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Re: [boost] [openmethod] On the unreasonable efficiency of fast_perfect_hash
by Joaquin M López Muñoz 12 May '25

12 May '25

El 12/05/2025 a las 12:06, Ivan Matek escribió: > > > On Sun, May 11, 2025 at 9:01 PM Joaquin M López Muñoz via Boost > <boost(a)lists.boost.org> wrote: > > > https://github.com/joaquintides/perfect_range_hash > > > Thank you for this. Unfortunately even this basic math is a bit too > complex for me > so few small comments: > > 1. stylistically I would link to particular hash of source code, > although documentation > link is nicer. Reason is that documentation link can die I can relink to a more stable doc if the lib is accepted. > 2. afaik C is not randomly chosen between 0<= C <= 2^64, i.e. it is > always an odd number. You're right. We'd have to ask the author, but I guess the "| 1" in "hash_mult = uniform_dist(rnd) | 1" is there to avoid the (very unlikely) case hash_mult = 0. In any case, this small adjustment doesn't change the big picture statistically --the output distribution is largely dominated by C/B, where C is the multiplier and B is 2^(64-m), so the least significant bit of C is mostly swallowed along the way. > 3. in my brief experiments I tried to add large gap, and randomly pick > subset of types, > and performance was still amazing. > > Example for 3.: [...] This case of 231 type_ids scattered in a range of 3000 positions is lower-bounded by (has better chances than) the baseline case with 3000 adjacent positions, which has a probability 0.14 of finding a perfect hash multiplier for 4096 buckets. Note that a "hard" case would involve having your type_ids distributed across the entire 64-bit space of possible numerical values, and in the example you wrote the values are confined to a range that is around 10^14 times smaller than 2^64 --this is the fact that the algorithm takes an implicit advantage of. Joaquin M Lopez Munoz

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[openmethod] On the unreasonable efficiency of fast_perfect_hash
by Joaquin M López Muñoz 12 May '25

12 May '25

Some days ago we discussed the fact that the perfect hashing machinery of candidate Boost.OpenMethod is apparently at odds with the theoretical probability of finding a viable function by chance (i.e. using brute force). I've taken a closer look at this issue and wrote a small article explaining why Boost.OpenMethod's fast_perfect_hash is so exceptionally successful: https://github.com/joaquintides/perfect_range_hash I hope the analysis is of interest to those puzzling over this. Joaquin M Lopez Munoz

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[graph] which algorithm/visitor to use?
by Andrzej Krzemienski 11 May '25

11 May '25

Hi All, I have a graph-related problem that I thought was easy, but I do not seem to be able to find the right tools (algorithms, visitors) in the library to solve it. Maybe someone in the list can give me a hint. I represent airports and their air connections as a graph: * Each airport is a vertex. * There are geographical coordinates associated with each airport. * When there is at least one flight operating between airports A and B, this is represented as an edge. The task is to enumerate all paths between two given vertices U and V, in an efficient way naturally, given the following constraints: 1. The path cannot be composed of more than N edges 2. The path cannot contain a vertex "too far away from U and V", that is, there would be a predicate P that tells for a given vertex W the value P(U, V, W) that tells if vertex W is acceptable. So, I would need a tool that instructs the algorithm that when a certain vertex or an edge is encountered, an algorithm should be skipped on a given branch of the graph, but resume from the next branch. Is there a tool like that in Boost.Graph? Regards, &rzej;

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[OpenMethod] Andrzej's review
by Andrzej Krzemienski 08 May '25

