tdesktop/Telegram/SourceFiles/base/optional.h

/*
This file is part of Telegram Desktop,
the official desktop application for the Telegram messaging service.

For license and copyright information please follow this link:
https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
*/
#pragma once

#include <gsl/gsl_assert>
#include "base/variant.h"

namespace base {

struct none_type {
	bool operator==(none_type other) const {
		return true;
	}
	bool operator!=(none_type other) const {
		return false;
	}
	bool operator<(none_type other) const {
		return false;
	}
	bool operator<=(none_type other) const {
		return true;
	}
	bool operator>(none_type other) const {
		return false;
	}
	bool operator>=(none_type other) const {
		return true;
	}

};

constexpr none_type none = {};

template <typename... Types>
class optional_variant {
public:
	optional_variant() : _impl(none) {
	}
	optional_variant(const optional_variant &other) : _impl(other._impl) {
	}
	optional_variant(optional_variant &&other) : _impl(std::move(other._impl)) {
	}
	template <typename T, typename = std::enable_if_t<!std::is_base_of<optional_variant, std::decay_t<T>>::value>>
	optional_variant(T &&value) : _impl(std::forward<T>(value)) {
	}
	optional_variant &operator=(const optional_variant &other) {
		_impl = other._impl;
		return *this;
	}
	optional_variant &operator=(optional_variant &&other) {
		_impl = std::move(other._impl);
		return *this;
	}
	template <typename T, typename = std::enable_if_t<!std::is_base_of<optional_variant, std::decay_t<T>>::value>>
	optional_variant &operator=(T &&value) {
		_impl = std::forward<T>(value);
		return *this;
	}

	bool has_value() const {
		return !is<none_type>();
	}
	explicit operator bool() const {
		return has_value();
	}
	bool operator==(const optional_variant &other) const {
		return _impl == other._impl;
	}
	bool operator!=(const optional_variant &other) const {
		return _impl != other._impl;
	}
	bool operator<(const optional_variant &other) const {
		return _impl < other._impl;
	}
	bool operator<=(const optional_variant &other) const {
		return _impl <= other._impl;
	}
	bool operator>(const optional_variant &other) const {
		return _impl > other._impl;
	}
	bool operator>=(const optional_variant &other) const {
		return _impl >= other._impl;
	}

	template <typename T>
	decltype(auto) is() const {
		return _impl.template is<T>();
	}
	template <typename T>
	decltype(auto) get_unchecked() {
		return _impl.template get_unchecked<T>();
	}
	template <typename T>
	decltype(auto) get_unchecked() const {
		return _impl.template get_unchecked<T>();
	}

private:
	variant<none_type, Types...> _impl;

};

template <typename T, typename... Types>
inline T *get_if(optional_variant<Types...> *v) {
	return (v && v->template is<T>()) ? &v->template get_unchecked<T>() : nullptr;
}

template <typename T, typename... Types>
inline const T *get_if(const optional_variant<Types...> *v) {
	return (v && v->template is<T>()) ? &v->template get_unchecked<T>() : nullptr;
}

template <typename Type>
class optional;

template <typename Type>
struct optional_wrap_once {
	using type = optional<Type>;
};

template <typename Type>
struct optional_wrap_once<optional<Type>> {
	using type = optional<Type>;
};

template <typename Type>
using optional_wrap_once_t = typename optional_wrap_once<std::decay_t<Type>>::type;

template <typename Type>
struct optional_chain_result {
	using type = optional_wrap_once_t<Type>;
};

template <>
struct optional_chain_result<void> {
	using type = bool;
};

template <typename Type>
using optional_chain_result_t = typename optional_chain_result<Type>::type;

template <typename Type>
class optional : public optional_variant<Type> {
public:
	using optional_variant<Type>::optional_variant;

	Type &operator*() {
		auto result = get_if<Type>(this);
		Expects(result != nullptr);
		return *result;
	}
	const Type &operator*() const {
		auto result = get_if<Type>(this);
		Expects(result != nullptr);
		return *result;
	}
	Type *operator->() {
		auto result = get_if<Type>(this);
		Expects(result != nullptr);
		return result;
	}
	const Type *operator->() const {
		auto result = get_if<Type>(this);
		Expects(result != nullptr);
		return result;
	}

};

template <typename Type>
optional_wrap_once_t<Type> make_optional(Type &&value) {
	return optional_wrap_once_t<Type> { std::forward<Type>(value) };
}

template <typename Type, typename Method>
inline auto operator|(const optional<Type> &value, Method method)
-> optional_chain_result_t<decltype(method(*value))> {
	if constexpr (std::is_same_v<decltype(method(*value)), void>) {
		return value ? (method(*value), true) : false;
	} else {
		return value ? make_optional(method(*value)) : none;
	}
}

} // namespace base