opaque_handle/include/opaque.hh

#pragma once

#include <utility>
#include <stdexcept>

#define $_oe_msg(msg) { inline const char* what() const noexcept override { return (msg); } }
struct opaque_exception : std::exception $_oe_msg("unspecified opaque_handle related error");

#define $_oe(msg) public opaque_exception $_oe_msg((msg))
struct opaque_not_copyable : $_oe("opaque_handle referred to a type that was not copyable");
struct opaque_handle_moved : $_oe("this opaque_handle was moved");
struct opaque_bad_cast : $_oe("bad opaque_handle pointer cast");

struct opaque_handle;
struct opaque_handle_impl;

struct opaque_handle_impl {
	friend class opaque_handle;

	virtual constexpr ~opaque_handle_impl() {}

	template<typename To>
	inline constexpr const To* try_cast() const noexcept { return static_cast<const To*>(as_raw_ptr()); }
	template<typename To>
	inline constexpr To* try_cast() noexcept { return static_cast<To*>(as_raw_ptr()); }


protected:
	constexpr virtual void* as_raw_ptr() const noexcept = 0;
	constexpr virtual opaque_handle_impl* try_clone() const = 0;

	inline virtual opaque_handle_impl& clone() const {
		if(auto* np = try_clone()) return *np;
		throw opaque_not_copyable{};
	}

	constexpr opaque_handle_impl() noexcept{}
};

struct opaque_handle final {
	constexpr explicit opaque_handle(opaque_handle_impl* ptr) noexcept : _impl(ptr){}

	constexpr opaque_handle(opaque_handle&& move) noexcept
		: _impl(std::exchange(move._impl, nullptr)){}
	inline opaque_handle(const opaque_handle& copy)
		: _impl(copy._impl 
			? copy._impl->try_clone() //(copy._impl->try_clone() ?: throw opaque_not_copyable{})
			: nullptr){}

	constexpr ~opaque_handle()
	{
		if(_impl) delete _impl;
	}

	constexpr const void* as_ptr() const noexcept { return _impl ? _impl->as_raw_ptr() : nullptr; }
	constexpr void* as_ptr() noexcept { return _impl ? _impl->as_raw_ptr() : nullptr; }


	///XXX: These don't work as expected. And we can't dynamic_cast<>(void*), so... What do we do to check the type of `as_ptr()` is `T` without embedding all this extra type info? Another layer of inheritence, and have `as_raw_ptr()` return a pointer to that base instead of `void*`? idk...
	template<typename T>
	/// WARNING: Unsafe, see above
	constexpr const T* try_cast() const noexcept { return static_cast<const T*>(as_ptr()); }
	template<typename T>
	/// WARNING: Unsafe, see above
	constexpr T* try_cast() noexcept { return static_cast<T*>(as_ptr()); }

	template<typename T>
	inline const T& cast() const { return *(try_cast<T>() ?: throw opaque_bad_cast{}); }
	template<typename T>
	inline T& cast() { return *(try_cast<T>() ?: throw opaque_bad_cast{}); }

	constexpr bool has_value() const noexcept { return bool(_impl); }
	constexpr explicit operator bool() const noexcept { return has_value(); }

	template<typename T>
	constexpr const T* operator->() const noexcept { return try_cast<T>(); }
	template<typename T>
	constexpr T* operator->() noexcept { return try_cast<T>(); } //XXX: TODO: throw if try_cast<T>() is null and T is not `void`.