08 May '25

Hi Everyone, This is a review of the proposed Boost.OpenMethod library. First of all, I want to thank Jean-Louis for writing and sharing this library, for undergoing the Boost Review process, and for reviving this interesting and important subject. My recommendation is to *reject* the Boost.OpenMethod library at this time. My decision is based on the design issues. I note that rejection is not a permanent verdict, and there are known Boost libraries that were initially rejected, and then were improved and passed the second review. The library claims to solve two problems: the "expression problem" and the "banana-gorilla-jungle problem". In my opinion, the first problem is not a technical problem worth solving at all. It is more like a logical challenge: can you provide a framework extensible in two dimensions? Maybe I can, but so what? What real program or library benefits from this? The second problem is real, and although the docs do not present it adequately, during the discussion, and also via links to YOMM2 library, Jean-Louis provided a very compelling example of printing a Matrix which is from a OO hierarchy. I am personally not a fan of OO. I use very sparingly anything that has `virtual` on it. For instance, for heavy matrix computations, I would never think of using OO-hierarchies of matrices. It looks to me that OOP is limited in a language without a garbage collection, and any attempts at fixing it can only be limited. The problem is real. I would call it differently: "I want a runtime-polymorphic behavior, but I do not want to commit to all polymorphic functions up front". The library provides a solution for it. It is not necessarily a good solution, and it is for sure not the only solution to the problem. Two other possible solutions: 1. Use std::variant and variant visitors. For a non-trivial motivating example see https://akrzemi1.wordpress.com/2016/02/27/another-polymorphism/ 2. Use OO-style class hierarchies with pattern-matching on top: either wait for the C++ pattern matching, or use a library like Mach7 ( https://github.com/solodon4/Mach7) The docs in OpenMethod are missing the discussion on these alternatives. Especially alternative #2 seems to offer a superior solution, simpler, and with fewer gotchas. It is not clear to me what is the relation between Boost.OpenMethod and N2216. Is the goal to implement N2216? Or is it just mentioned for context, or as a loose inspiration? I hear the replies in the review discussions, "but this is what N2216 said", also the docs say that the library implements N2216 with extensions. But note that N2216 calls itself "a report" and the authors never claimed that they had a solution suitable for standardization. It was an exploration. But the solution in N2216 has one important property: the program would never get a "runtime error" indicating a bad usage of the feature: it would either be a compiler/linker error or an arbitrary -- but *deterministic* -- selection of the overload. N2216 allows no "pure virtual" open methods: it requires that you provide a definition for the "top level" declaration. This way you always have an implementation to fall back to. Also its solution has a consequence that a different overrider can be chosen in different loaded DLLs for the same combination of concrete types. This solution is counterintuitive to me and looks similar to the ODR-violation. It is also possible that two DLLs provide a different overrider for the same combination of types. But this is not surprising. The problem of selecting "the right" overrider (whatever it means, given the two-dimensional flexibility) is not solvable. N2216 makes this lack of a good solution very clear. It also shows how other languages fail to solve it (for instance, requiring a "glue code" to be provided by the person who assembles the final program, which defeats the purpose of the "two dimensional flexibility" of open methods. Anyway, the solution for the overrider selection in N2216 is very different from the one in Boost.OpenMethod, so it is incorrect to say that Boost.OpenMethod implements N2216. It does not. It chose a different approach. In this different approach, you can get a runtime error. Worse still, you can sometimes get a runtime error and sometimes an arbitrary overrider, and it is difficult to remember when which happens. N2216 says that a new linker technology would be required to guarantee no-runtime-error. Boost.OpenMethod seems to lack it. As a consequence, we are getting a feature with a lot of type-safety holes. They are plugged by throwing exceptions and calling std::abort(), but they are still holes: you cannot preemptively prevent them, because of the announced flexibility: anyone can extend the type hierarchy or introduce new open-methods at run-time, via linking a DLL. Boost.OpenMethod is inherently type-unsafe and advertises this as flexibility. It is a common practice for library functions to have *preconditions* and to enforce them by calling std::abort() or throwing an exception, but this works only because the users have the means of controlling the correct usage of a function: sqrt(x); And I could have checked before the call that `x` is negative, or I could have made sure that `x` is the result of summing squares. Because I am in control. But in this call: to_string(matrix); I am not in control of satisfying a precondition, because I do not control what other matrix classes have been added to the hierarchy by other users in other DLLs that I may not even be aware of. This is why exceptions or std::abort() are not a good solution anymore. It is no longer a viable precondition. Another difference between Boost.OpenMethod and N2216 is that in the latter a programmer never says that they are overriding a virtual method. The compiler