gplib/include/gp/functional/optional.hpp

#pragma once

#include "gp_config.hpp"

#include "gp/functional/bind_front.hpp"
#include "gp/algorithms/move.hpp"
#include "gp/exception.hpp"

#include <type_traits>
#include <new>

namespace gp{
	struct nullopt_t final{};

	constexpr nullopt_t nullopt;

	template<typename T>
	class optional final{
		bool ready = false;
		alignas(T) char buffer[sizeof(T)];
	public:
		constexpr optional()
		: ready{false}
		{}

		constexpr optional(nullopt_t)
		: ready{false}
		{}

		constexpr optional(T& value)
		: ready{true}
		{
			new(buffer) T(value);
		}

		constexpr optional(T&& value)
		: ready{true}
		{
			new(buffer) T(gp::forward<T>(value));
		}

		~optional() {
			if(ready) {
				((T*)buffer)->~T();
			}
		}

		optional& operator=(nullopt_t) {
			if(ready) {
				((T*)buffer)->~T();
				ready = false;
			}
			return *this;
		}

		optional& operator=(T& value) {
			if(ready) {
				*(T*)buffer = value;
			} else {
				ready = true;
				new(buffer) T(value);
			}
			return *this;
		}

		optional& operator=(T&& value) {
			if(ready) {
				*(T*)buffer = gp::forward<T>(value);
			} else {
				ready = true;
				new(buffer) T(gp::forward<T>(value));
			}
			return *this;
		}


		constexpr bool has_value() const
		{
			return ready;
		}

		constexpr T& value()
		{
			if constexpr (gp_config::has_exceptions)
			{
				if(!ready)
				{
					throw bad_optional{};
				}
			} else {
				gp_config::assertion(ready, "bad optional access");
			}
			return *reinterpret_cast<T*>(buffer);
		}

		constexpr const T& value() const
		{
			if constexpr (gp_config::has_exceptions)
			{
				if(!ready)
				{
					throw bad_optional{};
				}
			} else {
				gp_config::assertion(ready, "bad optional access");
			}
			return *reinterpret_cast<const T*>(buffer);
		}
	};
}