commit 70d92c60b1bc2da030ad2cff886eaace00596a22
Author: Avril <flanchan@cumallover.me>
Date:   Sat Mar 12 16:44:01 2022 +0000

    Initial commit
    
    Added functional `opaque_object_handle<T>()` -> `opaque_handle`.
    
    Added auto-casting operators for `opaque_handle`.
    
    Fortune for opaque_handle's current commit: Blessing − 吉

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..8919ad2
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+*~
+*.o
+a.out
diff --git a/include/opaque.hh b/include/opaque.hh
new file mode 100644
index 0000000..1925892
--- /dev/null
+++ b/include/opaque.hh
@@ -0,0 +1,302 @@
+#pragma once
+
+#include <utility>
+#include <stdexcept>
+
+#if 0 //XXX: This doesn't work, we need `opaque_handle` to not be a template...
+namespace _opaque_impl {
+struct opaque_handle {
+
+	virtual constexpr ~opaque_handle();
+protected:
+	inline constexpr explicit opaque_handle(void* data) noexcept : _ptr(data){}
+	template<typename From>
+	inline constexpr static opaque_handle from_object(T& ptr) noexcept
+	{
+		return opaque_handle{dynamic_cast<void*>(&ptr) ?: static_cast<void*>(&ptr)};
+	}
+
+	inline constexpr void* ptr() const noexcept { 
+		return _ptr;
+	}
+
+	template<typename To>
+	inline constexpr T* downcast() const noexcept {
+		return dynamic_cast<To*>(_ptr);
+	}
+
+	template<typename To>
+	inline constexpr downcast_unsafe() const noexcept {
+		return static_cast<To*>(_ptr);
+	}
+
+	template<typename To>
+	inline T* ptr_as() { return std::bit_cast<To>(_ptr); }
+private:
+	void* _ptr;
+};
+
+template<typename T> 
+struct opaque_handle_for : opaque_handle
+{
+	constexpr opaque_handle_for(T&& value)
+		: opaque_handle(opaque_handle::template from_object<T>(*new T(std::move(value)))){}
+	constexpr virtual ~opaque_handle_for()
+	{
+		if(T* ptr = downcast_unsafe<T>()) {
+			delete ptr;
+		}
+	}
+}
+}
+
+struct opaque_handle
+{
+private:
+	
+};
+#endif
+
+struct opaque_not_copyable : std::exception{};
+struct opaque_handle_moved : std::exception{};
+struct opaque_bad_cast : std::exception{};
+
+struct opaque_handle;
+struct opaque_handle_impl;
+
+struct opaque_handle_impl {
+	friend class opaque_handle;
+
+	virtual constexpr ~opaque_handle_impl() noexcept {}
+
+	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 
+			? std::addressof(copy._impl->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; }
+
+	template<typename T>
+	constexpr const T* try_cast() const noexcept { return static_cast<const T*>(as_ptr()); }
+	template<typename T>
+	constexpr T* try_cast() noexcept { return static_cast<T*>(as_ptr()); }
+
+	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*>;
+
+
+/// 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
+{
+	constexpr bool is_nothrow = std::is_nothrow_invocable_v<HandleF, T*, opaque_handle_operation>;
+
+	struct object_handler 
+	{
+		constexpr object_handler(const object_handler& c) noexcept(is_nothrow)
+			: data(c.handler(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 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);
+		}
+
+		T* data;
+		const HandleF& handler;
+	};
+
+	return make_opaque_object_handle(object_handler{data, handler});
+}
+
+/// 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>)
+{
+	constexpr bool is_nothrow = std::is_nothrow_invocable_v<HandleF, T*, opaque_handle_operation>;
+	constexpr bool is_nothrow_ctor = std::is_nothrow_move_constructible_v<HandleF>;
+
+	struct object_handler 
+	{
+		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(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 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);
+		}
+
+		T* data;
+		HandleF handler;
+	};
+
+	return make_opaque_object_handle(object_handler{data, std::forward<HandleF>(handler)});
+}
+
+/// 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);
+	constexpr bool is_nothrow = std::is_nothrow_invocable_v<HandleF, T*, opaque_handle_operation>;
+
+	struct object_handler 
+	{
+		constexpr object_handler(const object_handler& c) noexcept(is_nothrow) 
+			: data(Func(data, opaque_handle_operation::Clone)){}
+
+		constexpr object_handler(object_handler&& m) noexcept
+			: data(std::exchange(m.data, nullptr)){}
+
+		constexpr object_handler(T* data) noexcept
+			: data(data){}
+
+		constexpr /*virtual*/ ~object_handler() noexcept(is_nothrow)
+		{
+			if(data) Func(data, opaque_handle_operation::Delete);
+		}
+
+		T* data;
+	};
+
+	return make_opaque_object_handle(object_handler{data});
+}
diff --git a/src/main.cpp b/src/main.cpp
new file mode 100644
index 0000000..43e03be
--- /dev/null
+++ b/src/main.cpp
@@ -0,0 +1,35 @@
+#include <cstdio>
+#include <unistd.h>
+
+#include "../include/opaque.hh"
+
+
+static void print_view(std::string_view ptr)
+{
+	//std::string_view ptr{string};
+
+	size_t sz;
+#if DIRECT_WRITE
+	if(__builtin_expect((sz = ptr.size()) > 0, true)) {
+		write(1, "> ", 2);
+		write(1, ptr.data(), sz);
+	} else	write(1, "! <null>\n", 8);
+	write(1, "\n", 1);
+#else
+	if(__builtin_expect((sz = ptr.size()) > 0, true)) {
+		printf("> %.*s\n", (int)sz, ptr.data());
+	} else puts("! <null>\n");
+#endif
+}
+static inline void print(const std::string& string)
+{
+	std::string_view sv{string};
+	return print_view(sv);
+}
+
+int main()
+{
+	const opaque_handle v{make_opaque_object_handle(std::string{"Hello world"})};
+	print(*v); //XXX: Why does converting it to string_view here break it?
+	return 0;
+}