit X3 has rules that do not compose well — the attributes produced by a rule can change depending on the context in which you use the rule. Spirit X3 is missing many of the convenient interfaces to parsers that Spirit 2 had. For instance, you cannot add parameters to a parser. All versions of Spirit have Unicode support, but it is quite difficult to get working. I wanted a library that does not suffer from any of the above limitations. It should be noted that while Spirit X3 only has a couple of flaws in the list above, the one related to rules is a deal-breaker. The ability to write rules, test them in isolation, and then re-use them throughout a complex parser is essential. Though no version of Boost.Spirit (Spirit 2 or Spirit X3) suffers from all those limitations, there also does not exist any one version that avoids all of them. Boost.Parser does so. However, there are a lot of great ideas in Boost.Spirit that have been retained in Boost.Parser. Both libraries: - use the same operator overloads to combine parsers; - use approximately the same set of directives to influence the parse (e.g. lexeme[]); - provide loosely-coupled rules that are separately compilable (at least for Spirit X3); and - are built around a flexible parse context object that has state added to and removed from it during the parse (again, comparing to Spirit X3). """ Zach

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Re: [boost] Interest in new parsing library?
by Zach Laine 29 Dec '23

29 Dec '23

On Fri, Dec 29, 2023 at 12:40 PM Zach Laine <whatwasthataddress(a)gmail.com> wrote: > > On Fri, Dec 29, 2023 at 3:53 AM Andrzej Krzemienski <akrzemi1(a)gmail.com> wrote: > > > > > > > > czw., 28 gru 2023 o 22:05 Zach Laine via Boost <boost(a)lists.boost.org> napisał(a): > >> > >> I'm trying to gauge interest in a parsing library to replace > >> Boost.Spirit 2/Spirit X3. I'm also looking for endorsements. > >> > [snip] > > > > Hi Zach, > > Thank you for writing and sharing this library. I intend to test it on my mini-language early next year. > > That sounds useful. Please let me know how it goes. > > > For now, let me dig a bit about the high-level differences between Boost.Parser and Boost.SpiritX3. > > > > Your introduction mentions "a separate lexer" as a feature that Boost.Spirit is missing. > > How does that square with the entire section for Spirit.Lex in Boost.Spirit docs? > > No, I meant the other way around. *Boost.Parser* is missing that. > Sorry for the confusion. > > > "Boost.Parser aims to be a superset of Boost.Spirit". But Boost.Spirit is also a generator. > > Quite right! I forgot about that; this is another thing that > Boost.Parser is missing vs. Spirit. > > > You mention that "Spirit X3 has rules that do not compose well". I personally never experienced this. Is there an example somewhere that would illustrate this problem? > > The issue for me is that the attribute types generated by X3 parsers > and rules are loose. This is by design. So, say you have a parser > that parses strings, producing a range of char: > > auto p = /* ... */; > > If you then use p to parse into some std::string variable in the > middle of the parse, everything works as you'd expect. But if you use > p to parse into a different std::vector<char> variable in the middle > of the parse, that also works. This is not by itself a problem. > However, I had a problem with this semantic, because there was no way > to lock down the attribute type to a specific, concrete type. So if > at some point in the parse I had: > > auto p_p = p >> p; > > That may be used to generate a std::variant<std::string, > std::vector<char>>, a std::variant<std::vector<char>, std::string>>, > etc. These types are not interconvertible. So, If you made a mistake > somewhere in writing a semantic action or something, you get an error > stating that std::variant<std::string, std::vector<char>> is not > assignable to std::variant<std::vector<char>, std::string>> or > whatever, and figuring out where the error actually lies is very > difficult. If you have a Boost.Parser rule, it has a concrete > attribute type A, and so it always produces As. I find this much, > much easier to diagnose and debug when I make a mistake. > > > What is the recommendation of Boost.Spirit authors to the programmers that need to do parsing? Is Boost.Parser simply the newer and improved version, or do they have disjoint sets of use cases? > > I think Joel answered that above. > > > Personally, skimming through the docs, I find the feature of producing custom error and warning messages very attractive. This is what I was always missing from the parsing libraries. > > > > Thanks again for your effort. > > Thanks for taking a look! Also, I should mention that this is the > easiest library I have ever written, given its complexity. That is > true due to all the very nice design work that came before it, from > Joel, Hartmut, and others. > > Zach I somehow sent only this one reply to Andrzej only; echoing here. Sorry for the borked formatting. Zach

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About weird behavior with boost.asio 1.63 on linux 2.6.15
by Scott Zhang 25 Dec '23

25 Dec '23

Dear All: I have been using boost over 10+ years. The asio library work quite well along with document. Today I get a problem: My development environment: gcc 4.7.3 OABI Linux 2.6.15 boost 1.63 The code looks like below: void myTask(const char *text) { sleep(5); /*for(;;) { //sleep(1); }*/ cout<<"text is "<<text<<endl; // cout<<"out"<<endl; } boost::asio::io_service ioService; ioService.post(boost::bind(myTask,"hello")); ioService.run(); I get 2 problems: 1. If I run program as above, the mytask get executed correctly, but as soon as it returns, the program get "illegal instruction", and program dies. If I call myTask directly, then everything is ok. So what's wrong with io_service.post 2. If I run ioservice within thread using threadpool.create_thread( boost::bind(&boost::asio::io_service::run, &ioService) Then after myTask executed, I get "Killed", using strace says program get SIGKILL. As the test code is so simple, I can't tell where goes wrong. I also noticed, if I don't call io_service.post, everything looks ok. And the threads, in linux ps, shows as many processes. I reread the document of asio 1.63, it says had beed tested against linux 2.4 and 2.6 kernel. so it should be ok. Please help. Thanks.

