May 2017 - Boost - lists.preview.boost.org

[yap] Re-announcing Yap
by Zach Laine 23 Feb '18

23 Feb '18

I posted 2-3 months ago about Yap, an expression template library I've written that I intend to propose for Boost. This is just a reminder that the library exists, and where to find it. I'm giving a talk about it at C++Now 2017, and some time after that I intend to submit it to the queue. Louis Dionne has offered to serve as review manager when the time comes. You can find the main repo on GitHub: https://github.com/tzlaine/yap And online docs are here: https://tzlaine.github.io/yap Zach

4 6

[Safe Numerics] Review
by John Maddock 09 Jul '17

09 Jul '17

OK, here's my review of Robert's library. By way of something different, I've tried to not read the docs (though I ended up scanning them in the end) and just tried to use the library. Here's what I've found: 1) Using: safe<int>(-1) & 1 I get a hard assert failure at safe_base_operations.hpp:1373. I'm assuming that this is a mistake, and the usual error handlers should be called? Bitwise or has the same issue. More tests required ;) 2) I'm not sure if this is an msvc issue but in: constexpr safe_base operator~() The static assert is triggered for *unsigned types* and not for signed. Even if this is a compiler issue, it indicates a missing test case or two. 3) If I assign a float to an integer, then I get the error: "conversion of integer to float loses precision" which isn't quite what you meant to say. More particularly, for float conversions I would expect: * Conversion from a float holding an integer value that's within the range of the target type to always succeed. * Conversion from a float holding a non-integral value to conditionally succeed (with trunction) depending on the policy in effect. * Conversion *to* a float may also fail if the integer is outside the range of the float (no checking may be required if the number of bits in the integer is less than the number of bits in the float). 4) Is constexpr arithmetic supposed to be supported? I tried: constexpr boost::numeric::safe<int> j = 0; constexpr boost::numeric::safe<int> k = 3; constexpr boost::numeric::safe<int> l = j + k; Which generates an internal compiler error for msvc, and for gcc-5.3 I get: ../../../include/safe_base_operations.hpp:332:35: error: call to non-constexpr function ‘void boost::numeric::dispatch(const boost::numeric::checked_result<R>&) [with EP = boost::numeric::throw_exception; R = int]’ dispatch<exception_policy>(r); Similarly if I try to add a literal to a safe<int>. Again as the tests all pass, so I'm assuming they're missing constexpr tests? 5) What is the purpose of class safe_literal? constepxr initialization seems to work just fine without it? 6) In the docs, the example of using error handler dispatch is nonesense (top of page 41)? 7) Related to the above, I see the need to be able to define your own primitives which behave just like yours. The example I have is for bit-scan-left/right which has as precondition that the argument is non-zero, but there are other bit-fiddling operations (bit count etc) which some folks may require. A "cookbook" example would be good. 8) In the tests, you have an undocumented function "log" in the library. Not sure if this is intended to be part of the API or not, but IMO it's misnamed. ilog2 seems to a semi-standard name for this. 9) By way of testing the libraries performance I hooked it into the Millar-Rabin primality test in the multiprecision lib (test attached, it counts how many probable primes there are below 30000000), here are the times for msvc: Testing type unsigned: time = 17.6503 seconds count = 1857858 Testing type safe<unsigned>: time = 83.5055 seconds count = 1857858 Testing type 32-bit cpp_int: time = 36.9026 seconds count = 1857858 and for GCC-MINGW-5.3.0: Testing type unsigned: time = 17.1037 seconds count = 1857858 Testing type unsigned: time = 18.4377 seconds count = 1857858 Testing type unsigned: time = 21.0592 seconds count = 1857858 I'm surprised by the difference in times between msvc and gcc, also deeply impressed that there is effectively no performance penalty with GCC (I was expecting there should be some!) 10) I don't find the scope of the library over-broad, in fact I probably find it about right wrt the policies etc. 11) There were a few traits and such that it would be nice to have, the problem as ever, is what to call them and where to put them: * A trait for extracting the underlying type of a safe<> - use case for example where you enable_if a function on is_safe so don't have the template argument list to safe<>. * Do we need a function for extracting the underlying type of a safe<>? Or is casting to the underlying type optimal? Actually, maybe having a function would negate the need for the above trait, as we could just write 'auto x = mysafe.value();' ? * Do we need traits equivalent to is_signed/is_unsigned/make_signed/make_unsigned? My gut feeling is yes we do, the issue is what to call them, and where to put them - we're not allowed to specialize the type_traits versions (either boost or std), but we would need a central "one true trait" version that can be specialized by other libraries as well (Multiprecision for example). We've backed ourselves into a corner over this I fear! 12) When calling boost::integer::gcd/lcm the sub-optimal(?) non-binary versions are called for safe<> - you might want to look into seeing what traits need to be defined to ensure that the same version as the underlying type is called. Those are all the issues I can see for now... in conclusion, as for whether the library should be accepted, yes I believe it should be, modulo the issues above. Regards, John. --- This email has been checked for viruses by AVG. http://www.avg.com

