Re: [OpenMethod] Andrzej's review

From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2025-05-08 08:37:12

J-L, thanks for the reply.
I appreciate this discussion. In fact, I believe the biggest value of the
Boost Review process is the discussions that get triggered.

* I am always open to changing my mind, given good arguments. I also know
> what
>   my goals are though.

The above is worth stressing for the wider audience. It is actually a proof
of good quality and a sound design when the author refuses to implement
some feedback because they have a clear picture of where they want to be,
and they detect incompatibilities with the main goal. I appreciate this
statement.

Indeed, a lot of my criticism goes against open methods in general. I could
summarize this as follows. the two dimensional (one dimension is the number
of sub-types, the other is the number of methods) run-time flexibility
necessarily results with runtime surprises and gotches. Runtime surprises
go against a robust system design.

Let me clarify: I am perfectly fine with Boost.OpenMethod not implementing
N2216. I think you are right that N2216 may have made wrong decisions. What
I am raising though, is that from reading the docs, one might get an
impression that implementing N2216 was the goal. Therefore, my suggestion
is to make it abundantly clear that Boost.OpenMethod does not implement
N2216.

You also made very good points why N2216 is wrong about some aspects, and
why you decided to provide a different solution. I strongly believe these
explanations should go into the documentation. Not the tutorials for the
novices, of course, but to the "in depth" part. The Boost guidelines for
documentation (
https://www.boost.org/development/requirements.html#Documentation)
recommend adding a section on the design rationale.

One other point, you explain a number of things by referring to YOMM2. This
is valuable background information. But when it comes to the documentation
or rationale, I think Boost.OpenMethod should be self-contained: I should
be able to understand the rationale without having to learn YOMM2.

I sort of get the idea behind virtual_ptr, but I still find the end result
not acceptable: there is `virtual_ptr` and there is `virual_`. Having the
two ways in one library is too much.

You asked about the reactions of the audience in my work. They were
interested until I showed them that they will get a `std::abort()` whenever
a match cannot be found. At that point I think I lost them. In my work we
try hard to minimize any run-time surprises. We prefer compile-time errors
to run-time errors. Open methods in general (this is not specific to
Boost.OpenMethod) prefer run-time errors.

And personally, I disagree that open-method run-time errors are like "pure
virtual function call". The latter is a very rare case, and you have to try
hard to get it. The former, on the other hand, is encouraged by the
two-dimensional run-time flexibility.

You also asked, what you should do with all this feedback. To a great
extent it is up to you. The review manager will tell you, if the library is
accepted, which changes are necessary to get the approval, and which
observations are just "worth considering". The reviews like mine are only
used to guide the manager to make their verdict.

Thanks again for your work on open methods.

Best regards,
&rzej;

czw., 8 maj 2025 o 07:33 Jean-Louis Leroy <jl_at_leroy.nyc> napisał(a):