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The master branch is now open for all changes
by Marshall Clow 14 Dec '23

14 Dec '23

Thanks to everyone who participated in the release! — Marshall

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[release] Boost 1.84.0
by Marshall Clow 14 Dec '23

14 Dec '23

Release 1.84.0 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 contain two new libraries, numerous enhancements and bug fixes for existing libraries. For details, including download links, see <http://www.boost.org/users/news/version_1.84.0> You can also download directly from: <https://boostorg.jfrog.io/artifactory/main/release/1.84.0/source> 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. -- The Boost Release Team Marshall Clow, Glen Fernandes

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[Fiber] aligned_storage is deprecated
by Niall Douglas 12 Dec '23

12 Dec '23

On Boost 1.81 with clang 18 in C++ 23 mode: ``` [build] /usr/include/boost/fiber/future/detail/shared_state.hpp:160:19: warning: 'aligned_storage<4, 4>' is deprecated [-Wdeprecated-declarations] [build] 160 | typename std::aligned_storage< sizeof( R), alignof( R) >::type storage_{}; [build] | ^ [build] /usr/include/boost/fiber/future/promise.hpp:28:22: note: in instantiation of template class 'boost::fibers::detail::shared_state<int>' requested here [build] 28 | typedef typename shared_state< R >::ptr_type ptr_type; ``` It could have been fixed upstream since, I haven't checked. Niall

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[release] Boost 1.84.0 Release Candidate 1 is available
by Marshall Clow 12 Dec '23

12 Dec '23

The first release candidates for the 1.84.0 release are now available at: <https://boostorg.jfrog.io/artifactory/main/release/1.84.0/source/> The SHA256 checksums are as follows: cc4b893acf645c9d4b698e9a0f08ca8846aa5d6c68275c14c3e7949c24109454 boost_1_84_0_rc1.tar.bz2 cc77eb8ed25da4d596b25e77e4dbb6c5afaac9cddd00dc9ca947b6b268cc76a4 boost_1_84_0_rc1.zip 81a4d10075731966477276c47324ecd9cac02da49899d36c48eba66e40014f25 boost_1_84_0_rc1.7z a5800f405508f5df8114558ca9855d2640a2de8f0445f051fa1c7c3383045724 boost_1_84_0_rc1.tar.gz As always, the release managers would appreciate it if you download the candidate of your choice and give building it a try. Please report both success and failure, and anything else that is noteworthy. -- The Release managers

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[scope] review
by Julien Blanc 07 Dec '23

