[openmethod] [policies]

From: Jean-Louis Leroy (jl_at_[hidden])
Date: 2025-05-18 17:31:48

Hi all, and in particular Joaquín, Christian, Ruben and Steven who contributed
remarks on the policy system.

I followed some of Ruben's suggestions. There is now a registry that contains
collections of classes and methods, and, after initialize(), dispatch data. It
also contains a collection of policies. In part this is "just" renaming: what
used to be a Policy is now a Registry, and what used to be a Facet is now a
Policy. But this is not just cosmetic. The registry mechanism is clearly
separated from the policies.

Many old facets were CRTP templates, but not always. Now a policy (ex-facet) is
- always - a Mp11 quoted meta-function that takes a registry as a single
argument. This complicates creating policies a bit, but makes using them much
easier, I think. CRTP is gone from the interface (in fact it is hidden). `fork`
is gone.

The debug and release stock policies looked like this:

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

    struct debug : release::fork<debug>::with<
        runtime_checks, basic_error_output<debug>,
        basic_trace_output<debug>> {};

Now they are:

    struct release : registry<
        std_rtti, fast_perfect_hash, vptr_vector,
        vectored_error_handler, stderr_output> {};

    struct debug : registry<
        std_rtti, fast_perfect_hash, vptr_vector, vectored_error_handler,
        stderr_output, runtime_checks, trace> {};

An application that wants to use a standard map instead of a fast hash
table, and keep all the rest the same, could say:

    struct my_registry : default_registry::with<vptr_map<>> {};

Or, using an unordered_flat_map:

    struct my_registry : default_registry::with<
        vptr_map<boost::mp11::mp_quote<boost::unordered_flat_map>>> {};

I also took Ruben's suggestion of having only one output policy, instead of one
for error reporting, and one for tracing.

Now, even in release builds, the default handler writes a diagnostic to stderr,
before calling abort().

Policies are not aggregated via multiple inheritance anymore.

At the moment, I am not convinced with Ruben's approach of making the registry
(formerly policy) an object. There would still need to be a static instance of
that object, which would contain instances of policy objects. Six of one or half
of a dozen of the other. From there Ruben suggested making the dependencies
between policies (ex-facets) explicit, as far as I can see this leads to a mess
of conditionally enabled constructors.

You can see the changes here: https://github.com/jll63/Boost.OpenMethod/pull/22
It's still a draft. The documentation is not yet updated, I'll do a big doc pass
when things stabilize again. If you want to post comments directly to the PRs,
drop me a line and I will invite you as a collaborator.

J-L