链表输出崩溃

程序在输出链表内容时挂起。我不能更改头文件，只能更改cpp文件。

playlist.h:

class PlayList {
    private:
    struct SongNode {
        Song data;
        SongNode* next;
        SongNode (const Song& s, SongNode* nxt = 0)
          : data(s), next(nxt)
        {}
    };
    friend std::ostream& operator<< (std::ostream& out, const PlayList& s);
    SongNode* first; // head of a list of songs, ordered by title, then artist
    //Snip...
    inline
    std::ostream& operator<< (std::ostream& out, const PlayList& pl)
    {
        for (PlayList::SongNode* current = pl.first; current != 0; current = current->next)
            out << current->data << std::endl;
        return out;
    }

playlist.cpp

using namespace std;

PlayList::PlayList()
    : totalTime(0,0,0)
{
}

void PlayList::operator+= (const Song& song)
{
    SongNode* newNode = new SongNode(song, first);
    first = newNode;
}

当输出列表时，所有应该打印的数据都打印出来了，但是随后程序挂起了