07 Dec '23

Hi, This is my review of the proposed boost scope library. * Does this library bring real benefit to C++ developers for real world use-case? Yes. Scope guards are a well-known and widely used feature, nearly every code-base has some implementation of it. The same goes for raii resource wrappers. * Do you have an application for this library? I would consider using it for new projects. It does not, however, offer significant value at the time which would justify migrating from our homebrew scope guard. Worse, it misses some guarantees that makes it less fitted (but arguably it is more generic than what we use now). * Does the API match with current best practices? The API is copied from a TS proposal, for some parts. Most of my griefs against the API also applies to the TS. While I was not fond of it at the start, i think there's a clear value in differentiating scope_success and scope_fail. The first one is allowed to throw, the second is not. This, however, is not a strong requirement, but a general advice. It thus raises the concern about why not simply giving a boolean parameter to the scope_guard handler function, and letting it act accordingly. I suggest making the noexcept-ness of scope_fail executor a strong requirement. scope_success / scope_fail are not default-constructible, although they have a valid empty state. While there's a rationale behind this (it is a bit uncommon to create a scope guard that does nothing), it is a limitation for the sake of nothing. Initializing an empty scope_exit allows scope escaping (useful when initializing the scope guard in the body of an if statement). There will always be the possiblity to do it with an optional, so it just seems that forbidding default construction has no value. To check whether a scope_xxx is active or not, one has to call the active() method. To check whether a unique_resource has a value, one has to call the allocated() method. These are additions compared to the TS. While they may be useful, the names are inconsistent, both are new, also inconsistent with what other nullable types (optional, unique_ptr, variant...) uses. While this is common practice accross the c++ standard, I would not called this *best*. This leads to problems for generic code, and proposals such as this one: https://herbsutter.com/2022/09/25/something-i-implemented-today-is-void/ . In both case, i think at least an operator bool() should be provided, because it's a more widespread and common way to check for emptiness / valid state. scope_success / fails / exit takes an additional parameter (extension over TS), which is the execution condition. scope_success executes when the condition returns false. This is unintuitive, and something i will usually get wrong. While i get the reasoning behind, i think it tells more in favor of a scope_conditional<SuccessHandler, FailureHandler> interface. I'm not sure there's a lot of use-cases for a success_only or fail_only scope guard, especially since you can release in case of success, to avoid the cost of the failure check. About this customizable failure detector, i'm not found of the idea. The default, detection of a current exception, is a sane one. And it allows us to require FailureHandler to be noexcept. That's no longer the case if you allow customizing the failure detector. I'm not sure it's worth the tradeoff, since the failure detection could always be done inside the body of a standard unconditional scope_exit. Finally, i don't get the point of set_active. Sure, it's cheap to implement, but do we really want to encourage dynamically enabling scope guards ? Disabling a scope guard makes a lot of sense, that's what you do to disable a rollback scope_guard upon successful operation. Dynamically enabling it looks more like being hostile to the reader to me. In all cases, it's possible to get this behaviour with a boolean inside the scope guard callback, so this use case is already addressed. There's no need for it to be a first-class citizen. I like the idea of the lighter, unconditional, scope_final, which is missing in the TS. * Is the scope of the library well-defined? unique_resource and scope_exit are quite different beasts. They only have in common being RAII. But that also include unique_lock, unique_ptr, and nobody thought about making them part of the same library. I think it is a mistake to mix them. unique_resource takes three template parameters, the third one being a predicate telling if the resource is valid. I understand the wish to avoid storing an extra information, when there's a possibility to do without by storing an invalid value. But to me, it looks like a more widespread problem already solved, for example by https://github.com/akrzemi1/markable . It would be far better IMHO to get markable in boost, and have unique_resource specialized for markable<X> than replicating most of its functionality into unique_resource. * Is the documentation helpful and clear? The documentation is clear enough, although i could not find some information that i think should be emphasized more (ie, what the library guarantees in terms of pre / post conditions, noexceptness, etc). I don't like the way the reference section subitems are links to the file headers descriptions, and not links to class descriptions. I'd prefer something like: Reference * template< > class scope_exit * template< > class scope_final * ... instead of the headers, for which i usually don't care (i just need to know which header i need to include, and this information is provided in the class description). There are a few examples in the documentation that can be improved. There's always room for improvement, the overall quality is good. * Did you try to use it? What problems or surprises did you encounter? How much time did you spent? I made some small experiments with it. I spent around 6-8 hours reviewing the library and writing this review. Most of the time was spent reading the documentation and checking the behaviour. No problems and no surprises so far. * What is your evaluation of the implementation? The library is big (~2,5k lines of code), given what it offers. Most of this comes from the fact that the library offers a lot of customization points, maybe too much. Others have pointed that it reimplements some standard features, for the sake of not including some headers. I don't like this choice, but I don't know what's boost policy in this regard is. In all cases, a reimplementation with a name is better than nothing. In particular, there are some static_cast<T&&> which would advantagely be replaced by a detail::forward<T>, the latter being far more readable than the former. It's a shame that compact_storage is not provided as a facility: i guess there are several similar implementation across boost. But that's not the library's fault. Outside that, the organisation is clean, and the code largely readable, given that it needs to be compatible with C++11. * What is you final evaluation of the library? My current recommandation is to CONDITIONNALY ACCEPT the library. I think the following should be adressed, but i'd note that several of these comments also applies to the TS: - scope_success and scope_fail should be unified into a single call, taking two callbacks. This will make a slightly bigger object, but if you plan on detecting exceptions then you don't mind the extra cost anyway. And it will allow to evaluate the condition only once, so that may prove a gain in several situations. - scope_success and scope_fail should not take an execution condition. Exception detection should be enough for everyone, or else you should resort to scope_exit. - the failure handler (scope_fail) should be enforced to be noexcept - scope_exit should be default constructible (released by default, equivalent to moved-from state). That does not hold for scope_final, which is not releasable. - the active / released name fiasco should be adressed. At least provide operator bool(). - set_active should be removed. This is encouraging bad practice. - unique_resource should be moved outside the library. I consider that it belongs to a separate library, and i think it has some flaws in its design that should be addressed. Finally, many thanks to Andrey for proposing this library, and for quickly replying to the questions during the review. Regards, Julien

3 5

[release] Master Branch is closed for ALL changes
by Marshall Clow 07 Dec '23

07 Dec '23

Please do not commit anything to the master branch. Other branches are always fine. The 1.84.0 release is scheduled for next Wednesday, 13-December. — Marshall

1 0

[scope] Scope review starts on November 26th
by Дмитрий Архипов 07 Dec '23

07 Dec '23

Dear Boost community. The peer review of the proposed Boost.Scope will start on 26th of November and continue until December 5th. Boost.Scope is a small library implementing utilities defined in <experimental/scope> from C++ Extensions for Library Fundamentals v3 with a few extensions. Namely, the library contains: * A number of scope guards for various use cases. * A unique resource wrapper that automatically frees the resource on destruction. * Utilities for wrapping POSIX-like file descriptors in the unique resource wrapper. You can find the source code of the library at https://github.com/Lastique/scope and read the documentation at https://lastique.github.io/scope/libs/scope/doc/html/index.html. The library is header-only and thus it is fairly easy to try it out. In addition, the library can be used with Conan and vcpkg (see its README for instructions on how to do that). Finally, there's a single header version suitable for Compiler Explorer (https://raw.githubusercontent.com/Lastique/scope/single-header/include/boos…) As the library is not domain-specific, everyone is very welcome to contribute a review either by sending it to the Boost mailing list, or me personally. In your review please state whether you recommend to reject or accept the library into Boost, and whether you suggest any conditions for acceptance. Thanks in advance for your time and effort! Dmitry Arkhipov, Staff Engineer at The C++ Alliance.

16 83

The standard library is better documented
by Niilo Huovila 07 Dec '23

07 Dec '23

By better I mean cppreference.com. There's a page for pretty much everything in the standard library. Related pages are linked. The preconditions and behavior of functions are specified. Consequentially I come to Boost only when the standard library doesn't do enough. If you are going to compete with the standard library instead of just supplementing it, you need outstanding documentation.