has to deduce the fact from the signatures. Another difference is that N2216 makes it an error to define a parameter as virtual when a corresponding class s not a reference to a polymorphic type. This is a feature (a safety feature). Boost.OpenMethod does not have it: I can pass any type I want in virtual_ptr only to get an abort at run-time: https://godbolt.org/z/b1Y5zMcfh Given this type of a library, where: * A language feature is implemented as a library * The feature is about declaring interfaces (rather than used for internal implementation, such as BOOST_FOREACH) * The feature itself is controversial and not well explored * It cannot be implemented as a library * The feature is complex I would expect a way more of a discussion and explanations in the documentation. More concepts and definitions. The lack of those, makes me further question the soundness of the design. For one instance, the statement, "Multiple and virtual inheritance are supported, with the exception of repeated inheritance." What does it mean "not to support a form of inheritance"? Compiler error? Run-time abort()? Undefined Behavior? I can check what the current implementation does, but is this guaranteed, or is it a happenstance of the today's implementation? Now, let's compare open methods with pattern matching that can distinguish different concrete classes in a type hierarchy. Regardless if they are language features or library emulations. The type-switch (as in wg21.link/n3449) or "match-expression" (as in wg21.link./p2688) is not devoid of similar problems: Someone may add a new class to the hierarchy after I have implemented my match-expression: double get_area(const Shape& shape) { return shape match { Circle: let [r] => 3.14 * r * r; Rectangle: let [w, h] => w * h; _: throw MyLib::Unimplemented(); }; } And if some new class Polygon now inherits from Shape, I will get a throw. But this time, it is plain clear, written down in my code, in what order the candidates will be considered (up-to-bottom, first match). No surprises of the form, "I thought that X would be a better match". Earlier, I made a claim that the authors of N2216 dropped the idea in favor of pursuing pattern matching. I cannot find any citation that would support this. I guess I got that from the fact that two of the authors of N2216 -- Bjarne Stroustrup and Yuriy Solodkyy -- later wrote papers only on pattern matching (wg21.link/n3449, https://www.stroustrup.com/OpenPatternMatching.pdf) Maybe I got this from the ISO C++ mailing lists. If I understood correctly, the idea behind virtual_ptr is that it should be created early to front-load the expensive hash table look-ups. This tells me that even though I have to use a lot of macros in my interfaces, they are still not able to hide the implementation details, and I am exposed to these details through the pieces of advice, such as "pass around virtual_ptr and store them as members". Then I am exposed to more things, like final_virtual_ptr (not to mention unique_virtual_ptr and shared_virtual_ptr). Here's an important point. Boost is known to deliver language features in form of libraries. But the ones I know were to be used inside function bodies: BOOST_FOREACH, BOOST_AUTO. So my callers do not have to know that I am even using them. It is my business. But with Boost.OpenMethod, we are talking about declaring interfaces. When I -- a library author -- make a choice to use a Boost.OpenMethod-style open method, how do I explain all these quirks to my users? Now I am spilling all those implementation details, micro-decisions, limitations to the poor users. Now, I have a more philosophical question. If we declare open-methods (or even pattern-matching-like type switches) non-intrusively on class hierarchies, these hierarchies no longer need to have virtual functions: he polymorphic behavior can now be implemented as open methods. In that case, what is the purpose of declaring class hierarchies? Previously the answer was clear: the types are in the OO-hierarchy because while they have different implementations, they provide the same interface. Now, with open methods they do not provide the same interface. So why put them into a hierarchy? Every library or a language feature or a tool is a burden: you have to learn it, your users have to learn it, you need to debug it. In order for it to be useful, the costs of usage/maintenance must be outweighed by the benefits of using it. I do not see this being the case for Boost.OpenMethod. This is my main objection. I have smaller observations that could be fixed: * There is a number of mysterious but unexplained terms used, such as "guide functions" or "with the virtual_ decorators stripped". * Things are imported into a global namespace (such as virtual_ptr). Don't do it by default. If users want to see virtual_ptr and final_virtual_ptr as keywords, let them type `using namespace boost::openmethod::keywords;`. * The framework behaves differently based on whether my types are copyable or not. Compare: ** https://godbolt.org/z/zKzYzbj3x ** https://godbolt.org/z/Ya8TrE79K For some standard review questions: * I have played with some toy examples using Clang on Compiler Explorer and using GCC on MinGW on Windows. No problems running the examples. * I spent a couple of days in total, studying the docs, re-reading related papers, running examples, filing bugs, and demonstrating the library in my workplace. * I have kept an eye on the papers on open-methods, open type-switch and pattern matching, playing with Mach7 library. Finally, this review may seem very negative, so let me stress again that I appreciate the work Jean-Louis has done to explore the subject. A critique can be seen as a form of appreciation. Also, thanks to Дмитрий for managing the review. Regards, &rzej;