4 14

[CallableTraits] Review results
by Louis Dionne 07 Jul '17

07 Jul '17

Dear Boost, I am pleased to announce that the CallableTraits library is CONDITIONALLY ACCEPTED into Boost. I counted the following formal reviews: - Bruno Dutra: Yes - Emil Dotchevski: Yes - John P. Fletcher: Yes - Klemens Morgenstern: Yes - Peter Dimov: Yes - Zach Laine: Yes The following people also participated to the review by providing informal feedback: - Edward Diener - Gavin Lambert - Niall Douglas - Paul Fultz II - Tim Song I also based my decision on previous experience I've had using the library, and some issues we faced in the Standard Committee where we could have used CallableTraits. I'd like to thank everyone who participated to the review for their feedback, and also Barrett Adair for his submission. The following lists the conditions for acceptance, and also summarizes the general feedback received during the review. GitHub issues are leveraged to make the tracking of these comments easier. Below, whenever I refer to #XYZ, the corresponding ticket can be found at https://github.com/badair/callable_traits/issues/XYZ To see all the issues that were discussed during the review, you can search for tickets labeled with `review` on GitHub (please allow some time for Barrett to update the new issues I just filed): https://github.com/badair/callable_traits/issues?page=1&q=+label%3Areview Conditions for acceptance ------------------------- The following are points that must be addressed for the library to be accepted. Once all of the associated GitHub issues are closed, I ask that the author post to this list with links to the closed issues. Unless major problems (having to do with the closed issues) are raised at that time, the library will be automatically accepted into Boost, no additional review required. - The library needs to clearly document the advantages of CallableTraits over FunctionTypes. Concrete examples must be given (at least one, preferably two). See #117. - The level of C++ conformance required for the whole library needs to be clearly stated in the documentation. Individual components should only document divergences from that base level of conformance. The current state of things is quite confusing for users. See #118. - The "algorithmic" parts of the library must be removed, since they are out of scope. This has already been partly done in issue #114, but the following algorithms must be discussed too (let's follow up in #114): + args_at + clear_args + pop_front_args + push_front_args + expand_args_right + expand_args_left - The lazy metafunctions and predicate traits should be SFINAE-friendly, just like the aliases are. I think this inconsistency is a big deal, enough to be a condition for acceptance. See #134. Summary of the feedback received -------------------------------- The following is a compilation of other points that were made during the review, but which are not requirements for acceptance. I trust, but do not require, that all of those points will be considered by the author: - There is a loss of information with the `args` metafunction. For example, `void(T)` and `void(T, ...)` both give the same result. See #132. - Consider adding a way of obtaining the parameter list of a function without the type of `*this`. See #133. - The documentation uses the terms of art "implementation defined" and "undefined behavior" incorrectly. See #103 and #131. - The non-reference documentation should present at least one real-world use case. See #116. - The library should handle top-level cv-qualifiers. See #106. - Consider supporting different calling conventions (e.g. `__cdecl`). See #102. - Consider adding a pre-C++17 implementation of `is_invocable` and friends. See #119. - The documentation should clearly state that the library can be used without the rest of Boost. See #99. - Consider combining `expand_args` and `args` into a single metafunction with a default "output" parameter of `std::tuple`. See #126. - Consider adding `make_fn` and `make_mem_fn` metafunctions to construct a signature from a return type and a tuple of the arguments. See #113. - Consider renaming some traits, like `is_reference_member`. See #108. - Reword some parts of the documentation that look odd due to the use of passive tense. See #105. - Consider adding a version of `apply_member_pointer` which carries cv-qualifiers. See #138. - Consider adding `remove_member_cv_ref` and `copy_member_cv_ref`. See #139. - Why does `qualified_parent_class_of` always returns a const& for PMDs? See #112. - Why does `qualified_parent_class_of` return a ref-qualified type even for unqualified PMFs? See #112. A few of my own comments: - Consider using alphabetical order in the table of contents of the reference. See #135. - Consider renaming `apply_member_pointer` to something like `make_member`. See #136. - Consider dropping the `parent_` in `parent_class_of`, or adding a note to justify the name. See #137. Again, thanks to everyone who participated in the review, and congratulations to Barrett for his work! Regards, Louis Dionne