14 26

Master branch is now open for bug fixes
by Marshall Clow 06 Dec '23

06 Dec '23

The master branch is now open for bug fixes and documentation changes. Other changes by release manager permission, as described in https://github.com/boostorg/wiki/wiki/Releases%3A-Beta-Merge-Policy The next deadline: On December 6th, master closes for all changes. As always, the calendar is at https://www.boost.org/development/ — Marshall

10 23

[release] Master Branch will close for the 1.84.0 release TOMORROW
by Marshall Clow 05 Dec '23

05 Dec '23

Don’t forget your release notes, too! — Marshall

1 0

[scope] Formal review - David Sankel
by David Sankel 05 Dec '23

05 Dec '23

- What is your evaluation of the design? I'm overall happy with the design. I like the API provided and think it is useful in many scenarios. The one question I have is why unique_resource::allocated instead of a bool conversion. This is the one aspect of the API that makes me hesitate using unique_resource for things like POSIX file descriptors as .allocated() would be quite foreign for users. - What is your evaluation of the implementation? I gave the implementation only a quick glance, but it looked fine to me. - What is your evaluation of the documentation? The documentation was good. I'd suggest the front page focuses on usage examples instead of a Boost.ScopeExit comparison though. Most people reading this page won't know about ScopeExit and are trying to evaluate whether not this library will solve their problem. - What is your evaluation of the potential usefulness of the library? I think this has wide applicability. - Did you try to use the library? With what compiler? Did you have any problems? I did not attempt to use it. - How much effort did you put into your evaluation? A glance? A quick reading? In-depth study? I spent about an hour working through the documentation. - Are you knowledgeable about the problem domain? Yes And finally, every review should answer this question: - Do you think the library should be accepted as a Boost library? Be sure to say this explicitly so that your other comments don't obscure your overall opinion. Yes, I do.

1 0

Proposal for Inclusion of Async.MQTT5 Library in Boost
by Ivica Siladic 05 Dec '23

05 Dec '23

Dear Boost Developers, we are writing to introduce a new C++ Async.MQTT5 library that we believe would be a valuable addition to Boost. MQTT has become the de-facto standard for IoT communication, with billions of IoT devices relying on it to publish or retrieve data from the cloud. Despite its widespread use, integrating MQTT into a C++ embedded application remains a complex and error-prone task. This challenge primarily arises from the absence of a higher-level library that offers a straightforward interface for message publishing and receiving while abstracting MQTT protocol intricacies and effectively managing various corner cases inherent in MQTT implementation. The primary objective behind developing Async.MQTT5 was to address these challenges comprehensively, allowing developers to focus on the messages they need to send without concerns about underlying network connectivity issues. Async.MQTT5 is a C++20 client implementation of the MQTT 5.0 protocol, leveraging Boost Asio and Boost Spirit. It fully adheres to the Asio asynchronous model, providing comprehensive support for all forms of asynchronous completion tokens, per-operation cancellation, and custom allocators. The library is designed to work seamlessly with any ordered network transport, typically TCP, SSL/TLS over TCP, or WebSockets (secured and unsecured). Our goal with Async.MQTT5 is to offer developers a simple and clear C++ interface for sending and receiving messages using the MQTT 5.0 protocol. Emphasizing code readability was a key design principle of Async.MQTT5. For instance, tedious protocol message parsing/composing is implemented declaratively (partially utilizing Boost Spirit). Additionally, Asio composed operations are implemented using a novel, clear, and highly efficient "chaining functional operators" approach. It is worth noting that the library is already written following Boost style guidelines, and the documentation is crafted and formatted in line with Boost conventions. You can find the library at https://github.com/mireo/async-mqtt5, and the documentation is available at https://spacetime.mireo.com/async-mqtt5/. We highly value your opinion and would appreciate any feedback you can provide on the Async.MQTT5 library. Thank you for your time and consideration. Best regards, Ivica Siladic Bruno Iljazovic Korina Simicevic

9 15

[scope] A few questions
by Julien Blanc 05 Dec '23

05 Dec '23

Hi, I have a few questions regarding the scope library currently being reviewed: * scope_exit is not default constructible. But it can be released and is movable, so a default constructed scope_exit state makes sense (this is the same as a moved-from scope_exit). I find this a rather strange design choice. What is the rationale behind this? There's even a section on the documentation on how to circumvent this: https://lastique.github.io/scope/libs/scope/doc/html/scope/scope_guards.htm… using cleanup_func_t = std::function< void() >; // Create an inactive scope guard first, since the cleanup function is not set yet boost::scope::scope_exit< cleanup_func_t > cleanup(cleanup_func_t(), false); Why can't i just write? boost::scope::scope_exit< std::function< void() > > cleanup; * the documentation says little about memory allocation. I finally found the answer i was looking for in the boost.ScopeExit comparison: scope_xxx objects do not do any heap allocations themselves. Is that a guarantee of the library? Such a guarantee is needed in some code i work with, i need to know if it is a feature of the library that i can rely upon, and not just an optimization that could go away in a further release. * unique_fd is a resource wrapper for files. But on all major systems, closing a file can fail and return an error. It may well be that the file is not completly written correctly, and this is reported not in the write call, but in the final close call. unique_fd silently drops the return value of close. I'm not sure we should encourage this behaviour. Another issue raises when we look at the example code: void write_string(std::string const& filename, std::string const& str) { // Create a unique resource for a file boost::scope::unique_resource< int, fd_deleter > file(open(filename.c_str(), O_CREAT | O_WRONLY)); if (file.get() < 0) { int err = errno; throw std::system_error(err, std::generic_category(), "Failed to open file " + filename); } // Use it to write data to the file const char* p = str.data(); std::size_t size_left = str.size(); while (size_left > 0) { ssize_t written = write(file.get(), p, size_left); if (written < 0) { int err = errno; if (err == EINTR) continue; throw std::system_error(err, std::generic_category(), "Failed to write data"); } p += written; size_left -= written; } } In case of a failure, what we got is an half-baked file, which we know nothing about. A proper write_string function should at least try to do its cleanup and delete the file in case of error. But that defeats the purpose of unique_resource... In my experience, automatically closing file handles is not error handling. But it has its use cases, maybe it would be better demonstrated by an example like: bool compare_files(std::string const& filename1, std::string const& filename2) { boost::scope::unique_resource< int, fd_deleter > file1(open(filename.c_str(), O_CREAT | O_WRONLY)); if (file1.get() < 0) { int err = errno; throw std::system_error(err, std::generic_category(), "Failed to open file " + filename1); } boost::scope::unique_resource< int, fd_deleter > file2(open(filename.c_str(), O_CREAT | O_WRONLY)); if (file2.get() < 0) { int err = errno; throw std::system_error(err, std::generic_category(), "Failed to open file " + filename2); } // do the comparison } Regards, Julien

