substate_manager_container.h

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#pragma once

#include "../../base/type_traits.h"
#include <variant>

namespace escad::detail
{

template<class States, template<typename> typename Manager>
class substate_manager_container {
public:
    template<class State, class... Args>
    void create(Args&... args) {
        constexpr auto Index = mpl::type_list_index_v<State, type_list>;
        managers_.template emplace<1 + Index>(args...);
    }

    template<class Fun>
    void visit(Fun&& fun) {
        std::visit(std::forward<Fun>(fun), managers_);
    }

private:
    // a list of states: type_list<A, B, C...>;
    using type_list = typename States::type_list;

    // meta-function extracting substate_type from a state
    // substate_type is a states<SubA1, SubA2, SubA3...> specialization
    template<class T> struct get_substate_manager_type {
        using type = Manager<typename T::substates_type>;
    };

    // use this meta-function to transform a list of states to a list of its substate managers in form of:
    // type_list<state_manager<states<SubA1, SubA2, SubA3...>>, state_manager<states<SubB1, SubB2, SubB3...>>...>
    using substates_manager_list = mpl::type_list_transform_t<type_list, get_substate_manager_type>;

    // prepend with monostate for default construction
    using substates_manager_list_fin = typename mpl::type_list_push_front<substates_manager_list, std::monostate>::result;

    // rename type_list<...> to std::variant<...> which will be the final storage type
    using substates_manager_variant = typename mpl::type_list_rename<
        substates_manager_list_fin,
        std::variant>::result;

    substates_manager_variant managers_;
};

} // namespace fsm::detail