> Hi Andrzej,
>
> Thank you for the 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.
> > [...]
> > Jean-Louis provided a very compelling example of printing a Matrix which
> is
> > from a OO hierarchy
>
> Instead of "expression problem", it can also be "marketed" as a way to
> deal with
> "cross-cutting concerns". The matrix example, or the rendering of an AST
> as a
> RPN string, are both cross-cutting concerns.
>
> > 1. 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)
>
> I would like to see both open-methods and pattern-matching in the
> language; and,
> in the meantime, in Boost. They exist in the same broad space, overlapping
> to an
> extent, yet each brings solutions that the other does not.
>
> Mach7 and the related papers contain a benchmark that shows that
> pattern-matching is slower than type-switch, which is slower than
> open-methods.
> But I reckon that, in presence of a single value subject to dynamic
> dispatch,
> pattern-matching is not horribly slower at all. I guess that, in many real
> programs, the difference would be unmeasurable.
>
> > 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?
>
> It started as a strong inspiration. I never implemented the attempt to
> resolve
> ambiguities at all cost in YOMM2. I did not implement covariant return
> types as
> a tie-breaker, which has been there from the very first paper, because I
> had
> reservations about it. The vocabulary (method declarations and method
> definitions) I use in YOMM2 is inspired more by CLOS and Clojure than by
> N2216.
>
> For OpenMethod, I thought I could get closer to consensus if I brought the
> library closer to N2216. In a way I did get a consensus: everybody,
> including
> myself, seems to strongly dislike "ambiguity is not an error".
>
> I already had some disagreements with N2216 (more below) and they grew as
> the
> review progressed.
>
> I am now strongly leaning towards making the YOMM2 way the default way of
> handling ambiguities, with N2216 an opt-in.
>
> > 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.
>
> I see it quite more than that. It is the last of a series of papers
> describing
> essentially the same design, with small variations over time. But here we
> are
> just trying to guess what is in other people's heads.
>
> > 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.
>
> Ambiguity handling in OpenMethod is deterministic as well. And just like in
> N2216, which overrider is picked is undocumented.
>
> > 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.
>
> This is one of the places where I dare disagree with our three luminaries
> ;-)
>
> You have to look hard to find an *example* of a base-method implementation
> (what
> you call a "top level declaration").
>
> In N2216:
>
>     bool intersect(virtual Shape&, virtual Shape&) { }
>
> This doesn't look good, it silently returns... what by the way? Isn't this
> UB?
> It's a warning at least. Anyway this will make it harder to troubleshoot a
> missing overrider.
>
> In Solodkyy's paper "Simplifying the Analysis of c++ Programs":
>
>     int eval (virtual const Expr&) { throw std::invalid argument ("eval");
> }
>
> This is better. I think that it is what most base-method implementations
> will
> look like. Again and again. A counter-example is my "animals meet"
> example: they
> ignore each other. But it's a made-up example. Reasonable fallback
> implementations will happen, but they will not be the most frequent.
>
> I have the feeling that Stroustrup & co wanted to avoid runtime errors so
> badly,
> while also refusing to throw an exception (I'm with them on this), that
> they
> convinced themselves that this is the right design.
>
> My take is that this is the same situation as calling a pure virtual
> function.
> You get a short message, if any, and then abort(). And there is no way to
> detect
> such a call through static analysis of a single translation unit.
>
> In OpenMethod, you can look at the report member returned by initialize()
> and
> find out, early, that you have virtual tuples without overriders.
>
> I could implement the N2216 way. What would it get us? Again, users would
> write
> silly catch-alls like the above. And then checking the report would be
> pointless, because I can detect missing overriders, but not silly ones.
>
> I am changing my mind about something though. In release variants, by
> default, I
> should output a diagnostic. You could opt out via a policy facet if you
> want
> minimal footprint.
>
> Above is my position on *missing* overriders, which I implemented. Indeed
> that
> is not the N2216 way.
>
> The N2216 part I implemented when transitioning from YOMM2 to OpenMethod
> is how
> *ambiguities* are handled, i.e. bring that part is in line with N2216. I
> don't