2 7

[scope] Mr. Docs rendered reference
by Krystian Stasiowski 05 Dec '23

05 Dec '23

Hi everyone, My name is Krystian, and I'm a Staff Engineer at The C++ Alliance. I’ve been working on Mr. Docs, a clang based C++ documentation generator intended to serve as an alternative to Doxygen. Although Mr. Docs is not yet ready for release, I noticed that there was an issue with rendering the exception specification of destructors due to a bug in Boostbook. Thus, I would like to present Mr. Docs as a potential future alternative, and have generated a multi-page reference for Boost.Scope to demonstrate its capabilities: https://mrdocs.com/demos/develop/boost-scope/multi/adoc-asciidoc/boost/scop… The output was created using the built-in Asciidoc generator and converted to HTML with asciidoctor. Since the Asciidoc generator uses handlebars templates to format the output, customizing the rendered documentation is a matter of modifying the templates. Unlike Doxygen, Mr. Docs operates directly on the clang AST. This means that support for new/obscure language features is only limited by the extent of clang’s conformance to the latest C++ standard. Any feedback is greatly appreciated. 🙂 Best, Krystian Stasiowski

5 6

Re: [boost] The standard library is better documented
by Niilo Huovila 05 Dec '23

05 Dec '23

On 12/5/23 1:43 AM, boost-request(a)lists.boost.org wrote: > Date: Mon, 4 Dec 2023 12:36:27 > -0800 From: Marshall Clow <mclow.lists(a)gmail.com> To: > boost(a)lists.boost.org Subject: Re: [boost] The standard library is > better documented Message-ID: > <B135B251-3578-40CA-A9D8-34E165EE6C14(a)gmail.com> Content-Type: > text/plain; charset=utf-8 On Dec 4, 2023, at 12:21 PM, Mateusz Loskot > via Boost <boost(a)lists.boost.org> wrote: >> On Mon, 4 Dec 2023 at 21:05, Niilo Huovila via Boost >> <boost(a)lists.boost.org <mailto:boost@lists.boost.org>> wrote: >>> On 12/4/23 4:36 PM, Hans Dembinski wrote: >>>>> On 2. Dec 2023, at 16:57, Ren? Ferdinand Rivera Morell via Boost<boost(a)lists.boost.org> wrote: >>>>> >>>>> It would be useful if you could point to instances of Boost libraries >>>>> that could use better documentation. >>>> Can we organise a user poll that ranks all Boost libraries on how well documented they are? >>>> >>>> Might be a fun way to approach this and it provides an opportunity to create some publicity for Boost when we advertise this. I can see the news headline: "Boost devs finally respond to user cries for better docs" >>> The secret of cppreference seems to be that it is a wiki. How about >>> that? Users could put their energy to improving the docs instead of >>> complaining. :) >> Users can already do that by submitting pull requests on GitHub. > Or, if they don?t grok Quickbook, they can open a GitHub issue and suggest better wording. They seem relatively heavy-handed ways to add a link or correct spelling. > Date: Tue, 5 Dec 2023 00:12:17 +0300 > From: Andrey Semashev<andrey.semashev(a)gmail.com> > To:boost@lists.boost.org > Subject: Re: [boost] The standard library is better documented > Message-ID:<9e217656-9b05-4e82-a5a3-67c55d949b4f(a)gmail.com> > Content-Type: text/plain; charset=UTF-8 > > On 12/4/23 20:53, Niilo Huovila via Boost wrote: >> On 12/4/23 4:36 PM, Hans Dembinski wrote: >>>> On 2. Dec 2023, at 16:57, Ren? Ferdinand Rivera Morell via Boost >>>> <boost(a)lists.boost.org> wrote: >>>> >>>> It would be useful if you could point to instances of Boost libraries >>>> that could use better documentation. >>> Can we organise a user poll that ranks all Boost libraries on how well >>> documented they are? >>> >>> Might be a fun way to approach this and it provides an opportunity to >>> create some publicity for Boost when we advertise this. I can see the >>> news headline: "Boost devs finally respond to user cries for better docs" >> The secret of cppreference seems to be that it is a wiki. How about >> that? Users could put their energy to improving the docs instead of >> complaining. :) > Wiki is prone to vandalism and edit wars. > > Also, aside from stylistic and wording edits, the documentation is > supposed to be written by people who are very knowledgeable in the > subject. Those people are likely very few, often maintainers only. > > Wiki is good for "public knowledge" kind of documentation. The official > library documentation is very different from that. In fact, it is the > ground truth upon which that public knowledge is built. Would separating the reference from other documentation also separate it from those concerns? Then it could be a wiki and editing the other stuff can be restricted to maintainers.

