Re: [OpenMethod] Questions about custom policies.

From: Ivan Matek (libbooze_at_[hidden])
Date: 2025-04-30 23:10:12

On Wed, Apr 30, 2025 at 4:32 PM Jean-Louis Leroy via Boost <
boost_at_[hidden]> wrote:

> Hi,
>
> > 1. I wanted to get hash map stats, but they are not printed in debug
> > policy, even when env variable is set
>
> There is a small bug here. If you try with the "review" branch, or use
> <
> https://raw.githubusercontent.com/jll63/Boost.OpenMethod/refs/heads/review/flat/boost/openmethod.hpp
> >
> in compiler explorer, you will get the hash stats. See
> https://godbolt.org/z/bneKT6s7v
>
> Now an important thing: if you build with NDEBUG defined, the default
> policy is
> debug. Otherwise it is release. I suspect that in your examples, NDEBUG is
> not
> set. Thus the classes and methods are added to the *debug* policy.
>
> > 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>,
>
> This creates a new policy, from scratch. Its catalog of classes and
> methods is
> distinct from the debug policy's - and it's empty.
>
> You need to use my_release consistently. For example:
>
>     BOOST_OPENMETHOD(
>         poke, (std::ostream&, virtual_ptr<Animal, my_release>), void,
> my_release);
>                                                 ^^^^^^^^^^^^
> ^^^^^^^^^^^^
>     BOOST_OPENMETHOD_CLASSES(Animal, Cat, Dog, Bulldog, my_release);
>                                                       ^^^^^^^^^^^^
>
>     int main() {
>         boost::openmethod::initialize<my_release>();
>                                      ^^^^^^^^^^^^
>         ...
>     }
>
> Compiler Explorer: https://godbolt.org/z/MGs1eMd8W
>
> Of course it is rather cumbersome to add the policy as a second argument to
> virtual_ptr everywhere. At this point, you can create an alias:
>
>     template<typename T>
>     using my_ptr = virtual_ptr<T, my_release>;
>
>
Hi, thank you for the answer, review branch prints out hashing stats.

Few follow up questions.

   1. Why virtual_ptr doesn't just pick up the policy that was used in
   initialize? I presume to avoid some runtime performance overhead.
   2. Should this behavior be more explicitly mentioned in the
   documentation. I was reading documentation and assumed I can just provide
   my policy in initialize, it was not clear to me  what you explained here.
   I mean now I know how things work it is obvious
   https://jll63.github.io/Boost.OpenMethod/#tutorials_policies_and_facets
   is mentioning that initialize and virtual_ptr  are "linked", but I think
   it is not obvious when reading it for the first time.
   3. Why are we breaking here out of just 1 for loop?
   https://github.com/jll63/Boost.OpenMethod/blob/4e4dff8059f94b07c201036c0e91856af86e8a2c/include/boost/openmethod/policies/fast_perfect_hash.hpp#L135