Re: [OpenMethod] Questions about custom policies.

From: Jean-Louis Leroy (jl_at_[hidden])
Date: 2025-04-30 14:31:41

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>;

    BOOST_OPENMETHOD(
        poke, (std::ostream&, my_ptr<Animal>), void, my_release);

You can also globally override the policy:

    #include <boost/openmethod/policies.hpp>

    using namespace boost::openmethod::policies;

    struct my_release
        : basic_policy<
              my_release, std_rtti, fast_perfect_hash<my_release>,
              vptr_vector<my_release>, vectored_error_handler<my_release>> {};

    // BEFORE including <boost/openmethod.hpp>
    #define BOOST_OPENMETHOD_POLICY my_release

    #include <boost/openmethod.hpp>

> 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...

If you mean what I think ;) it is because `debug` is the default policy in the
absence of NDEBUG.

> Looks to me I am doing something that is not allowed, but it is not clear
> what I am doing wrong.

Policies and facets are definitely an advanced feature.

J-L

On Wed, Apr 30, 2025 at 7:36 AM Ivan Matek via Boost
<boost_at_[hidden]> wrote:
>
> 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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