1 0

[scope] some more thoughts
by Andrzej Krzemienski 04 Dec '23

04 Dec '23

Hi Everyone, I want to share some thoughts regarding the review of Boost.Scope. The fact that it wants to implement 1-to-1 a feature from a TS -- not from the standard -- impedes the review process. The reviewer says, "this feature is bad". The author responds, "but this is how the TS requires it to be". And now what? Should we accept a bad feature because the TS requires so? Being TS-conformant is very constraining for the author. Maybe this is not a worthy goal. Also, my observation from recent years is that putting stuff into a TS may be a polite way of saying, "we respect you, but we do not want this." Also, the goal of having an implementation for a proposed feature is to check if the interface has been battle-tested to be sound. The Boost Review process is such a test. When the feedback is, this feature is wrong, then this feedback should affect the STD proposal. But Boost shouldn't be compromised by an STD proposal that was submitted without a battle-tested implementation. The move assignment operator in scope_exit and friends. It looks like its existence is against the goal of the feature. You define a guard and then you use it. Why would you assign a different guard to it? Why do we have scope_success? An alternative to using scope success is to just put your instructions at the end of the function. Do you have multiple return statements in your function and at the same time you need the same sequence of operations to be performed at each end? This is a good motivation to restructure your function to have a single return. Multiple returns are generally OK, but needing the same sequence warrants a single return. scope_success seems like a solution that is looking for a problem. "But the TS has scope_success" -- this is not a good argument for me. In Boost Review we should be evaluating a library based on its own merit, not based on a similarity with some other design, which can have its own flaws. Mixing unique_resource with scope guards is not a good idea. The STD tends to use big aggregate headers. Boost does not. The solution from the TS should not be ported 1-to-1 to Boost. Boost.Scope offers at least three ways to do the same thing. For instance, to execute some code only upon some inconvenient condition, I can use scope_fail with my condition, or scope_exit with my condition, or I can add set_active on top of either. I consider this to be a bad design. Source code is not only for the flexibility of writing, but also for the ease of reading and learning. Having multiple ways to do the same thing impedes the understanding of the code. While I understand the existence of the triplet `scope_exit`, `scope_fail` and `scope_success` (D uses an analogous triplet), it takes a while to understand the difference between scope_exit and scope_final. And the choice of name doesn't help there. The Standard library chose to use term guard (std::lock_guard) to indicate something small and cheap. Maybe use this name instead. Not perfect either, but at least consistent with what we have in STD. Also, my experience is that one uses scope guards only in rare situations where you need to satisfy complex guarantees, like maintaining the same size of two vectors. And in those situations one is already very cautious, and tries to structure the code for the minimum surprises. One does not use multiple returns or complex control flows. The justification I heard for so many features in Boost.Scope is that there may be complex situations that require it. I think that in these situations restructuring the code is better than adding more features to Boost.Scope. This looks like a lot of harsh words, but I find a subset of the library very useful: * the unconditional execution of the function * the execution based on manual deactivation (active being the initial state) * execution based on the uncaught exception count, with the caveat that it sometimes doesn't work, and it is not cheap. Regards, &rzej;

2 5

[scope] unique_resource vs unique_ptr
by Bruno Martínez 04 Dec '23

04 Dec '23

Hi, unique_ptr can emulate unique_resource: template<auto nullvalue, auto delete_> class unique { using T = decltype(nullvalue); struct generic_delete { class pointer { T t; public: pointer(T t) : t(t) {} pointer(std::nullptr_t = nullptr) : t(nullvalue) { } explicit operator bool() { return t != nullvalue; } friend bool operator ==(pointer lhs, pointer rhs) { return lhs.t == rhs.t; } friend bool operator !=(pointer lhs, pointer rhs) { return lhs.t != rhs.t; } operator T() { return t; } }; void operator()(T p) { delete_(p); } }; public: using type = std::unique_ptr<struct not_used, generic_delete>; }; int main() { using unique_fd = unique<-1, close>::type; static_assert(sizeof(unique_fd) == sizeof(int), "bloated unique_fd"); unique_fd fd1(open("fd.txt", O_WRONLY | O_CREAT | O_TRUNC)); write(fd1.get(), "hello\n", 6); } The std papers say While std::unique_ptr can be tweaked by using a custom deleter type to almost a perfect handler for resources, it is awkward to use for handle types that are not pointers. But what's the awkwardness? Kind regards, Bruno

3 2

Re: [boost] The standard library is better documented
by Niilo Huovila 03 Dec '23