2 3

Re: [boost] Boost CRC maintainer (Daryle Walker) missing in action?
by Olaf van der Spek 19 Jun '17

19 Jun '17

Hi Daryle, Any luck on your new setup? Gr, Olaf On Mon, Nov 28, 2011 at 1:11 AM, Daryle Walker <darylew(a)gmail.com> wrote: > I can look at it once I've finished my new setup. > > I had to reset my computer after a partial crash years ago. I never > got around to recreating my programming setup. My computer had been > at its maximum Operating System level back then, which is now 3 > versions behind. For the past few months, I've had my sister's old > laptop, with Windows Vista, and am trying to add a programming setup > to it now. > > > On Tue, Nov 22, 2011 at 7:18 AM, Beman Dawes <bdawes(a)acm.org> wrote: >> Hi Daryle, >> >> This guy has volunteered to take over maintenance of Boost CRC. Should >> we take him up on his offer? > >> ---------- Forwarded message ---------- >> From: >> Date: Wed, Nov 2, 2011 at 7:51 AM >> Subject: [boost] Boost CRC maintainer (Daryle Walker) missing in action? >> To: boost(a)lists.boost.org >> >> >> Hi, >> >> It looks like the Boost CRC maintainer (Daryle Walker) is missing in action? >> >> No response to tickets from the past 3 years. What's the procedure in this case? >> >> https://svn.boost.org/trac/boost/ticket/2492 >> https://svn.boost.org/trac/boost/query?component=crc&col=id&col=summary&col… > > -- > Daryle Walker > Mac, Internet, and Video Game Junkie > darylew AT gmail DOT com -- Olaf

2 2

[review] Review of PolyCollection starts today (May 3rd)
by Ion Gaztañaga 15 Jun '17

15 Jun '17

Hi everyone, The formal review of Joaquín M. López Muñoz's PolyCollection library starts today. I'd like to encourage your participation as the proposed library is small and focused, and reviewers don't need to be domain experts to appreciate the potential usefulness of the library and propose improvements. PolyCollection implements fast containers of polymorphic objects. Typically, polymorphic objects cannot be stored directly in regular containers and need be accessed through an indirection pointer, which introduces performance problems related to CPU caching and branch prediction. Boost.PolyCollection implements a novel data structure that is able to contiguously store polymorphic objects without such indirection, thus providing a value-semantics user interface and better performance. Three polymorphic collections are provided: * boost::base_collection * boost::function_collection * boost::any_collection dealing respectively with classic base/derived or OOP polymorphism, function wrapping in the spirit of std::function and so-called duck typing as implemented by Boost.TypeErasure. The library can be found here: Incubator: http://blincubator.com/bi_library/polycollection/?gform_post_id=1643 Github: https://github.com/joaquintides/poly_collection and the documentation here: http://rawgit.com/joaquintides/poly_collection/website/doc/html/index.html Please post your comments and review to the boost mailing list (preferably), the Boost Library Incubator. or privately to the Review Manager (to me ;-). Here are some questions you might want to answer in your review: - 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? And most importantly: - Do you think the library should be accepted as a Boost library? For more information about Boost Formal Review Process, see: http://www.boost.org/community/reviews.html Waiting your reviews! Ion

