poco/Foundation/include/Poco/Optional.h

//
// Optional.h
//
// Library: Foundation
// Package: Core
// Module:  Optional
//
// Definition of the Optional class template.
//
// Copyright (c) 2012, Applied Informatics Software Engineering GmbH.
// and Contributors.
//
// SPDX-License-Identifier:	BSL-1.0
//


#ifndef Foundation_Optional_INCLUDED
#define Foundation_Optional_INCLUDED


#include "Poco/Foundation.h"
#include "Poco/Exception.h"
#include <optional>


namespace Poco {


template <typename C>
class Optional
	/// Optional is a simple wrapper class for value types
	/// that allows to introduce a specified/unspecified state
	/// to value objects.
	///
	/// An Optional can be default constructed. In this case,
	/// the Optional will have a Null value and isSpecified() will
	/// return false. Calling value()(without default value) on
	/// a Null object will throw a NullValueException.
	///
	/// An Optional can also be constructed from a value.
	/// It is possible to assign a value to an Optional, and
	/// to reset an Optional to contain a Null value by calling
	/// clear().
	///
	/// For use with Optional, the value type should support
	/// default construction.
	///
	/// Note that the Optional class is basically the same as
	/// Nullable. However, serializers may treat Nullable
	/// and Optional differently. An example is XML serialization based
	/// on XML Schema, where Optional would be used for an element with
	/// minOccurs == 0, whereas Nullable would be used on an element with
	/// nillable == true.
{
public:
	Optional()
		/// Creates an empty Optional.
	{
	}

	Optional(const C& value):
		/// Creates a Optional with the given value.
		_optional(value)
	{
	}

	Optional(C&& value):
		/// Creates a Optional by moving the given value.
		_optional(std::forward<C>(value))
	{
	}

	Optional(const Optional& other):
		/// Creates a Optional by copying another one.
		_optional(other._optional)
	{
	}

	Optional(Optional&& other) noexcept:
		/// Creates a Optional by moving another one.
		_optional(std::move(other._optional))
	{
	}

	~Optional()
		/// Destroys the Optional.
	{
	}

	Optional& assign(const C& value)
		/// Assigns a value to the Optional.
	{
		_optional.emplace(value);
		return *this;
	}

	Optional& assign(C&& value)
		/// Moves a value into the Optional.
	{
		_optional.emplace(std::move(value));
		return *this;
	}

	Optional& assign(const Optional& other)
		/// Assigns another Optional.
	{
		Optional tmp(other);
		swap(tmp);
		return *this;
	}

	Optional& operator = (const C& value)
	{
		return assign(value);
	}

	Optional& operator = (C&& value)
	{
		return assign(std::move(value));
	}

	Optional& operator = (const Optional& other)
	{
		return assign(other);
	}

	Optional& operator = (Optional&& other) noexcept
	{
		_optional = std::move(other._optional);
		return *this;
	}

	void swap(Optional& other) noexcept
	{
		using std::swap;
		swap(_optional, other._optional);
	}

	const C& value() const
		/// Returns the Optional's value.
		///
		/// Throws a Poco::NullValueException if the value has not been specified.
	{
		if (_optional.has_value())
			return _optional.value();

		throw Poco::NullValueException();
	}

	const C& value(const C& deflt) const
		/// Returns the Optional's value, or the
		/// given default value if the Optional's
		/// value has not been specified.
	{
		if (_optional.has_value())
			return _optional.value();

		return deflt;
	}

	bool isSpecified() const
		/// Returns true iff the Optional's value has been specified.
	{
		return _optional.has_value();
	}

	void clear()
		/// Clears the Optional.
	{
		_optional.reset();
	}

private:

	std::optional<C> _optional;
};


template <typename C>
inline void swap(Optional<C>& n1, Optional<C>& n2) noexcept
{
	n1.swap(n2);
}


} // namespace Poco


#endif // Foundation_Optional_INCLUDED