public_cast

public_cast.hpp

#include <cstddef>
#include <meta>
#include <string_view>
#include <utility>

#define FWD(...) static_cast<decltype(__VA_ARGS__)&&>(__VA_ARGS__)

#define ARROW(...) \
	noexcept(noexcept(__VA_ARGS__)) \
	-> decltype(auto) \
	requires(requires { __VA_ARGS__; }) \
	{ return (__VA_ARGS__); }

#define LIFT(...) ([&](auto&&... _args) ARROW(__VA_ARGS__(FWD(_args)...)))

namespace detail {
	struct fixed_string {
		const char* _data;
		std::size_t _size;

		template<std::size_t size>
		consteval fixed_string(const char (&data)[size])
		: _data(std::define_static_string(data)), _size(size - 1) {}

		constexpr operator std::string_view() const {
			return std::string_view(this->_data, this->_size);
		}
	};

	template<detail::fixed_string name, typename Args>
	struct public_cast_call {
		Args _args;

		friend constexpr decltype(auto) operator->*(auto&& obj, public_cast_call<name, Args> self) {
			template for (constexpr auto member : std::define_static_array(members_of(remove_cvref(^^decltype(obj)), std::meta::access_context::unchecked()))) {
				if constexpr (has_identifier(member) && (identifier_of(member) == name)) {
					auto&& [...args] = self._args;
					return (FWD(obj).*&[:member:])(FWD(args)...);
				}
			}
			std::unreachable();
		}
	};

	template<detail::fixed_string name>
	struct public_cast {
		friend constexpr decltype(auto) operator->*(auto&& obj, public_cast<name>) {
			template for (constexpr auto member : std::define_static_array(members_of(remove_cvref(^^decltype(obj)), std::meta::access_context::unchecked()))) {
				if constexpr (has_identifier(member) && (identifier_of(member) == name)) {
					return FWD(obj).*&[:member:];
				}
			}
			std::unreachable();
		}

		constexpr auto operator()(auto&&... args) const {
			struct Args;
			consteval {
				define_aggregate(^^Args, {
					data_member_spec(^^decltype(args)&&, { .name = "_" })...
				});
			}
			return public_cast_call<name, Args>({ FWD(args)... });
		}
	};
}

template<detail::fixed_string name>
constexpr auto public_cast = detail::public_cast<name>();