03 Dec '23

On 12/3/23 3:37 AM, boost-request(a)lists.boost.org wrote: > Date: Sat, 2 Dec 2023 17:44:05 +0100 > From: Andrzej Krzemienski <akrzemi1(a)gmail.com> > To: boost(a)lists.boost.org > Subject: Re: [boost] The standard library is better documented > Message-ID: > <CAOenAXhr9fu6jo7PoFC6JTKBhD7Kdw6yxWJUEFxtcPgYkujHwQ(a)mail.gmail.com> > Content-Type: text/plain; charset="UTF-8" > > sob., 2 gru 2023 o 17:21 Andrey Semashev via Boost <boost(a)lists.boost.org> > napisa?(a): > >> On 12/2/23 03:35, Niilo Huovila via Boost wrote: >>> By better I mean cppreference.com. There's a page for pretty much >>> everything in the standard library. Related pages are linked. The >>> preconditions and behavior of functions are specified. Consequentially I >>> come to Boost only when the standard library doesn't do enough. If you >>> are going to compete with the standard library instead of just >>> supplementing it, you need outstanding documentation. >> >> To be fair, as useful as cppreference.com is, it is not quite the same >> kind of documentation as Boost documentation is. It is mostly similar to >> the reference section that you can find in most libraries' docs, as it >> basically translates the standard wording to human language and gives a >> single usage example in the end. There is no rationale, discussion, or >> usage recommendations. It's a feature: there's a time for discussion and rationale, but not when I just want to look up something quickly. >> The unquestionable advantage of cppreference.com is that it has lots of >> cross-links, which makes it very easy to navigate and find what you're >> looking for. Cross-linking is definitely something that Boost docs could >> use more. ^ This. The easy to navigate reference is what I miss the most. Cppreference can be explored like a retailer's magazine. Items are usually sorted to categories. It makes them more discoverable. > True, but for that we would have to have a uniform documentation experience > for all the libraries. And I do not think we are ever going to get there. I agree that it's a daunting task. Actually I thought that the first reaction to my message would be pondering whether competing with the standard library is worth the effort or even feasible. I were surprised when the first reaction was asking which docs need improvement, because cppreference sets the bar so high.

1 0

Re: [boost] [scope] Scope review starts on November 26th
by Janko Dedic 02 Dec '23

02 Dec '23

Thanks for proposing this library. Scope guards are a natural and much needed addition to Boost. The following is my review of the proposed Boost.Scope. The first thing I looked for in this library was a convenience SCOPE_EXIT { ... }; macro that would provide the functionality in the form of a control flow construct. I've managed to quickly find it, but it wasn't exactly what I was expecting. BOOST_SCOPE_FINAL [] { std::cout << "Hello world!" << std::endl; }; I found it rather odd that the capture list is not included in the macro. I looked a bit further into the documentation to find this: BOOST_SCOPE_FINAL std::ref(cleanup_func); Is this use case the reason why the capture list is required? It seems like a very rare one that shouldn't pessimize the interface for 99% of use cases. Performance shouldn't be a concern because the compiler sees all this code and inlines it anyway. In my view, this just makes the interface worse and less pleasant to use. scope_success doesn't seem terribly useful. I have yet to see a legitimate use case of this type. I haven't found a motivating example in the documentation. There should either be one, or this type should be removed from the library. There is no BOOST_SCOPE_FAIL macro. Why is it missing? I found the scope_fail example pretty bad: class collection { std::set< std::shared_ptr< object > > objects; public: void add_object(std::shared_ptr< object > const& obj) { // Create a deactivated scope guard initially std::set< std::shared_ptr< object > >::iterator it; boost::scope::scope_fail rollback_guard{[&, this] { objects.erase(it); }, false}; bool inserted; std::tie(it, inserted) = objects.insert(obj); if (inserted) { // Activate rollback guard rollback_guard.set_active(true); } obj->on_added_to_collection(*this); } }; It could be simply rewritten to not use set_active (which should be discouraged, because it makes code less declarative and harder to follow): class collection { std::set< std::shared_ptr< object > > objects; public: void add_object(std::shared_ptr< object > const& obj) { // Create a deactivated scope guard initially std::set< std::shared_ptr< object > >::iterator it; bool inserted; std::tie(it, inserted) = objects.insert(obj); if (inserted) { boost::scope::scope_fail rollback_guard{[&, this] { objects.erase(it); }}; obj->on_added_to_collection(*this); } } }; Could be made even cleaner by using an early return: if (!inserted) return; All destructor declarations in the documentation lack noexcept specification, which by default means that they're noexcept, but they're actually not if you look at the Throws section, or the implementation. Also, the question presents itself: why are these destructors allowed to throw? I feel like it is common knowledge today that destructors should not throw, and this is heavily frowned upon. I can see the want to "be generic" here, but I don't see why a library should entertain broken code. I also noticed that certain scope guards are move constructible. Why? I cannot imagine myself ever approving of someone moving a scope guard on code review. Scope guards are to be used exclusively as local variables and don't need to be moved. This just opens up room for misuse. I don't see any compelling use cases for scope_exit::set_active in the documentation. The only example I find motivating is this one: void push_back(int x, std::vector<int>& vec1, std::vector<int>& vec2) { vec1.push_back(x); // Revert vec1 modification on failure boost::scope::scope_exit rollback_guard{[&] { vec1.pop_back(); }}; vec2.push_back(x); // Commit vec1 modification rollback_guard.set_active(false); } This could as well have used rollback_guard.release();. This matches my personal experience: only dismiss/release/cancel is necessary for real use cases ("commiting" something and thus disabling the "rollback" action). absl::Cleanup for example, supports only Cancel(). There is value in preventing users from shooting themselves in the foot. I don't like the name scope_exit::release personally. I feel like "dismiss" or "cancel" would be much better. I know that std::experimental::scope_exit calls it "release", but we shouldn't appeal to false authority. I did not like the locked_write_string example. Reading this code locally: // Lock the file while (flock(fd, LOCK_EX) < 0) { err = errno; if (err != EINTR) return; } it's hard to get what's happening, why we're setting err and how big of an impact that has on function behavior. A better way to write this code would be to simply extract this piece into its own function: err = errno; if (err != EINTR) return; together with the ec = std::error_code(err, std::generic_category()), which doesn't require a scope guard. fd_deleter and unique_fd provide some nice handling for EINTR, which wasn't required but I think it's nice to have and it's good that it's a part of this library! I think make_unique_resource_checked approaches the problem from the wrong direction. This code: // Create a unique resource for a file auto file = boost::scope::make_unique_resource_checked( open(filename.c_str(), O_CREAT | O_WRONLY) -1, fd_deleter()); if (!file.allocated()) { int err = errno; throw std::system_error(err, std::generic_category(), "Failed to open file " + filename); } could be this: // Create a unique resource for a file int file = open(filename.c_str(), O_CREAT | O_WRONLY); boost::scope::scope_final close_file(fd_deleter{}); if (file != -1) { int err = errno; throw std::system_error(err, std::generic_category(), "Failed to open file " + filename); } This has the advantage of being able to use `file` directly for the rest of the code instead of `file.get()`. I think a similar utility would be more useful in form of a class template, for example: resource<HANDLE, INVALID_HANDLE_VALUE, ::CloseHandle> windows_handle; resource<SDL_Window*, nullptr, ::SDL_DestroyWindow> sdl_window_handle; This would be very useful for concisely implementing RAII wrappers over C handles (you don't have to manually write destructors, move constructors and move assignment operators). For local use (within a function), I feel like a local handle + scope guard is just more ergonomic than the whole make_unique_resource_checked thing. My overall impression is that this library is trying to be too generic when it's not really necessary. There's definitely useful stuff here that I want to see included, but also a lot of stuff that is too much. I don't personally see the need for any scope guard beyond scope_exit with a release. That covers all the use cases I've ran into very nicely. scope_fail can be more error-prone since the "commit" is not explicit, so you may end up doing rollback even when it's unnecessary. Using the modified example from the documentation: void push_back(int x, std::vector<int>& vec1, std::vector<int>& vec2) { vec1.push_back(x); // Revert vec1 modification on failure boost::scope::scope_fail rollback_guard{[&] { vec1.pop_back(); }}; vec2.push_back(x); do_something_else(); // if this throws, rollback_guard runs } If you use scope_exit with an explicit release, it's clearer IMO: void push_back(int x, std::vector<int>& vec1, std::vector<int>& vec2) { vec1.push_back(x); // Revert vec1 modification on failure boost::scope::scope_exit rollback_guard{[&] { vec1.pop_back(); }}; vec2.push_back(x); rollback_guard.release(); // commit do_something_else(); // if this throws, rollback_guard won't run } The documentation states that "exception_checker is incompatible with C++20 coroutines", which is true in general, but it doesn't have to be (depending on the coroutine library implementation). However, I bring this up because using scope_exit+release is an entirely viable alternative for coroutines. scope_exit+release will work whether you use exceptions, error codes, std::expected, coroutines or all of them combined. This is the beauty of it, which makes me feel like scope_fail is unnecessary. I vote to ACCEPT this library with the following CONDITIONS: - BOOST_SCOPE_FINAL includes the lambda capture list OR another macro that does is provided; - noexcept specification on destructors is properly documented; - move constructors on all scope guard types are deleted; - a motivating example for scope_success is provided, or the type is removed. These are my conditions, but I'm hoping the full review is taken as a recommendation in improving the library and the documentation. Best regards, Janko Dedic

