C++ 类中的循环依赖

Question

I am relatively new to C++, and facing a circular dependency problem. Can someone please help me solve this?

class Vertex {
    string name;
    int distance;
    //Vertex path;
    int weight;
    bool known;
    list<Edge> edgeList;
    list<Vertex> adjVertexList;

public:
    Vertex();
    Vertex(string nm);
    virtual ~Vertex();
};

class Edge {
    Vertex target;
    int weight;

public:
    Edge();
    Edge(Vertex v, int w);
    virtual ~Edge();

    Vertex getTarget();
    void setTarget(Vertex target);
    int getWeight();
    void setWeight(int weight);
};

Answer 1

All you need is to forward-declare the Edge class before it's used within the Vertex :

class Edge;

class Vertex {
    string name;
    int distance;
    ...
};

class Edge { ... };

You can't put members of type Vertex instead of the declaration of Vertex itself, since C++ doesn't allow recursive types. Within the semantics of C++, the size of such type would need to be infinite.

You can, of course, put pointers to Vertex within Vertex .

What you want, in fact, within Vertex 's edge and adjacency lists, is pointers, not copies of objects. Thus, your code should be fixed like below (assuming you use C++11, which you in fact should be using now):

class Edge;

class Vertex {
    string name;
    int distance;
    int weight;
    bool known;
    list<shared_ptr<Edge>> edgeList;
    list<shared_ptr<Vertex>> adjVertexList;

public:
    Vertex();
    Vertex(const string & nm);
    virtual ~Vertex();
};

class Edge {
    Vertex target;
    int weight;

public:
    Edge();
    Edge(const Vertex & v, int w);
    virtual ~Edge();

    Vertex getTarget();
    void setTarget(const Vertex & target);
    int getWeight();
    void setWeight(int weight);
};

Answer 2

If you think about it, instantiating a single Vertex<\/code> or Edge<\/code> object would instantiate an infinite amount of more Vertex<\/code> and Edge<\/code> objects because each of them contain instances of each other.

class Edge; // This is a forward declaration

class Vertex {
    string name;
    int distance;
    //Vertex path;
    int weight;
    bool known;
    list<Edge*> edgeList;
    list<Vertex*> adjVertexList;

public:
    Vertex();
    Vertex(string nm);
    virtual ~Vertex();
};

class Edge {
    Vertex* target;
    int weight;

public:
    Edge();
    Edge(Vertex* v, int w);
    virtual ~Edge();

    Vertex* getTarget();
    void setTarget(Vertex* target);
    int getWeight();
    void setWeight(int weight);
};