initialising array of vectors in constructor

I have a class named Graph. There this vertices member of the class. I have initialized vertices in the constructor. Also, There is a member array of vectors. I want the number of vectors to be equal to vertices. for example if vertices = 5 then my array of vectors should look like this. vector v[5]; How can I do this in the constructor as I will only know the value of vertices in the constructor?

class Graph
{

private:
    int vertices;
    std::vector<int> adj[];
public:
    Graph(int v); //constructor
    // add an edge
    void addEdge(int u, int v);
    //print bfs traversal of graph
    void bfs(int s); // s is a source from where bfs traversal should 
                     //start

};

Graph :: Graph(int v)
{
   vertices = v;
}