C++调试报错 has incomplete type

这样写 不报错

template<typename E,typename W>
class ListGraph{
    class Edge; 
    class Vertex;
public:
    //unordered_map<E,Vertex> vertices; //用来存放 顶点

    ListGraph(){}
    ~ListGraph(){}
    int VertexSize();
    int EdgeSize();
    void addVertex(E element);
    void addEdge(E from,E to,W weight);
    void addEdge(E from,E to);
    void removeVertex(E element);
    void removeEdge(E form,E to);
};

template<typename E,typename W> class ListGraph<E,W>::Edge{
    Vertex m_from,m_to;
    W m_weight;
    Edge(Vertex from,Vertex to,W weight):m_from(from),m_to(to),m_weight(weight){}
    ~Edge(){}
};
template<typename E,typename W> class ListGraph<E,W>::Vertex{
    E m_element;
    unordered_multiset<Edge >m_inEdges,m_outEdges;
    Vertex(E element):m_element(element){}
    ~Vertex(){}
};

如果 我把 注释的那一句

unordered_map<E,Vertex> vertices; //用来存放 顶点

就报错了
错误 ：

你这里Edge和Vertex类相互依赖，只能通过指针来实现

#include <iostream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <memory>
#include <vector>

using namespace std;

template <typename E, typename W>
class ListGraph
{
    class Edge;
    class Vertex;

public:
    unordered_map<E, unique_ptr<Vertex>> vertices; //用来存放 顶点
    vector<unique_ptr<Edge>> edges;

    ListGraph() {}
    ~ListGraph() {}
    int VertexSize();
    int EdgeSize();
    void addVertex(E element);
    void addEdge(E from, E to, W weight);
    void addEdge(E from, E to);
    void removeVertex(E element);
    void removeEdge(E form, E to);
};

template <typename E, typename W>
class ListGraph<E, W>::Edge
{
public:
    Vertex *m_from, *m_to;
    W m_weight;
    Edge(Vertex *from, Vertex *to, W weight) : m_from(from), m_to(to), m_weight(weight) {}
    ~Edge() {}
};

template <typename E, typename W>
class ListGraph<E, W>::Vertex
{
public:
    E m_element;
    unordered_multiset<Edge *> m_inEdges, m_outEdges;
    Vertex(E element) : m_element(element) {}
    ~Vertex() {}
};

int main()
{
    ListGraph<string, int> gp;
    return 0;
}