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Re: [boost] [OpenMethod] review starts on 28th of April
by Ruben Perez 07 May '25

07 May '25

<CC-ing the mailing list, since you just replied me, and answering inline> On Tue, 6 May 2025 at 01:35, Jean-Louis Leroy <jl(a)leroy.nyc> wrote: > > Your example is very similar to > https://jll63.github.io/Boost.OpenMethod/#tutorials_custom_rtti. > > virtual_ptr does not try to use boost_openmethod_vptr in its constructors. That > is an oversight. It is a recent invention. YOMM2 tests for the presence of a > boost_openmethod_vptr instance variable in the object directly. In that context, > the function is very simple and inlinable, but if it is user-defined, there is > no reason to assume that. So I will make virtual_ptr consider it as well. > > Regardless, the easiest way to handle your example is to put the vptr > acquisition in the policy. dynamic_type returns a type_id, not a vptr. The > function for that is dynamic_vptr. It is not part of a facet, eventhough facets > can provide one. The dispatch mechanism is described here: > https://jll63.github.io/Boost.OpenMethod/#ref_description_17 > > WARNING: if you try that code with the "review" branch, make sure to pull the > latest changes, because they fix a bug I introduced while re-establishing checks > for non-polymorphic classes. > > Thus: > > > // ------------------------ > // you > > #include <cstdint> > > enum class kind : std::uintptr_t { unknown, n1, n2 }; > > class base_node { > kind k_; > > protected: > base_node(kind k) noexcept : k_(k) { > } > > public: > kind getKind() const { > return k_; > } > }; > > class node1 : public base_node { > public: > node1() noexcept : base_node(kind::n1) { > } > }; > > class node2 : public base_node { > public: > node2() noexcept : base_node(kind::n2) { > } > }; > > // ------------------------ > // me > > // Get kind from static type, as you RTTI system does not provide it as a > // static member. > > template<typename T> > struct kind_of { > static constexpr kind value = kind::unknown; > }; > > template<> > struct kind_of<node1> { > static constexpr kind value = kind::n1; > }; > > template<> > struct kind_of<node2> { > static constexpr kind value = kind::n2; > }; > > #include <boost/openmethod/policies/basic_policy.hpp> > > struct custom_rtti : boost::openmethod::policies::rtti { > template<class Node> > static constexpr bool is_polymorphic = > std::is_base_of_v<base_node, Node>; > > using type_id = boost::openmethod::type_id; > > template<typename T> > static auto static_type() { > return type_id(kind_of<T>::value); > } > > template<typename T> > static auto dynamic_type(const T& obj) { > if constexpr (is_polymorphic<T>) { > return type_id(obj.getKind()); > } else { > return kind::unknown; > } > } > }; > > struct custom_policy > : boost::openmethod::policies::basic_policy<custom_policy, > custom_rtti> { > > static auto dynamic_vptr(const base_node& b) { > switch (b.getKind()) { > case kind::n1: > return custom_policy::static_vptr<node1>; > case kind::n2: > return custom_policy::static_vptr<node2>; > default: > return custom_policy::static_vptr<base_node>; > } > } > }; > > #define BOOST_OPENMETHOD_DEFAULT_POLICY custom_policy > > #include <iostream> > > #include <boost/openmethod.hpp> > #include <boost/openmethod/compiler.hpp> > > BOOST_OPENMETHOD(process, (std::ostream&, virtual_ptr<base_node>), void); > > BOOST_OPENMETHOD_OVERRIDE( > process, (std::ostream & os, virtual_ptr<node1> node1), void) { > os << "process node1\n"; > } > > BOOST_OPENMETHOD_OVERRIDE( > process, (std::ostream & os, virtual_ptr<node2> node2), void) { > os << "process node2\n"; > } > > BOOST_OPENMETHOD_CLASSES(base_node, node1, node2); > > auto main() -> int { > boost::openmethod::initialize(); > > auto a = std::make_unique<node1>(); > auto b = std::make_unique<node2>(); > > process(std::cout, *a); > process(std::cout, *b); > > return 0; > } > > We could also take advantage of the fact that type_ids fall in a short, compact > range. We can use the vptr_vector facet, but there is no need to hash the > type_ids, we can use them as straight indexes. So we don't use a type_hash > facet: > > struct custom_policy > : boost::openmethod::policies::basic_policy< > custom_policy, custom_rtti, > boost::openmethod::policies::vptr_vector<custom_policy>> {}; > > This time dynamic_vptr() is provided by a facet. Everything else stays the same. For the reference, this is what I ended up doing: enum class kind : std::uintptr_t { unknown, n1, n2 }; class base_node { kind k_; protected: base_node(kind k) noexcept : k_(k) { } public: kind getKind() const { return k_; } }; class node1 : public base_node { public: static constexpr kind node_kind = kind::n1; node1() noexcept : base_node(node_kind) { } }; class node2 : public base_node { public: static constexpr kind node_kind = kind::n2; node2() noexcept : base_node(node_kind) { } }; constexpr const char* kind_to_string(kind k) { switch (k) { case kind::n1: return "node1"; case kind::n2: return "node2"; default: return "<unknown node type>"; } } struct custom_rtti : boost::openmethod::policies::rtti { template<class T> static constexpr bool is_polymorphic = std::is_base_of_v<base_node, T>; template<typename T> static auto static_type() -> std::uintptr_t { if constexpr ( std::is_base_of_v<base_node, T> && !std::is_same_v<base_node, T>) { return static_cast<std::uintptr_t>(T::node_kind); } else { return static_cast<std::uintptr_t>(kind::unknown); } } template<typename T> static auto dynamic_type(const T& obj) -> std::uintptr_t { if constexpr (is_polymorphic<T>) { return static_cast<std::uintptr_t>(obj.getKind()); } else { return static_cast<std::uintptr_t>(kind::unknown); } } template<class Stream> static void type_name(std::uintptr_t type, Stream& stream) { stream << kind_to_string(static_cast<kind>(type)); } }; struct custom_policy : boost::openmethod::policies::basic_policy< custom_policy, custom_rtti, boost::openmethod::policies::runtime_checks, boost::openmethod::policies::basic_error_output<custom_policy>, boost::openmethod::policies::vptr_vector<custom_policy>> {}; BOOST_OPENMETHOD_CLASSES(base_node, node1, node2, custom_policy) BOOST_OPENMETHOD(print, (virtual_<base_node>), void, custom_policy); BOOST_OPENMETHOD_OVERRIDE(print, (base_node&), void) { /* whatever */ } I've just realized that I've disabled hashing by accident, and this happened to work by accident. I think that this implicit dependency between the vptr_vector and the hashing policy is a problem - I prefer explicit dependencies. > > As you see in this example, we don't have to fork policies, we can just > construct them from scratch. Forking was introduced with the idea of tuning > default_policy, without having to replicate it from scratch. Also the > with/without setters are less verbose than the add/replace/remove ones. > > Elsewhere I said that facets categories are a closed set. That is not correct. > The dispatcher and the compiler look only at a closed set of facets > (using if constexpr (Policy::has_facet>)). But some facets, like vptr_vector and > error_handler, look for other facets in the policy, which are not necessarily > referenced directly by the compiler or the dispatcher. In that sense, the set of > facets is actually open. > > I am going to think about your suggested alternative to the facet system. > Seriously, thus it will take me a few days. In the meantime, how would you see > the two example policies above, expressed with your system? This is what it'd look like with my proposal: struct custom_rtti : openmethod::policies::rtti { template<class T> static constexpr bool is_polymorphic = std::is_base_of_v<base_node, T>; template<typename T> auto static_type() const -> std::uintptr_t { /* same definition */ } template<typename T> auto dynamic_type(const T& obj) const -> std::uintptr_t { /* same definition */ } template<class Stream> void type_name(std::uintptr_t type, Stream& stream) const { /* same definition */ } }; struct custom_policy : boost::openmethod::policies::abstract_policy { // enable runtime checks openmethod::policies::runtime_checks runtime_checks; // enable the default error output openmethod::policies::basic_error_output<> error_output; // pass the hashing policy to vptr_vector explicitly, rather than implicitly via the entire Policy // disable_hashing should be either a marker type to tell vptr_vector "I don't have any hashing available" // or implement the type_hash facet and be a no-op (I guess the 1st option is better) openmethod::policies::vptr_vector<openmethod::policies::disable_hashing> extern_vptr; }; // Now register the classes. See below for the definition of registry BOOST_OPENMETHOD_CLASSES(base_node, node1, node2, openmethod::registry<custom_policy>) BOOST_OPENMETHOD(print, (virtual_<base_node>), void, openmethod::registry<custom_policy>); BOOST_OPENMETHOD_OVERRIDE(print, (base_node&), void) { /* whatever */ } // This type needs to be provided by the library, and is in charge of creating the required static storage namespace boost::openmethod { template <class Policy> struct registry { using policy_type = Policy; static Policy policy; }; } There are likely things I'm not seeing now that will probably complicate this. If the review is favorable, I can write a PR with the proposed changes and see if it's viable. > > I would love to ship a fully functional clang RTTI policy right off the bat; it > is an important use-case. > > J-L Thanks, Ruben.