13 51

[beast] Request for Discussion
by Vinnie Falco 12 Jun '17

12 Jun '17

Beast provides low level HTTP and WebSocket interfaces built on Boost.Asio, the project is here: https://github.com/vinniefalco/Beast The plan is to solicit feedback from the Boost community before submitting a request for a formal review. First a brief update: I have attended CppCon 2016 and provided a 15 minute Lightning Talk on the library. There should be a video up at some point with all the talks, including mine. I spread the word about Beast and discovered that there are several folks in the wild already using the library and happy with it. There are also a couple of commercial products that are using it. One point brought up during the conference is that Beast does not offer sufficient higher level HTTP functionality. This omission is intentional. It is my idea to offer the building blocks (serializing and deserializing HTTP/1 messages, in synchronous and asynchronous flavors, and a universal HTTP message model), upon which other developers more knowledgeable than me could build those interfaces. The higher level pubic operations requested were: 1. Perform HTTP GET or POST and receive the response. Presumably this would handle both plain and TLS connections, handle redirects, and handle HEAD requests. Not sure about traversing proxies. 2. Expose an asynchronous HTTP server class which callers can instantiate with some sort of handler that returns the HTTP response given a request. It should be possible, in a minimal number of lines, to quickly get a server up and running. The requestor informed me that this functionality did not need to provide every possible feature or handle every use case. It just needed to handle the common use-cases. For the record, I am against this higher level functionality for a number of reasons. The higher the level of functionality, the more contentious the feature becomes, and the harder it is to satisfy a quorum of reviewers. No one would dispute that we need the ability to read a complete HTTP message from a socket. But not everyone will agree that a HTTP server needs an option to allow multiple listening sockets with different level of permissions. Another problem with offering higher level functionality is that it could be less likely to be approved for standardization in the C++ standard library. For example, the attributes of ease of use and fully functional oppose each other. If the functionality offered by a hypothetical std::http::client is not sufficient, a developer would be forced to write their own with little possibility to reuse parts of the implementation from the stdlib. I doubt the committee would approve of such a thing. I know I wouldn't. Admittedly I have little experience or knowledge on what would or would not get approved. A point was raised that a low-level-only HTTP implementation in Boost has the potential to discourage new users that need to perform simple operations like HTTP GET from using Boost in the future, given that robustly handling such operations would require the developer to write a significant amount of code. I can see some validity here. On the other hand, if Boost.Asio was submitted for a formal review today, would those same points be raised? Would Asio get rejected because it doesn't offer public interfaces for doing simple tasks like requesting an object from a web server? I wonder if perhaps the Boost formal review process has become defective. For reference here is what the current API offers: Example program that performs HTTP GET, then receives and prints the response: https://github.com/vinniefalco/Beast/blob/master/examples/http_example.cpp This is a high performance asynchronous HTTP server that serves local files as static pages. It is not an official API, it is part of the examples. Users who want a server need to copy the source code and modify it to suit their needs: https://github.com/vinniefalco/Beast/blob/master/examples/http_async_server… My initial purpose for requesting this discussion is two-fold: 1. Determine the level of consensus on the issue that Beast needs to do more for HTTP than the lowest level of functionality (reading and writing messages). I'm open to hearing all arguments for and against. 