> see the difference between OpenMethod and N2216. As long as you don't
> dlopen() a
> library that adds methods, overriders or classes, one of the ambiguous
> overriders is called, unspecified but always the same. Call dlopen(), and
> everything is reshuffled. The only difference is that, OpenMethod being a
> library, you have to call initialize() again.
>
> Since we're at it, I also have doubts about using covariant return types as
> ambiguity tie-breakers. It's a conditional mechanism. It kicks in only if
> the
> return types are classes. It's getting too complicated. When I present
> YOMM2, I
> say: "I don't need to explain the resolution rules, you already know them.
> Same
> as overload resolution, only it's at runtime". I find it more coherent,
> less
> surprising.
>
> > 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.
>
> This is a criticism of open-methods in general, correct?
>
> > Another difference between Boost.OpenMethod and N2216 is that in the
> latter
> > a programmer never says that they are overriding a virtual method.
>
> Doesn't it? It's BOOST_OPENMETHOD and BOOST_OPENMETHOD_OVERRIDE. It's more
> explicit than the N2216 syntax. Which of course I cannot reproduce,
> because it's
> a library.
>
> Here's another - a better? - one. In N2216, an overrider can even override
> multiple base-methods. I cannot emulate that. And anyway, what does `next`
> mean
> in that case?
>
> > 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
>
> We discussed this before, a compile-time check used to exist in YOMM2. It
> was
> lost when I added support for custom RTTI and non-polymorphic types.
> Following
> your remark, I re-introduced the compile-time test in the "review" branch.
>
> > 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.
>
> The documentation needs improvement, right. I wouldn't drown "normal"
> users with
> information on advanced features though. It's not an easy piece to
> document.
>
> The design gives you (or whomever wants it) a flexibility that N2216
> doesn't.
> You can work with non-polymorphic types. Or rather, they will be
> polymorphic
> (because open-methods are), without requiring any virtual functions in your
> hierarchies.
>
> > 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?
>
> A compile-time error. Consider:
>
>     #include <boost/openmethod.hpp>
>     #include <boost/openmethod/compiler.hpp>
>     #include <iostream>
>
>     struct Repeated {
>         virtual ~Repeated() {}
>     };
>
>     struct Base1 : Repeated {};
>     struct Base2 : Repeated {};
>
>     struct Derived : Base1, Base2 {};
>
>     BOOST_OPENMETHOD_CLASSES(Repeated, Base1, Base2, Derived);
>
>     BOOST_OPENMETHOD(wrong, (virtual_ptr<Repeated>), void);
>
>     BOOST_OPENMETHOD_OVERRIDE(wrong, (virtual_ptr<Derived>), void) {
>         std::cout << "Derived\n";
>     }
>
>     int main() {
>         boost::openmethod::initialize();
>         Derived d;
>         Repeated& r = d;
>         wrong(r);
>     }
>
> https://godbolt.org/z/6bvTahhqb
>
> Thank you for pushing on this though. Having thought about it, I am
> pretty confident
> that I can detect this situation in use_classes/BOOST_OPENMETHOD_CLASSES
> and
> generate a better error message.
>
> > 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.
>
> Stroustrup:
>
> > In retrospect, I don’t think that the object-oriented notation (e.g.,
> x.f(y))
> > should ever have been introduced. The traditional mathematical notation
> f(x,y)
> > is sufficient. As a side benefit, the mathematical notation would
> naturally
> > have given us multi-methods, thereby saving us from the visitor pattern
> > workaround.
> > Stroustrup, 2020, Thriving in a Crowded and Changing World: C++
> 2006–2020.
>
> I am not a mind-reader, but it seems to me that he still liked
> open-methods as
> of 2020, some years after the pattern matching papers.
>
> > 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.
>
> Yes. It's one of the ways of using the library. You can also use an
> embedded
> vptr (see with_vptr).
>
> There is an ongoing debate about implementing virtual functions the C++ way
> (embedded vptr) vs the Go/Rust fat pointer. I believe that in practice it
> cannot
> be decided either way. Anyway...
>
> > 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".
>
> virtual_ptr is not an implementation detail. It's an interface. While
> turning
> YOMM2 into OpenMethod, at one point I decided to bring virtual_ptr - an
> afterthought in YOMM2 - to front stage. One of my motivations was
> clarity. If you
> use virtual_, you use it in the base-method declaration, but not in the
> overriders. That was confusing.
>
> Also note that virtual_ptr at all levels resembles N2216 syntax more.
>
> > Then I am exposed to more things, like final_virtual_ptr
>
> It is an optimization (similar to make_shared) that gives open-methods the