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In 1.88.0, BCP creates project that doesn't build
by Mason Loyet 04 May '25

04 May '25

Hi, I am attempting to use BCP to put boost into a different namespace as below: mkdir myboost_1_88_0 curl -L https://archives.boost.io/release/1.88.0/source/boost_1_88_0.tar.bz2 | tar -xvjf - cd boost_1_88_0 && ./bootstrap.sh && ./b2 tools/bcp && ./dist/bin/bcp --namespace=myboost . ../myboost_1_88_0 cd ../myboost_1_88_0 ./b2 This results in: … error: Unable to find file or target named error: '/boost/align//boost_align' error: referred to from project at error: '/home/mloyet/external/boost-test/myboost_1_88_0/libs/atomic/build' The snippet appears to work for 1.84.0. Has something changed? Am I invoking BCP incorrectly? ________________________________ Note: This email is for the confidential use of the named addressee(s) only and may contain proprietary, confidential, or privileged information and/or personal data. If you are not the intended recipient, you are hereby notified that any review, dissemination, or copying of this email is strictly prohibited, and requested to notify the sender immediately and destroy this email and any attachments. Email transmission cannot be guaranteed to be secure or error-free. The Company, therefore, does not make any guarantees as to the completeness or accuracy of this email or any attachments. This email is for informational purposes only and does not constitute a recommendation, offer, request, or solicitation of any kind to buy, sell, subscribe, redeem, or perform any type of transaction of a financial product. Personal data, as defined by applicable data protection and privacy laws, contained in this email may be processed by the Company, and any of its affiliated or related companies, for legal, compliance, and/or business-related purposes. You may have rights regarding your personal data; for information on exercising these rights or the Company’s treatment of personal data, please email datarequests(a)jumptrading.com.