2. Get feedback on what official public interfaces to higher level functionality might look like. Specifically, a client that handles GET and POST, and a generic asynchronous server. And what features those interfaces would support. For example, is traversing a proxy a necessary feature for an official client interface? The best answers would provide function signatures or class declarations using the existing Beast HTTP types. In case its not clear, I am asking stakeholders and reviewers to help design the library. I'll open a discourse on the HTTP client to illustrate the proposed style of discussion: We want to provide a HTTP client implementation that lets users perform simple tasks like fetch a resource from a remote host. Calling code should be compact, letting people get their job done without getting mired in boilerplate. Here's a possible signature with example use: response_v1<string_body> get(std::string const& url, error_code& ec); error_code ec; auto res = get("http://boost.org", ec); if(! ec) std::cout << res; else std::cerr << ec.message() << '\n'; The get function would automatically perform the name resolution, connect the socket, request the resource with uri "/", and return the result. There's a problem here though, the caller is forced to use a message with std::string as the container for the body. An obvious fix is to add a template parameter: template<class Body = string_body> response_v1<Body> get(std::string const& url, error_code& ec); Better, but what if the body is not DefaultConstructible? We can add some parameters to forward to the Body constructor: template<class Body = string_body, Args&&... args> response_v1<Body> get(std::string const& url, error_code& ec, Args&&... args); We're getting somewhere now. Most users will use the default body, while an option exists for advanced users. But the implementation of get now has a complication. If an error occurs, it still has to construct a return value. This problem is likely surmountable but lets table it and start a new line of reasoning. What if the URI indicates https, e.g. "https://boost.org"? We can hide that under the hood by using a separate code path that uses asio::ssl::stream instead of ip::tcp::socket. But how does the caller provide the root CA certificates? How does the caller provide a client certificate? We are left with a client that satisfies two thirds of users but is unusable for the remaining one third. Is this Boost worthy? Is this std-lib worthy? I think not. Lets revisit the SSL issues later and think about a new problem. What happens to the socket when the request is done? In the proposed interface, the socket is destroyed. This means you can only make one request on a connection. At this point, I think its clear that the current interface is a non-starter. Here's a better idea. Lets define a class called client and make get a member function. Now we can have state, including configurable settings with reasonable defaults. class client { public: template<class Option> void set_option(Option&&); template<class Body = string_body, Args&&... args> response_v1<Body> get(std::string const& url, error_code& ec, Args&&... args); }; client c; error_code ec; auto res = c.get("http://boost.org", ec); if(! ec) std::cout << res; else std::cerr << ec.message() << '\n'; We still have some of the original problems, but other problems are solved. The class can keep the socket around, so that a subsequent call to get with the same server will use the already-open socket. The caller can set options on the client object instance before invoking get, such as any TLS certificates. Unfortunately we have new problems. Where is the io_service? If the class creates it for you, how many threads? How do I use my existing io_service, or is that even necessary? The class interface allows the connection to be kept alive but what if now the caller wants to close it (i.e. specify "Connection: close" in the request)? And there are some problems which are applicable to any interface. How does the caller adjust the parser options, for example change the limit on the size of the body? What if the caller wants the request made using a specific HTTP version? How does this handle basic authentication? How can the caller adjust the headers in the request? etc... This is why I think that offering a HTTP client is a rabbit hole which can only lead to a rejected formal review. Or maybe I am overthinking this? I'd love to hear proposed solutions to the issues (which could be just to say "forget about those features"). Despite my objection I have started work on these features anyway. Here are the beginnings of a HTTP client: https://github.com/vinniefalco/Beast/blob/88b485df7e6216282842f40cf99cc75de… Here is some work on a generic server: https://github.com/vinniefalco/Beast/blob/server/extras/beast/http/async_se… Please reply to this thread instead of starting a new thread, and keep the subject line prefix (e.g. "Re: [boost] [beast] ...") Thank you