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[scope] The comparison with Boost.ScopeExit is unfair
by Andrzej Krzemienski 02 Dec '23

02 Dec '23

Hi All, This is not a review yet. I am just looking at the front page of the docs, and I think the comparison with Boost.ScopeGuard is unfair. You are comparing C++03 Boost.ScopeGuard with C++11 Boost.Scope. Try comparing it with C++03 Boost.Scope. Otherwise compare C++11 to C++11, and there Boost.ScopeGuard has a nicer syntax: int compute() { // Reset variables on return BOOST_SCOPE_EXIT(this_) { this_->x = 0; this_->y = 0; }; return x + y; } Regarding the example with exceptions, nobody would use Boost.ScopeGuard like this. A robust usage is in Boost.ScopeGuard's docs: void world::add_person(person const& a_person) { bool commit = false; persons_.push_back(a_person); // (1) direct action // Following block is executed when the enclosing scope exits. BOOST_SCOPE_EXIT(&commit, &persons_) { if(!commit) persons_.pop_back(); // (2) rollback action }; // ... // (3) other operations commit = true; // (4) disable rollback actions } Which is longer, but usable in coroutines. Whereas Boost.Scope's solution is not usable in coroutines. Only after these changes are applied, I think is the comparison fair. Regards, &rzej;

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Boost Culture
by Vinnie Falco 02 Dec '23

02 Dec '23

It occurs to me that newcomers to the mailing list will not have any understanding of Boost culture or its development process. For example there is a recurring theme where a new person will ask about "what documentation toolchain does Boost use." The answer of course is that every library has considerable autonomy to define how it wants to generate its documents. The only requirement is that the final result is rendered as HTML. Another example is the idea that there is a "CEO of Boost." Or rather that there are particular individuals who have some sort of decision-making authority for the entire libraries. Or another example, why we use mailing lists over dedicated forum software. We might consider putting together a document which centralizes all of the collected wisdom and knowledge about Boost culture including the history of its evolution and why some things are the way that they are. This would facilitate the entry of newcomers to places like the mailing list which assume that participants already know a lot about Boost culture. This could become a chapter in the new Contributor Guide: https://www.preview.boost.org/doc/contributor-guide/index.html Thanks

4 3

Dead Link
by Brian Salehi 01 Dec '23

01 Dec '23

There is a dead link on page [Machine Learning](https://www.preview.boost.org/doc/user-guide/task-machine-learnin… refering to Boost.uBLAS library. I guess the typo is an extra s letter after /doc which seems to point to the correct page when removed. Regards, Brian

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