shared_value

#pragma once

#include <boost/optional.hpp>

namespace util {

namespace shared_value_detail {
    using boost::optional;

    // Some values that we'd like to put into shared_value don't have the move
    // assign operator. As long as they have the move constructor we can still
    // use them. 
    template<typename T, bool has_move_assign_op> struct assign;

    template<typename T>
    struct assign<optional<T>, true> {
        static void move(optional<T>& dst, optional<T>& src) {
            dst = std::move(src);
        }
    };

    template<typename T>
    struct assign<optional<T>, false> {
        static void move(optional<T>& dst, optional<T>& src) {
            // Destroying should be cheap because we always destroy
            // non initialized optional<T>.
            assert(dst == boost::none);
            dst.~optional<T>();
            new (&dst) optional<T>(std::move(src));
        }
    };
};

template<typename T>
class shared_value {
public:
    using value_type = std::decay_t<T>;

    shared_value() = default;

    shared_value(const shared_value&);
    shared_value& operator=(const shared_value&);

    template<typename K> shared_value(K);

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

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

    ~shared_value();

private:
    void remove_self();
    void append_self_to_end(const shared_value&);
    value_type* value();

private:
    shared_value* _prev = nullptr;
    shared_value* _next = nullptr;

    // We use optional to handle the case where value_type
    // doesn't have the default constructor.
    boost::optional<value_type> _value;
};

template<typename T>
inline
shared_value<T>::shared_value(const shared_value& other)
{
    append_self_to_end(other);
}

template<typename T>
template<typename K>
inline
shared_value<T>::shared_value(K value)
    : _value(std::move(value))
{
}

template<typename T>
inline
shared_value<T>& shared_value<T>::operator=(const shared_value& other)
{
    // Do nothing if this and other already share the same value.
    if (value() == other.value()) return *this;

    remove_self();
    append_self_to_end(other);

    return this;
}

template<typename T>
inline
void shared_value<T>::append_self_to_end(const shared_value& other)
{
    shared_value* last = const_cast<shared_value*>(&other);
    while (last->_next) { last = last->_next; }
    last->_next = this;
    _prev = last;
    assert(_next = nullptr);
}

template<typename T>
inline
void shared_value<T>::remove_self()
{
    if (_prev) {
        // We're not the first, so we don't have the value
        assert(!_value);
        _prev->_next = _next;
        if (_next) { _next->_prev = _prev; _next = nullptr; }
        _prev = nullptr;
        return;
    }

    if (_next == nullptr) {
        // We're the value owners, but we're not sharing it with anyone else.
        // Thus destroy the value and exit.
        _value = boost::none;
        return;
    }

    // We're the owners of the value and we're sharing it with others. Move the
    // value to the next one in line.
    shared_value_detail::assign< boost::optional<value_type>
                               , std::is_move_assignable<value_type>::value
                               >::move(_next->_value, _value);

    _value = boost::none;
    _next->_prev = nullptr;
    _next = nullptr;
}

template<typename T>
inline
typename shared_value<T>::value_type* shared_value<T>::value()
{
    shared_value* first = this;
    while (first->_prev) { first = first->_prev; }
    if (!first->_value) return nullptr;
    return &*first->_value;
}

template<typename T>
inline
shared_value<T>::~shared_value()
{
    remove_self();
}

} // namespace