A dirty sketch of using root_t<T> and node_t<D, B> to traverse type hierarchy with respect to message mutability

upcastable_root_and_node_20240724_002.cpp

#include <iostream>
#include <typeindex>
#include <stdexcept>

enum class message_mutability_t
{
	immutable_message,
	mutable_message
};

template< typename Msg >
struct mutable_msg {};

class message_upcaster_t;
using upcaster_factory_t =
	message_upcaster_t (*)(message_mutability_t) noexcept;

class message_upcaster_t
{
	std::type_index m_self_type;

	upcaster_factory_t m_parent_factory;

public:
	message_upcaster_t(
		std::type_index self_type,
		upcaster_factory_t parent_factory )
		: m_self_type{ std::move(self_type) }
		, m_parent_factory{ parent_factory }
	{}

	[[nodiscard]] const std::type_index &
	self_type() const noexcept
	{
		return m_self_type;
	}

	[[nodiscard]] bool
	has_parent_factory() const noexcept
	{
		return nullptr != m_parent_factory;
	}

	[[nodiscard]] message_upcaster_t
	parent_upcaster( message_mutability_t mutability ) const
	{
		if( !has_parent_factory() )
			throw std::runtime_error{
					"message_upcaster_t::parent_upcaster: "
					"no parent upcaster_factory"
			};

		return (*m_parent_factory)( mutability );
	}
};

class message_t
{
public:
	virtual ~message_t() = default;
};

class root_base_t : public message_t
{
	upcaster_factory_t m_factory{};

public:
	[[nodiscard]] upcaster_factory_t
	so_message_upcaster_factory() const noexcept
	{
		return m_factory;
	}

	void
	so_set_message_upcaster_factory(
		upcaster_factory_t factory) noexcept
	{
		m_factory = factory;
	}
};

template<typename Base>
class root_t : public root_base_t
{
public:
	[[nodiscard]] static message_upcaster_t
	so_make_upcaster_root( message_mutability_t mutability ) noexcept
	{
		if( message_mutability_t::mutable_message == mutability )
			return { typeid(mutable_msg<Base>), nullptr };
		else
			return { typeid(Base), nullptr };
	}

public:
	root_t()
	{
		this->so_set_message_upcaster_factory( &root_t::so_make_upcaster_root );
	}
};

template< typename B, typename = std::void_t<> >
struct has_so_make_upcaster_method : public std::false_type {};

template< typename B >
struct has_so_make_upcaster_method<
	B, std::void_t< decltype(&B::so_make_upcaster) >
> : public std::true_type {};

template<typename Derived, typename Base>
class node_t
{
public:
	[[nodiscard]] static message_upcaster_t
	so_make_upcaster( message_mutability_t mutability ) noexcept
	{
		if constexpr( has_so_make_upcaster_method<Base>::value )
		{
			const auto upcaster = &Base::so_make_upcaster;
			if( message_mutability_t::mutable_message == mutability )
				return { typeid(mutable_msg<Derived>), upcaster };
			else
				return { typeid(Derived), upcaster };
		}
		else
		{
			const auto upcaster = &Base::so_make_upcaster_root;
			if( message_mutability_t::mutable_message == mutability )
				return { typeid(mutable_msg<Derived>), upcaster };
			else
				return { typeid(Derived), upcaster };
		}
	}

public:
	node_t(Derived & derived)
	{
		static_assert(
				std::is_base_of_v<root_base_t, Derived> &&
				std::is_base_of_v<Base, Derived> );

		derived.so_set_message_upcaster_factory( &node_t::so_make_upcaster );
	}
};

class image_base : public root_t<image_base>
{
public:
	image_base() = default;
};

class image_vendor_1
	: public image_base
	, public node_t<image_vendor_1, image_base>
{
public:
	image_vendor_1()
		: image_base{}
		, node_t<image_vendor_1, image_base>{ *this }
	{}
};

class image_vendor_2
	: public image_base
	, public node_t<image_vendor_2, image_base>
{
public:
	image_vendor_2()
		: image_base{}
		, node_t<image_vendor_2, image_base>{ *this }
	{}
};

class special_image
	: public image_vendor_2
	, public node_t<special_image, image_vendor_2>
{
public:
	special_image()
		: image_vendor_2{}
		, node_t<special_image, image_vendor_2>{ *this }
	{}
};

void handle(
	const image_base & base,
	message_mutability_t mutability)
{
	auto upcaster = (base.so_message_upcaster_factory())( mutability );
	std::cout << "self_type: " << upcaster.self_type().name() << std::endl;
	while( upcaster.has_parent_factory() )
	{
		upcaster = upcaster.parent_upcaster( mutability );
		std::cout << "base_type: " << upcaster.self_type().name() << std::endl;
	}
}

int main()
{
	special_image img;
	handle( img, message_mutability_t::immutable_message );
	handle( img, message_mutability_t::mutable_message );

	return 0;
}