Variadic, minimally type-checked unrestricted union in C++

unrestricted_union.hh

/* Copyright 2021 Stuart Olsen
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to
 * deal in the Software without restriction, including without limitation the
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 */

#ifndef UNRESTRICTED_UNION_HH
#define UNRESTRICTED_UNION_HH

#include <utility>
#include <type_traits>

typedef unsigned int union_index_type;



template <typename...>
struct variadic_union;

template <typename Head_>
struct variadic_union <Head_>
{
	typedef Head_ Head;
	alignas (Head) unsigned char data [sizeof (Head)];

	//

	template <typename T>
	void construct (T&& head_init, union_index_type& union_index)
	{
		static_assert (std::is_same <T, Head>::value, "Tried to access non-existent type in union");
		new (data) Head (std::forward <T> (head_init));
		union_index = 0;
	}

	void destruct (union_index_type)
	{
		static_cast <Head*> (static_cast <void*> (data))->~Head ();
	}
};

template <typename Head_, typename... Tail>
struct variadic_union <Head_, Tail...>
{
	typedef Head_ Head;
	union
	{
		alignas (Head) unsigned char data [sizeof (Head)];
		variadic_union <Tail...> tail;
	};

	//

	void construct (const Head& head_init, union_index_type& union_index)
	{
		new (data) Head (head_init);
		union_index = 0;
	}

	void construct (Head& head_init, union_index_type& union_index)
	{
		const auto& head_cref = head_init;
		construct (head_cref, union_index);
	}

	void construct (Head&& head_init, union_index_type& union_index)
	{
		using std::move;
		new (data) Head (move (head_init));
		union_index = 0;
	}

	template <typename T>
	void construct (T&& t, union_index_type& union_index)
	{
		tail.construct (std::forward <T> (t), union_index);
		++union_index;
	}

	//

	void destruct (union_index_type union_index)
	{
		if (union_index == 0)
			static_cast <Head*> (static_cast <void*> (data))->~Head ();
		else
			tail.destruct (union_index - 1);
	}
};



template <typename...>
struct variadic_union_get;

template <typename Head, typename... Tail>
struct variadic_union_get <Head, variadic_union <Head, Tail...>>
{
	static
	Head& get (variadic_union <Head, Tail...>& vu)
	{
		return *static_cast <Head*> (static_cast <void*> (vu.data));
	}

	static
	const Head& get (const variadic_union <Head, Tail...>& vu)
	{
		return *static_cast <Head*> (static_cast <void*> (vu.data));
	}

	static constexpr
	const union_index_type index = 0;
};

template <typename Target, typename Head, typename... Tail>
struct variadic_union_get <Target, variadic_union <Head, Tail...>>
{
	static
	Target& get (variadic_union <Head, Tail...>& vu)
	{
		return variadic_union_get <Target, variadic_union <Tail...>>::get (vu.tail);
	}

	static
	const Target& get (const variadic_union <Head, Tail...>& vu)
	{
		return variadic_union_get <Target, variadic_union <Tail...>>::get (vu.tail);
	}

	static constexpr
	const union_index_type index = variadic_union_get <Target, variadic_union <Tail...>>::index + 1;
};



#include <stdexcept>

template <typename... Types>
class unrestricted_union
{
	variadic_union <Types...> data;
	union_index_type          union_index;

public:

	template <typename T>
	unrestricted_union (T&& t)
	{
		data.construct (std::forward <T> (t), union_index);
	}

	~unrestricted_union ()
	{
		data.destruct (union_index);
	}

	//

	template <typename T>
	T& get ()
	{
		if (variadic_union_get <T, variadic_union <Types...>>::index != union_index)
			throw std::logic_error ("Accessed wrong type in union");
		return variadic_union_get <T, variadic_union <Types...>>::get (data);
	}

	template <typename T>
	const T& get () const
	{
		if (variadic_union_get <T, variadic_union <Types...>>::index != union_index)
			throw std::logic_error ("Accessed wrong type in union");
		return variadic_union_get <T, variadic_union <Types...>>::get (data);
	}
};

#endif