value&
value::
operator=(value const& other)
{
    struct undo
    {
        union
        {
            value v;
        };
        value* pv;
        bool commit = false;

        explicit
        undo(value* pv_)
            : pv(pv_)
        {
            relocate(&v, *pv);
        }

        ~undo()
        {
            if(commit)
                v.~value();
            else
                relocate(pv, v);
        }
    };

    if(this != &other)
    {
        undo u(this);
        copy(other,
            u.v.get_storage());
        u.commit = true;
    }

    return *this;
}