	inline auto operator*() const noexcept { 
		return _impl_deref_const{*this};
	}
	inline auto operator*() noexcept { 
		return _impl_deref{*this};
	}
/*
	template<typename T>
	inline auto operator*() noexcept { return *(try_cast<T>() ?: throw opaque_bad_cast{}); }
*/
	template<typename T>
	constexpr operator T*() noexcept { return try_cast<T>(); }
	template<typename T>
	constexpr operator const T*() const noexcept { return try_cast<T>(); }

private:
	struct _impl_deref_const {
		const opaque_handle& ptr;

		template<typename T>
		inline operator const T&() && {
			return  *(ptr.try_cast<T>() ?: throw opaque_bad_cast{});
		}
		template<typename T>
		inline operator const T&() const&& {
			return  *(ptr.try_cast<T>() ?: throw opaque_bad_cast{});
		}
	};
	struct _impl_deref {
		opaque_handle& ptr;

		template<typename T>
		inline operator T&() && {
			return  *(ptr.try_cast<T>() ?: throw opaque_bad_cast{});
		}
		template<typename T>
		inline operator const T&() const&& {
			return  *(ptr.try_cast<T>() ?: throw opaque_bad_cast{});
		}
	};
	opaque_handle_impl* _impl;
};


template<typename T>
struct opaque_handle_object : public opaque_handle_impl
{
	friend class opaque_handle;

	inline constexpr opaque_handle_object(T&& value) noexcept(std::is_nothrow_move_constructible_v<T>)
		: _obj(std::move(value)){}


	inline constexpr virtual ~opaque_handle_object() {}
protected:
	inline constexpr void* as_raw_ptr() const noexcept override { return const_cast<void*>(static_cast<const void*>(&_obj) /*?: (static_cast<void*>(&_obj) ?: &_obj)*/); }
	inline constexpr opaque_handle_impl* try_clone() const override {
		if constexpr(std::is_copy_constructible_v<T>) {
			return new opaque_handle_object<T>{T{_obj}};
		} else return nullptr;
	}
private:
	T _obj;
};
template<typename T>
opaque_handle_object(T&&) -> opaque_handle_object<T>;

/// Make an opaque_handle from an object.
template<typename T>
constexpr inline opaque_handle make_opaque_object_handle(T&& value) noexcept(std::is_nothrow_move_constructible_v<T>)
{
	return opaque_handle{static_cast<opaque_handle_impl*>(new opaque_handle_object<T>(std::forward<T>(value)))};
}

enum class opaque_handle_operation
{
	Clone,
	Delete,
};


template<typename F, typename T>
concept OpaqueHandleFunc = std::is_invocable_v<F, T*, opaque_handle_operation> 
				&& std::is_convertible_v<std::invoke_result_t<F, T*, opaque_handle_operation>, T*>;

//TODO: Turn all local `constexpr is_nothrow` in the following functions into #define macros. The constexpr locals being used in a local class cause the compiler to segfault for some reason...

//XXX: TODO: `make_opaque_handle()`'s local classes cannot be casted back to `T*`, adding operator T* won't help here because they are static/dynamic casted. We need them to inherit `opaque_handle_impl`

/// Create and opaque_handle from a data pointer and a lambda which handles the copying (if possible) and deleting of the object
///
/// `handler` should be in the form: `auto* (T*, opaque_handle_operation) [noexcept]` and the reference must outlive the returned object. It should handle a `null` argument, and it can return `null`.
template<typename T, OpaqueHandleFunc<T> HandleF> // Note: This should be a lambda, or a free-function. it's lifetime must not end before the `opaque_handle` returned from this function does.
constexpr inline opaque_handle make_opaque_handle(T* data, const HandleF& handler) noexcept
{
#define is_nothrow (std::is_nothrow_invocable_v<HandleF, T*, opaque_handle_operation>)

	struct object_handler final : opaque_handle_impl
	{
		constexpr object_handler(const object_handler& c) noexcept(is_nothrow)
			: data(c.handler(c.data, opaque_handle_operation::Clone))
			, handler(c.handler){}
		constexpr object_handler(object_handler&& m) noexcept
			: data(std::exchange(m.data, nullptr))
			, handler(m.handler){}
		constexpr explicit object_handler(T* _data, const HandleF& handler) noexcept
			: data(_data)
			, handler(handler){}

		constexpr virtual ~object_handler() //noexcept(is_nothrow)
		{
			if(data) handler(data, opaque_handle_operation::Delete);
		}

		inline constexpr void* as_raw_ptr() const noexcept override final
		{ return const_cast<T*>(data); }
		constexpr opaque_handle_impl* try_clone() const override final 
		{ return data ? new object_handler(*this) : nullptr; }