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Library submission, checking out the interest
by Artem Shapovalov 04 May '25

04 May '25

Hello Boost dev community. Hope you are doing well and I'm doing right. I want to make my small contribution with my small library. It's a minimalistic implementation of a message bus that may connect together the pieces of code inside the same binary. Idea is: bring a microservices-like approach and the Publish-Subscribe pattern into a micro-level and allow all of the people not to reinvent the bicycle. Several times, I've re-implemented it because it's useful and had no reference implementation, and this is why I finally decided to make it, and I did. Here, you may check it out, it's generally satisfy some standards of Boost: https://github.com/Artem-Shapovalov/Independency Thanks for your attention. I hope you'll like it and find it useful and worth for Boost Libraries' family. Sincerely,Artem Shapovalov.

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[OpenMethod] A pre-release feedback
by Andrzej Krzemienski 03 May '25

03 May '25

Hi Everyone, The library review starts still in two weeks, but I wanted to give some early feedback. First, I wanted to thank Jean-Louis for exploring the subject, writing and sharing the library, and for submitting it for the Boost Review. My feedback is based on the reading of the documentation, I haven't played with the library just yet. [1]. Animals as examples There are already a number of things in the library that suggest it might not be suitable for production use (I describe them below), but using class Animal as explanatory examples amplifies the impression that this is going to be a toy library. Half of the books I have read on OO use the Animal (or Employee) example, but never in my C++ career did I have to use OO to model something close to "animal". I know they are just examples, but picking one so detached from programming reality is in my estimation harmful. The arithmetic expression example is way better. I, for one, use OO for representing dynamically-configurable parts of the applications, such as plugins or user-selectable sorting or selection predicates. [2]. The motivation In C++ we already have a dynamic-dispatch tool for "closed set of methods with open set of types": virtual functions. We also have a dynamic-dispatch tool for "open set of methods with closed set of types": the visitation of std::variant. You need to provide a motivating example (more convincing than the zoo) that needs exactly this: "open set of methods with the open set of types". The arithmetic expression example doesn't do the job: there is a fixed and well known list of concrete types. Also, the fact that Bjarne Stroustrup wanted to add open methods long ago is insufficient a motivation. [3]. Are open methods compatible with static type checking? It looks that when one of the two things -- number of types, number of methods -- is known statically, the compiler can verify if the programmer didn't forget to take some type or method into account. (Well, sometimes we get the "pure virtual function call" error, but these are very rare cases.) The same seems not to be the case for open methods. What this library advertises as flexibility would be the source of many programmer bugs detected only at runtime, possibly by end users. Imagine that programmer Alice is adding her new type A and plugs it into open methods known to her, and at the same time programmer Bob in another file adds a new open method B known only to him. Bob doesn't know about class A, and Alice doesn't know about open method B. The ISO C++ proposals quoted in the docs mentioned solving this by a mandated or encouraged linker error. Is it possible for this library to diagnose the incomplete specification of an open method in the `initialize()` function? The docs use an expression, "when an error is encountered [...]". But note that in C++ we often use the term "error" to express the situation where a _correct_ program encounters a hostile situation in the environment (lack of resources, bad user input). But openmethod docs actually mean something different: a programmer bug. It is very easy for a programmer to plant a bug in this flexible design. [4] In a similar vein, the decision to select an arbitrary specialization of an open method upon a tie, is another runtime surprise: not even a reported error. For multi methods it is not even clear how a static check for exhaustive unambiguous specialization should work. [5] It is concerning that so many things are defined in the global namespace. The docs say, "First we need to include the library’s main header. It defines a few macros, and injects a name - virtual_ptr - in the global namespace." But then the reference part says that virtual_ptr is in the namespace boost::openmethod. Who is right? The fact that as common a name as `next` is reserved inside open methods is also uncomfortable. Why not use a namespace-scoped name? Next, the docs use the using-directive: `using namespace boost::openmethod;`. This is a bad idea: both for real programs and for the documentation: it is not clear if the library was designed with namespaces in mind. Also, all the examples define open methods in the global namespace. I instead would like the docs to show how to declare open methods in my namespace, as this is a far more probable use case. [6] Friendship It puzzles me how the notion of friendship can be combined with this idea of flexibility in open methods. It is my understanding that open methods are added on top of the existing classes. That is: classes do not have to know that some open method will be added in the future. In other words, open methods are non-intrusive, right? So how would a class know who to grant the friendship to? Also, the examples use the declaration `friend struct`, even though we are talking about methods. I think docs should mention the existence of these structs more prominently. [7] Function signatures Some macros in the library use function types in declarations: `BOOST_OPENMETHOD_OVERRIDERS(poke)<void(std::ostream&, virtual_ptr<Cat>)>` Others specify the type separately: `BOOST_OPENMETHOD_OVERRIDE(value, (virtual_ptr<Plus> node), int)` Is there a reason for this? Can the clever use of macros not offer the same function signature syntax? [8] Passing virtual pointers to const objects If `virtual_ptr<Cat>` is an analogue of this pointer, could we see an example in the docs that shows a virtual pointer to const objects? It would be `virtual_ptr<const Cat>`, I guess? [9] Is this library intended for production use? The library comes with a lot of clever macros, a lot of pointer type templates, a lot of new syntax and runtime surprises (mentioned earlier). I am concerned about how the debugging would look if something goes wrong with the usage of this library. Would you recommend it for use in production systems? Or is it for experimentation? [10] Minor things Section Custom RTTI has sentence: > Now we can include the "core" header and then header `boost/openmethod/core.hpp` is not included. Is it a bug? Regards, &rzej;

