在 c++ 中，如何定义模板类的嵌套类构造函数，该构造函数将同级类作为参数？

问：

给出的例子：DirectedGraph

两个嵌套类的构造函数的定义是什么？根据我在下面的观察，我猜这与构造函数的参数是兄弟类有关。

DirectedGraph.tpp

template < class Key,
           class Compare   = std::less<Key>,
           class Allocator = std::allocator<Key> >
class DirectedGraph
{
public:
    // only used for returns/queries
    class Edge
    {
    public:
        Edge(Vertex&, 
             Vertex&);

    private:
        Vertex& from;
        Vertex& to;
    };


    class Vertex
    {
    public:
        Vertex(Key&);
        ~Vertex(void);

    private:
        value T&;
        std::set<Vertex *> edges;
    };

    DirectedGraph(void);
    ~DirectedGraph(void);

    <...lots of methods...>

private:
    size_t num_edges;
    std::set<DirectedGraph<Key, Compare, Allocator>::Vertex *> vertices;
};

Visual Studio 给我一个错误（声明与”E0147DirectedGraph<Key, Compare, Allocator>::Edge::Edge(<error-type> &, <error-type> &)" )

DirectedGraph.cpp

template <class Key, class Compare, class Allocator>
DirectedGraph<Key, Compare, Allocator>::Edge::Edge
(
    Vertex& from,
    Vertex& to
)
{

}

但奇怪的是，不是使用 Vertex 构造函数：

template<class Key, class Compare, class Allocator>
DirectedGraph<Key, Compare, Allocator>::Vertex::Vertex
(
    Key& value
)
{

}

我猜这与后者不需要其兄弟嵌套类作为构造函数参数有关？

提前致谢，我完全被难住了。

编辑 #1

根据 quimby 的评论或 user7860670 的评论转发声明（或更改位置）解决了错误。经过一些调整，从 168 个错误变成了 1 个。

DirectedGraph.tpp

template < class Key,
           class Compare   = std::less<Key>,
           class Allocator = std::allocator<Key> >
class DirectedGraph
{
public:
    // only used for returns/queries
    class Edge;
    class Vertex;

    class Edge
    {
    ... everything else identical to above
}

DirectedGraph.cpp

template<class Key, class Compare, class Allocator>
DirectedGraph<Key, Compare, Allocator>::Vertex::Vertex
(
    Key& value
)
{

}

template <class Key, class Compare, class Allocator>
DirectedGraph<Key, Compare, Allocator>::Edge::Edge
(
    Vertex& from,
    Vertex& to
)
{

}