21 58

[outcome] Second high level summary of review feedback accepted so far
by Niall Douglas 11 Jun '17

11 Jun '17

Firstly thanks to everyone who has contributed no less than **618** emails at the time of writing to the Outcome review. Despite some on Reddit calling this review a "train wreck" and indeed some lovely things about me personally, I have found this review deeply helpful, and I believe so have most of its contributors in helping us all firm up our differing opinions about what such a vocabulary type ought to be. With regard to the previous high level summary at http://boost.2283326.n4.nabble.com/outcome-High-level-summary-of-review-fee…, I have made only these changes to the changes in that summary: 1. I have been persuaded to use longer more appropriate naming for result<T> and outcome<T>, so now the typedefed varieties with implicit conversions to their empty-capable form indicated by "=>" are: - static_checked_outcome<T> => static_checked_optional_outcome<T> static_checked_result<T> => static_checked_optional_result<T> These have narrow observers which reinterpret_cast and are therefore undefined behaviour if the state does not match, but use __builtin_unreachable() to help static analysis tooling like clang-tidy spot UB where it can be statically deduced. Conceptually, they are a very thin convenience wrap of std::variant<...>. - runtime_checked_outcome<T> => runtime_checked_optional_outcome<T> runtime_checked_result<T> => runtime_checked_optional_result<T> The runtime_checked_optional_* varieties are identical and unchanged to the outcome<T> and result<T> in the submitted library. They have wide observers with a formal empty state and it is never undefined behaviour to call any observer. This allows the programmer to save typing considerable boilerplate knowing that the runtime checks save the programming needing to spell things out. I have not been persuaded to change a single thing in their design yet apart from removing unnecessary member functions, but thanks to everybody who has tried, and if you think I am making a critical mistake, you still have a few days left to persuade me of it! For anyone not familiar with the discussion and feels that the names are excessively long, be aware they are just convenience typedefs of a more complex to configure "template<...> class outcome_impl". It is expected that end users of the library will choose the varieties they want for their code and typedef them into their local namespace as "result<T>" or whatever. Nobody is proposing that end users actually type out the full name each and every time they use them, and the documentation will make that clear. 2. I have been persuaded to replace the implicit conversion from static_checked_* varieties to runtime_checked_* varieties with explicit conversion instead as the implicit conversion could produce runtime failure where the programmer did not expect that to occur. Regarding other changes that I have accepted since the previous high level summary: 1. Outcome's STL exception throw catch to result<T> super-macro was found to be overkill given that current compilers won't replace simple throw...catch sequences with a branch, and still invoke the full throw-catch runtime machinery whatever the case. Therefore an exception to error code conversion function makes more sense than a macro. https://github.com/ned14/boost.outcome/issues/50 2. Outcome's usage of git submodules is felt by some reviewers to be a showstopper for acceptance as they could bring in non-Boost code unexpectedly in the future. I have agreed to make a cronjob script generated Boost.Outcome from standalone Outcome in the same fashion as Boost.ASIO is script generated from standalone ASIO. https://github.com/ned14/boost.outcome/issues/51 - Licensing to be BSL only - No cmake, just bjam - No git submodules at all - std-flavoured, not boost-flavoured - No code to be brought in except that written by me i.e. a hardcoded whitelist applies. - Contents of Boost repo to only contain material directly relevant to Boost. Nothing else. 3. Under the assumption that error_code_extended's use of a static global ring buffer would be controversial in this review, I hacked a quick and dirty solution expecting to have to remove it. Now we know that few disapprove, a more serious implementation will be needed. https://github.com/ned14/boost.outcome/issues/52 4. Given where Vicente appears to be heading next with expected<T, E1, E2, ...> it may be wise to make the static_checked_* varieties use std::variant<...> for storage when being compiled on C++ 17. https://github.com/ned14/boost.outcome/issues/53 Thanks, Niall -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/

10 52

[outcome] High level summary of review feedback accepted so far
by Niall Douglas 05 Jun '17

