class node :
        public std::enable_shared_from_this<node>
{
public:
    using ptr  = std::shared_ptr<node>;
    using cref = const ptr &;

    void add_child(cref n)
    {
        if(std::find(childs.begin(), childs.end(), n) != childs.end()) return;
        auto par = n->parent.lock();
        if(par)
            par->remove_child(n);
        n->parent = shared_from_this();
        childs.push_back(n);
    }
    void remove_child(cref n)
    {
        if(n)
        {
            childs.erase(std::remove(childs.begin(), childs.end(), n), childs.end());
            n->parent.reset();
        }
    }
    ptr get_child(size_t n)
    {
        return childs.at(n);
    }
    void set_parent(cref n)
    {
        auto par = parent.lock();
        if (par == n || !n)
            return;
        n->add_child(shared_from_this());
    }
private:
    std::weak_ptr<node> parent;
    std::vector<ptr>    childs;
};