		T* data;
		const HandleF& handler;
	};

	return opaque_handle(static_cast<opaque_handle_impl*>(new object_handler(data, handler)));
#undef is_nothrow
}

/// Create and opaque_handle from a data pointer and a move-constructible functor which handles the copying (if possible) and deleting of the object.
///
/// `handler` should be in the form: `auto* (T*, opaque_handle_operation) [noexcept]`. Its lifetime is managed by the returned opaque_handle
template<typename T, OpaqueHandleFunc<T> HandleF> 
constexpr inline opaque_handle make_opaque_handle(T* data, HandleF&& handler) noexcept(std::is_nothrow_move_constructible_v<HandleF>)
{
#define is_nothrow (std::is_nothrow_invocable_v<HandleF, T*, opaque_handle_operation>)
#define is_nothrow_ctor (std::is_nothrow_move_constructible_v<HandleF>)

	struct object_handler final : opaque_handle_impl 
	{
		constexpr object_handler(const object_handler& c) 
				noexcept(is_nothrow && std::is_nothrow_copy_constructible_v<HandleF>) requires(std::is_copy_constructible_v<HandleF>)
			: data(c.handler(c.data, opaque_handle_operation::Clone))
			, handler(c.handler){}
		constexpr object_handler(object_handler&& m) noexcept
			: data(std::exchange(m.data, nullptr))
			, handler(std::move(m.handler)){}

		constexpr explicit object_handler(T* _data, HandleF&& handler) noexcept(is_nothrow_ctor)
			: data(_data)
			, handler(std::move(handler)){}

		constexpr virtual ~object_handler() //noexcept(is_nothrow)
		{
			if(data) handler(data, opaque_handle_operation::Delete);
		}

		inline constexpr void* as_raw_ptr() const noexcept override final
		{ return const_cast<T*>(data); }
		constexpr opaque_handle_impl* try_clone() const override final 
		{
			if constexpr(std::is_copy_constructible_v<HandleF>) { 
				return data ? new object_handler(*this) : nullptr; 
			} else return nullptr;
		}

		T* data;
		HandleF handler;
	};

	return opaque_handle(static_cast<opaque_handle_impl*>(new object_handler(data, std::forward<HandleF>(handler))));
#undef is_nothrow
#undef is_nothrow_ctor
}

/// Create and opaque_handle from a data pointer and a constexpr handler funcion template param (e.g. a lambda)
///
/// `Func` should be in the form: `auto* (T*, opaque_handle_operation) [noexcept]`. It must be a constant expression.
template<typename T, OpaqueHandleFunc<T> auto Func>
constexpr inline opaque_handle make_opaque_handle(T* data) noexcept
{
	using HandleF = decltype(Func);
#define is_nothrow (std::is_nothrow_invocable_v<HandleF, T*, opaque_handle_operation>)

	struct object_handler final : opaque_handle_impl
	{
		constexpr object_handler(const object_handler& c) noexcept(is_nothrow) 
			: data(Func(c.data, opaque_handle_operation::Clone)){}

		constexpr object_handler(object_handler&& m) noexcept
			: data(std::exchange(m.data, nullptr)){}

		constexpr explicit object_handler(T* data) noexcept
			: data(data){}

		constexpr virtual ~object_handler() //noexcept(is_nothrow)
		{
			if(data) Func(data, opaque_handle_operation::Delete);
		}

		inline constexpr void* as_raw_ptr() const noexcept override final
		{ return const_cast<T*>(data); }
		constexpr opaque_handle_impl* try_clone() const override final 
		{ return data ? new object_handler(*this) : nullptr; }

		T* data;
	};

	return opaque_handle(static_cast<opaque_handle_impl*>(new object_handler(data)));
#undef is_nothrow
}

#undef $_oe_msg
#undef $_oe