05 Jun '17

Some reviewers have asked what form Outcome will take in the near future. The following is a high level summary of the changes already made on develop branch or will be made according to https://github.com/ned14/boost.outcome/issues logged from review feedback. I have left out some minor issues, this lists just the major changes. This should help reviewers to decide whether to recommend acceptance or rejection. If there are any questions, please do ask. Major design changes: ===================== - option<T> to be removed https://github.com/ned14/boost.outcome/issues/48 - outcome<T> and result<T> will have their empty state removed, and all observers gain narrow contracts. Default construction is disabled. https://github.com/ned14/boost.outcome/issues/44. Example: ``` result<Foo> v(make_errored_result(std::errc::invalid_argument)); assert(v.has_error()); // true assert(!v.has_value()); // true // As if Foo(reinterpret_cast<Foo &&> // (error_code_extended(std::errc::invalid_argument)); Foo f(std::move(v.value())); assert(v.has_error()); // still true assert(!v.has_value()); // still true ``` (NOTE: expected<T, E> will track whatever LEWG Expected does, but it differs from what will become result<T> by having a wide contract on .value() and narrow contracts on operator*(), .error(), operator->(). result<T> will have narrow contracts on everything, it is basically a thin wrap of std::variant<T, error_code_extended> except with strong never empty warranty) - New typedefs outcome_e<T> and result_e<T> are identical to outcome<T> and result<T> except for adding a formal empty state. Observer contract slightly widens, an attempt to use an empty object throws a bad_outcome_access exception. Implicit conversion from non-empty-capable varieties is permitted to empty-capable varieties, but not the other way round. Default construction is to **empty**. https://github.com/ned14/boost.outcome/issues/44 - New typedefs checked_outcome<T>/checked_result<T>, checked_outcome_e<T>/checked_result_e<T> are added. These mirror the editions just described, but checks and default actions occur on all observer usage so hidden reinterpret_cast<> never occurs. Implicit conversion from non-checked varieties is permitted to checked varieties, but not the other way round. https://github.com/ned14/boost.outcome/issues/47. Examples: ``` // Note result<T> implicitly converts to checked_result<T>, but not // the other way round. So we can use same make_errored_result(). checked_result<Foo> v(make_errored_result(std::errc::invalid_argument)); assert(v.has_error()); // true assert(!v.has_value()); // true // .value() throws std::system_error( // std::make_error_code(std::errc::invalid_argument)); Foo f(std::move(v.value())); ``` ``` checked_result<Foo> v(make_valued_result(Foo())); assert(!v.has_error()); // true assert(v.has_value()); // true // .error() returns a default constructed (null) error_code_extended // when result is valued to indicate "no error here" error_code_extended ec(std::move(v.error())); assert(!ec); // true ``` - The presented library self generates using lots of preprocessor based multiple #include's to stamp out finished editions of implementation. The git repo partially preprocesses this into a nearly finished edition for minimum compile time load for end users. Due to the many more varieties of outcome<T> and result<T> provided, the preprocessor machinery would be replaced with a template based machinery instead. I will look into making optional use of extern template and a static library to retain minimum compile time load, but header only usage will remain possible. Smaller design changes: ======================= - tribool stuff to go away in default configured build. https://github.com/ned14/boost.outcome/issues/22 - .get*() functions will be removed. https://github.com/ned14/boost.outcome/issues/24 - Don't include <windows.h> on winclang. https://github.com/ned14/boost.outcome/issues/39 Documentation changes: ====================== - Licence boilerplate missing on some files. https://github.com/ned14/boost.outcome/issues/38 - Reference API docs need to spell out exact preconditions, postconditions and semantics per API. https://github.com/ned14/boost.outcome/issues/26 - Reference API docs need to describe types void_rebound, *_type and raw_*_type in detail. https://github.com/ned14/boost.outcome/issues/29 - User overridable macros which throw exceptions and provide customisation points have vanished from the docs somehow. https://github.com/ned14/boost.outcome/issues/35 - Landing page and introduction to be broken up into bitesize single page chunks. https://github.com/ned14/boost.outcome/issues/41 Still to be decided: ==================== - Should *_e() varieties provide convenience .get(), .get_error(), .get_exception() which returns T, error_code_extended and std::exception_ptr by value moved from internal state, resetting state afterwards to empty? These would mirror future.get()'s single shot observers. Niall -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/

