Optional.h

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/*
Minetest
Copyright (C) 2021  rubenwardy
 
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
 
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Lesser General Public License for more details.
 
You should have received a copy of the GNU Lesser General Public License along
with this program; if not, write to the Free Software Foundation, Inc.,
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/
 
#pragma once
 
#include "debug.h"
 
struct nullopt_t
{
};
constexpr nullopt_t nullopt{};
 
template <typename T>
class Optional
{
    bool m_has_value = false;
    T m_value;
 
public:
    Optional() noexcept {}
    Optional(nullopt_t) noexcept {}
    Optional(const T &value) noexcept : m_has_value(true), m_value(value) {}
    Optional(const Optional<T> &other) noexcept :
            m_has_value(other.m_has_value), m_value(other.m_value)
    {
    }
 
    void operator=(nullopt_t) noexcept { m_has_value = false; }
 
    void operator=(const Optional<T> &other) noexcept
    {
        m_has_value = other.m_has_value;
        m_value = other.m_value;
    }
 
    T &value()
    {
        FATAL_ERROR_IF(!m_has_value, "optional doesn't have value");
        return m_value;
    }
 
    const T &value() const
    {
        FATAL_ERROR_IF(!m_has_value, "optional doesn't have value");
        return m_value;
    }
 
    const T &value_or(const T &def) const { return m_has_value ? m_value : def; }
 
    bool has_value() const noexcept { return m_has_value; }
 
    explicit operator bool() const { return m_has_value; }
};