> same
> speed as native virtual functions. It just loads the vptr from a static
> variable. When an object is created, we don't have to find its type, we
> know it.
> Why not give the *option* of using that knowledge?
>
> > (not to mention unique_virtual_ptr and shared_virtual_ptr).
>
> Copied from my reply to Joaquin on April 30.
>
> > "Design and evaluation of C++ open multi-methods", 5.3. Smart pointers
> states:
> >
> > > Defining open-methods directly on smart pointers is not possible. In
> the
> > > following example, (1) yields an error, as ptr1 is neither a reference
> nor a
> > > pointer type. The declaration of (2) is an error, because shared_ptr
> is not
> > > a polymorphic object (it does not define any virtual function). Even
> when
> > > shared_ptr were polymorphic, the open-method declaration would be
> > > meaningless. A shared_ptr<B> would not be in an inheritance
> relationship to
> > > shared_ptr<A>, thus the compiler would not recognize foo(virtual
> > > shared_ptr<B>&) as an overrider.
> > >
> > > void foo(virtual shared_ptr<A> ptr1); // (1) error
> > > void foo(virtual shared_ptr<A>& ptr2); // (2) error
> >
> > I don't buy this. How is it different from plain pointers? A Cat* is not
> in an
> > inheritance relationship to Animal* either: Cat and Animal are, thus
> there is
> > a standard _conversion_ from one to the other.
> >
> > My intuition is that virtual smart pointer arguments must be forbidden
> because
> > it would require the compiler to be aware of the standard library.
>
> I don't want to over-quote myself, please see my full answer in that post.
>
> > Now, I have a more philosophical question. [...] 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 the impression that these paragraphs, while interesting, are general
> considerations about the principle of open-methods, and the value of
> libraries;
> and only in the last sentence you connect them to OpenMethod.
>
> I don't perceive Boost as a monolithic message on how programming should be
> done. I see it more as a toolbox, full of *good* *quality* tools, and it
> is up
> to users to pick what tool they like to use. Or not.
>
> > * 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;`.
>
> I won't die on that hill. This has been suggested earlier in the review,
> and I
> considered it (positively) before the review even began. Of course I
> realise
> it's borderline. Please consider it done.
>
> > * The framework behaves differently based on whether my types are
> copyable
> > or not. Compare:
>
> > ** https://godbolt.org/z/zKzYzbj3x
> > ** https://godbolt.org/z/Ya8TrE79K
>
> There are a few things going on here.
>
> What you should get is a runtime error: unknown class YNode
>
> And that is the root of the problem. *Not* constructors.
>
> So what's happened?
>
> 1. You are compiling with NDEBUG, so you get no checks, and no
> diagnostics. That
>    is a mistake on my part. I focused too much on the smallest possible
>    footprint in release builds. I will change it to: diagnostic by
> default, with
>    the possibility to opt out.
>
> 2. In the conversion from YOMM2, I introduced a bug. Fixed in the "review"
>    branch, see https://godbolt.org/z/6v8v5ce4b
>
> > * 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 am curious, what were the reactions?
>
> > 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.
>
> I got valuable feedback, which triggered new ideas, in particular about the
> customization points, and the defaults (like, emit diags even in release
> mode by
> default, N2216 and ambiguity handling). And I still have to read Steven
> Watanabe's review...So I am happy I took the plunge.
>
> One final thing: upon re-reading my reply, I realize that it looks like I
> am
> willing to change my mind on a lot of things. Should I be sent back to the
> drawing board then? Am I shooting myself in the foot?
>
> So:
>
> * I am always open to changing my mind, given good arguments. I also know
> what
>   my goals are though.
> * This is the first time my work on open-methods gets a review. I had
> users and
>   even contributors for a decade. This time I get a review and I find the
>   feedback very valuable. The conversations I am having on the policy/facet
>   system are guiding me to something better, simpler.
> * Many of the "doubts" I expressed are related to picking the best
> *defaults*
> * I intend my library to be production-ready. I also want it to enable
> research
>   and experimentation. The first motivation for policies was unit tests.
> Then it
>   was experimenting with memory layouts for dispatch data. Then hashing.
> And
>   finally, accepting that in the C++ world, there is also a reasonable
> need to
>   use custom RTTI; to disable exceptions; to operate with limited
> resources.
>
> J-L
>