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[OpenMethod] Few questions about unreasonable effectiveness of fast_perfect_hash and other minor things
by Ivan Matek 02 May '25

02 May '25

Hi everyone, few questions: 1. fast_perfect_hash achieves performance(load factor) I would assume is impossible, based on perfect hashing talk <https://www.youtube.com/watch?v=yOo6GnbKzp8>. I presume reason why it works so well that compiler places type info data clustered in same region of addresses... Is that correct? 2. regarding static_list: would Boost.Intrusive list work here also, and if so was this considered? 3. basic_error_output does not seem to need the Policy template parameter, is this left for consistency or am I wrong that it is not needed? 4. dynamic_cast_ref docs <https://github.com/jll63/Boost.OpenMethod/blob/ea7dbe86b511797f0281ef07b3ad…> mention that D must match the reference category of B. Would it be possible to static_assert that?

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[OpenMethod] Questions about custom policies.
by Ivan Matek 30 Apr '25

30 Apr '25

Hello, I just started with review and I have a cluster of questions, but it is not clear what the root cause of my issues is so apologies if ordering of questions is wrong. 1. I wanted to get hash map stats, but they are not printed in debug policy, even when env variable is set, I must hack release policy like this: - vptr_vector<release>, vectored_error_handler<release>> {}; + vptr_vector<release>, vectored_error_handler<release>, basic_trace_output<release>> {} Is this expected? IDK why debug prints some trace, but not hash info 2. While playing with this I get crashes when I just c/p release policy and rename it, e.g. +struct my_release : basic_policy< + my_release, std_rtti, fast_perfect_hash<my_release>, + vptr_vector<my_release>, vectored_error_handler<my_release>> {}; crashes in template<bool Indirect> inline auto box_vptr(const vptr_type& vp) { if constexpr (Indirect) { return &vp; } else { return vp; } } but when I switch boost::openmethod::initialize<boost::openmethod::policies::my_release>(); to boost::openmethod::initialize<boost::openmethod::policies::release>(); it works fine. It also exibits some other weird behavior, e.g. if I switch debug inheritance from release to my_release crash goes away, even when not using debug... Looks to me I am doing something that is not allowed, but it is not clear what I am doing wrong. regards, Ivan

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