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;