看一些代码看到源码中模板的写法：

// DoublyLinkedListLink
template<typename Element>
class DoublyLinkedListLink {
public:
    Element*    next;
    Element*    previous;
};

// DoublyLinkedListMemberGetLink
template<typename Element,
    DoublyLinkedListLink<Element> Element::* LinkMember = &Element::fLink>
class DoublyLinkedListMemberGetLink {
private:
    typedef DoublyLinkedListLink<Element> Link;

public:
    inline Link* operator()(Element* element) const
    {
        return &(element->*LinkMember);
    }

    inline const Link* operator()(const Element* element) const
    {
        return &(element->*LinkMember);
    }
};

这行代码:

template<typename Element,
    DoublyLinkedListLink<Element> Element::* LinkMember = &Element::fLink>

模板的第二个参数是什么用法，没搞懂什么意思..
没搞懂模板这种用法的目的是啥