optional_reference.hpp

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/*
    Copyright (C) 2021 - 2022
    Part of the Battle for Wesnoth Project https://www.wesnoth.org/

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 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.

    See the COPYING file for more details.
*/

#pragma once

#include <functional>
#include <optional>

namespace utils
{
/**
 * A simple wrapper class for optional reference types.
 *
 * Since std::optional (as of C++17 at least) does not support reference types (see [1]),
 * the only way to use those is std::optional<std::reference_wrapper<T>>. However, this makes
 * the interace messy, as to access the referenced object you need an extra get() call to
 * access the value stored in the reference wrapper.
 *
 * This rebinds operator=() as boost::optional does. Assigning a value to this wrapper will
 * simply change the object to which it points instead of assigning a value to the referenced
 * object. To change the value of the referenced object, perform an assignment on value()
 * or operator*.
 *
 * [1] https://www.fluentcpp.com/2018/10/05/pros-cons-optional-references/
 */
template<typename T>
class optional_reference
{
public:
    optional_reference() = default;

    optional_reference(T& ref)
        : opt_(ref)
    {
    }

    optional_reference(std::nullopt_t)
        : opt_()
    {
    }

    T& value() const
    {
#ifndef __APPLE__
        return opt_.value().get();
#else
        if(opt_) {
            return opt_->get();
        } else {
            // We're going to drop this codepath once we can use optional::value anyway, but just
            // noting we want this function to ultimately throw std::bad_optional_access.
            throw std::runtime_error("Optional reference has no value");
        }
#endif
    }

    optional_reference<T>& operator=(T& new_ref)
    {
        opt_ = new_ref;
        return *this;
    }

    explicit operator bool() const
    {
        return opt_.has_value();
    }

    /** Returns a pointer to the referenced object or nullptr if no reference is held. */
    T* ptr() const
    {
        if(opt_) {
            return &value();
        } else {
            return nullptr;
        }
    }

    T* operator->() const
    {
        return &value();
    }

    T& operator*() const
    {
        return value();
    }

private:
    std::optional<std::reference_wrapper<T>> opt_;
};

} // namespace utils