c++ - boost ptr_vector 迭代器

Question

我正在遍历树节点的子节点。子级存储在 ptr_vector 中，在整个迭代过程中的某个时刻我陷入无限递归，但我不明白为什么。

这里是无限递归发生的方法(这个方法只是用来打印树结构到cout):

std::ostream& operator<<(std::ostream &strm, const node &n) {
    if (n.children_.empty())
        return strm << "[]";

    for (boost::ptr_vector<node::edge>::const_iterator iter = n.children_.begin(); iter != n.children_.end(); ++iter)
    {
        if (iter != n.children_.begin())
            strm << ", ";
        strm << "-" << iter->distance << "->[ " << *(iter->dest) << "]";
    }

    return strm;
}

这是我正在导航的树结构(请注意，嵌套 edge 的目的是表示父节点和子节点之间的距离):

class node
{
public:
    node(void);
    ~node(void);

    node* add_child(unsigned int d);
    node* get_closest(void);

    friend std::ostream& operator<<(std::ostream&, const node&);

private:
    class edge
    {
    public:
        edge(node* n, unsigned int d);
        ~edge(void);

        unsigned int distance;
        node* dest;
    };

    boost::ptr_vector<edge> children_;
};

此外，我注意到这种无限递归仅在调用以下方法之后发生:

node* node::get_closest(void) const
{
    if (children_.empty())
        return NULL;

    boost::ptr_vector<node::edge>::const_iterator iter = children_.begin();
    node::edge closest = *iter;
    ++iter;

    if (iter != children_.end())
    {
        for (; iter != children_.end(); ++iter)
        {
            if (iter->distance < closest.distance)
                closest = *iter;
        }
    }

    return closest.dest;
}

为什么这个方法会导致无限递归？谢谢!

Answer 1

根据@RobI 的建议，原始指针和共享指针的混合导致了问题。我将 node::edge closest = *iter; 更改为 const node::edge* closest = &(*iter);。这是该方法的新(有效)版本:

node* node::get_closest(void) const
{
    if (children_.empty())
        return NULL;

    boost::ptr_vector<node::edge>::const_iterator iter = children_.begin();
    const node::edge* closest = &(*iter);
    ++iter;

    if (iter != children_.end())
    {
        for (; iter != children_.end(); ++iter)
        {
            if (iter->distance < closest->distance)
                closest = &(*iter);
        }
    }

    return closest->dest;
}