10 104

[outcome] non-interface-related concerns
by Andrzej Krzemienski 05 Jun '17

05 Jun '17

The discussion mostly revolves around the design of the interface of `outcome::expected` and even `std::expected`. My questions here are about the way this library will be distributed to users. And I am uncomfortable with the situation. I am not necessarily saying it is wrong, but I do not understand what is going on here. 1. The current GitHub repository consists of sub-repos. Do you (Niall) plan to have it like this when the library is accepted? Is it relevant? 2. This macro BOOST_OUTCOME_DISABLE_PREPROCESSED_INTERFACE_FILE, it controls whether I use this "preprocessed" file versus if I use... what? Normal header files? What is gained by this preprocessed file? Not having to include more than one header, or something else? Can I get different results when compiling with and without this macro? 3. I am recommended to use this macro BOOST_OUTCOME_V1_NAMESPACE. I am told it has something to do with ABI stability? Can you elaborate on how this guarantees ABI stability. Or is its purpose different? I have a certain problem with understanding this philosophy. A couple of days back, I reported the compilation failure in MinGW. I can see that the issue (https://github.com/ned14/boost.outcome/issues/36) is reorted as fixed in branch 'develop'. I would like to confirm that the bug is fixed. I checkout branch develop. There is no branch develop for the sub-repos, so what am I supposed to checkout, master, or some particular commit? I went with master. I do not understand a lot of CMake, but since it is a header-only library, I assume that I can just #include the file "include/boost/outcome.hpp", and my programs (compiled manually) should compile. But the only change in the commit that was supposed to fix the bug, was in a .cmake file, so I cannot see how it would help fix a header only library. Also, would the fix be visible in a "precompiled" header, or normal headres, or both? I am not complaining about this particular fix. I would like to know how in this system of mcros, sub-modules and preprocessed headers, I can figure out what changes will be observed in my program, when I see a particular change in git sub-repo. Regards, &rzej;

10 42

[outcome] To variant, or not to variant?
by Gavin Lambert 03 Jun '17

03 Jun '17

I probably should have thought of asking this earlier, but it occurs to me now that my own mental model of how an "outcome"-ish type should act is probably not suited to variant storage at all. So just out of curiosity I thought I'd ask whether people prefer this sort of interface: template<typename T> class outcome { public: bool has_value() const { return m_storage.has<T>(); } T& value() { return m_storage.get<T>(); } bool has_error() const { return m_storage.has<error_code>(); } error_code& error() { return m_storage.get<error_code>(); } bool has_exception() const { return m_storage.has<exception_ptr>(); } exception_ptr& exception() { return m_storage.get<exception_ptr>(); } void set(none_t) { m_storage = none; } void set(const T& val) { m_storage = val; } void set(error_code err) { m_storage = err; } void set(exception_ptr ep) { m_storage = ep; } private: variant<none_t, T, error_code, exception_ptr> m_storage; }; Or this sort of interface: template<typename T> class outcome { public: bool has_value() const { return !!m_value; } T& value() { ensure(); return m_value.value(); } //bool has_error() const { return !!m_error; } const error_code& error() const { return m_error; } //bool has_exception() const { return !!m_exception; } const exception_ptr& exception() const { return m_exception; } void ensure() const { if (m_exception) { rethrow_exception(m_exception); } if (m_error) { throw system_error(m_error); } } void set(none_t) { m_value = none; m_error = error_code(); m_exception = nullptr; } void set(const T& val) { m_value = val; m_error = error_code(); m_exception = nullptr; } void set(error_code err) { m_value = none; m_error = err; m_exception = nullptr; } void set(exception_ptr ep) { m_value = none; m_error = ep ? error_code(errc::has_exception) : error_code(); m_exception = ep; } private: optional<T> m_value; error_code m_error; exception_ptr m_exception; }; (Don't get too hung up on the specifics. This is a sketch, not a real implementation. I've obviously omitted things that a real implementation would need such as more const methods and move support, and construction and assignment rather than using set methods. The focus is on variant vs. non-variant.) Anyway, the point is that this could actually transport multiple things; in particular as above both an error_code and an exception_ptr; perhaps set(error_code) could construct an exception as well, although I've explained elsewhere why I don't like that option. Perhaps this could even be exposed even more so that user code could explicitly provide both a value and an error -- think "here's a value, but it was truncated" or "here's the 5 values you asked for, but there's more", both fairly common in system APIs. Or for things like ambiguous matches where you still return the most likely candidate. Or dictionary insertion (duplicate key, but here's the value that's already there). Or many more such examples. (Though obviously in this case value() couldn't call ensure(). But that should make some people happy.) Also note has_error() and has_exception() don't even need to be provided in this version since you can always call error() and exception() in a boolean context anyway, with the same result. (Or actually a slightly better result than the implementation shown here.) The obvious downside is that it now uses more storage, which might hinder some inlining cases.

9 63