试图显示我的Graph类，有STL映射迭代器的问题

Question

我的图形类基本上由值Vertices之间的map组成，其中每个Vertex本身就是一个类。每个顶点都有一个值和一个相邻列表，它被实现为相邻的Vertices和边权重之间的map。我试图显示图中的每个顶点及其相邻的顶点以及将其连接到其相邻顶点的边的权重。 （对不起，所有的代码）

template <class VertexType> 
void Graph<VertexType>::display() const 
{ 
    typedef map<VertexType, Vertex<VertexType> >::iterator vertices_iter; 
    typedef map<Vertex<VertexType>, int>::iterator adjList_iter; 

    for (vertices_iter v_iter = vertices.begin(); v_iter != vertices.end(); v_iter++) 
    { 
     cout << "Vertex: " << v_iter->second.value << endl; 
     cout << setw(25) << left << "Adjacent to: " << "Edge weight:\n"; 
     for (adjList_iter a_iter = vertices[v_iter->first].adjList.begin(); a_iter != vertices[v_iter->first].adjList.end(); a_iter++) 
      cout << " " << a_iter->first.value << "       " << a_iter->second << endl; 
     cout << endl; 
    } 
} 

，但我得到了以下错误：

错误1：

error C2440: 'initializing' : cannot convert from 'std::_Tree_const_iterator<_Mytree>' to 'std::_Tree_iterator<_Mytree>' 
1>   with 
1>   [ 
1>    _Mytree=std::_Tree_val<std::_Tmap_traits<unsigned int,Vertex<unsigned int>,std::less<unsigned int>,std::allocator<std::pair<const unsigned int,Vertex<unsigned int>>>,false>> 
1>   ] 
1>   No constructor could take the source type, or constructor overload resolution was ambiguous 
1>   c:\users\user\desktop\cs m20a\topic g project 2\topic g project 2\graph.h(241) : while compiling class template member function 'void Graph<VertexType>::display(void) const' 
1>   with 
1>   [ 
1>    VertexType=unsigned int 
1>   ] 

错误2：

error C2678: binary '[' : no operator found which takes a left-hand operand of type 'const std::map<_Kty,_Ty>' (or there is no acceptable conversion) 
1>   with 
1>   [ 
1>    _Kty=unsigned int, 
1>    _Ty=Vertex<unsigned int> 
1>   ] 
1>   c:\program files (x86)\microsoft visual studio 10.0\vc\include\map(164): could be 'Vertex<VertexType> &std::map<_Kty,_Ty>::operator [](unsigned int &&)' 
1>   with 
1>   [ 
1>    VertexType=unsigned int, 
1>    _Kty=unsigned int, 
1>    _Ty=Vertex<unsigned int> 
1>   ] 
1>   c:\program files (x86)\microsoft visual studio 10.0\vc\include\map(209): or  'Vertex<VertexType> &std::map<_Kty,_Ty>::operator [](const unsigned int &)' 
1>   with 
1>   [ 
1>    VertexType=unsigned int, 
1>    _Kty=unsigned int, 
1>    _Ty=Vertex<unsigned int> 
1>   ] 
1>   while trying to match the argument list '(const std::map<_Kty,_Ty>, const unsigned int)' 
1>   with 
1>   [ 
1>    _Kty=unsigned int, 
1>    _Ty=Vertex<unsigned int> 
1>   ] 

，这里是我的Graph类，如果有帮助。

template <class VertexType> 
class Graph 
{ 
private: 
    // list of all vertices in the graph. assumes non-duplicate data. 
    map< VertexType, Vertex<VertexType> > vertices; 

    const unsigned MAX_VERTICES; // Maximum number of vertices the graph can hold. 
    unsigned numVertices;   /** Current number of vertices in the graph. */ 
    unsigned numEdges;   /** Number of edges in the graph. */ 

    typename map<VertexType, int>::iterator findEdge(const VertexType& v, const VertexType& w) const; 

public: 
    Graph(unsigned max); 

    unsigned getNumVertices() const; 
    unsigned getMaxNumVertices() const; 
    unsigned getNumEdges() const; 
    int getWeight(const VertexType& v, const VertexType& w) const; 

    Graph<VertexType>& addVertex(const VertexType& newValue); 
    Graph<VertexType>& addEdge(const VertexType& v, const VertexType& w, int weight); 
    void removeEdge(const VertexType& v, const VertexType& w); 
    void BFS(const VertexType& v) const; 
    void display() const; 
}; // end Graph 

Answer 1

你声明方法const，也就是说，它不会改变任何一个成员变量：

void Graph<VertexType>::display() const 

这意味着你必须只使用const底层成员变量的方法;因此const_iterator为了你们的基本类型：

typedef map<VertexType, Vertex<VertexType> >::const_iterator vertices_iter; 
typedef map<Vertex<VertexType>, int>::const_iterator adjList_iter; 

更重要的是，如果你使用C++11，利用auto。

至于你第二个错误，我无法看到究竟是什么原因造成的，但是，你正在做的工作，查找无故：

for(auto a_iter = vertices[v_iter->first].adjList.begin(); 
    a_iter != vertices[v_iter->first].adjList.end(); 
    a_iter++) 

说v_iter是在原环vertices.begin()。然后vertices[v_iter->first]与*v.begin()实际上是一样的。你可以取代这个：

for(auto a_iter = v_iter->first.adjList.begin(); 
    a_iter != v_iter->first.adjList.end(); 
    a_iter++) 

Answer 2

的问题是，

void Graph<VertexType>::display() const 

是一个const方法，所以图中的每个成员也const的在里面。

在这里，您使用非常量迭代

for (vertices_iter v_iter = vertices.begin(); v_iter != vertices.end(); v_iter++) 

尝试常量迭代器，而不是

typedef map<VertexType, Vertex<VertexType> >::const_iterator vertices_iter;