Vector of list in c++ for implementing adjacency lists

Question

I am trying to implement a graph using adjacency list, using vector of lists, but the code is not compiling.

class Graph
{
    public:
        vector<list<int> > adj;
        int V;
        int E;

        Graph(int N);
        void addEdge(int u, int v);
};

Graph::Graph(int N)
{
    adj = new vector<list<int> >(N+1);
}

void Graph::addEdge(int u, int v)
{
    adj[u].push_back(v);
}

Answer 1

C++ is not Java or C#, you don't need to use new to create new objects.

Instead use a member initializer list to construct the adj member:

Graph::Graph(int N)
    : adj(N + 1)
{
}

Answer 2

vector<list<int> > adj; Defines a member object of your Graph . You don't need to use new to initialize it (this isn't Java). Just call the vector c'tor in the member initializer list.

Graph::Graph(int N) : adj(N+1)
{
}

Answer 3

Using new , you have to assign to a pointer. I modified your code such that it now compiles fine:

class Graph
{
    public:
        vector<list<int> > * adj;
        int V;
        int E;

        Graph(int N);
        void addEdge(int u, int v);
};

Graph::Graph(int N)
{
    adj = new vector<list<int> >(N+1);
}

void Graph::addEdge(int u, int v)
{
    (*adj)[u].push_back(v);
}

Edit

As others have pointed out, it's better to just do:

class Graph
{
public:
    vector<list<int> > adj;
    int V;
    int E;

    Graph(int N) : adj(N+1) {}
    void addEdge(int u, int v);
};

void Graph::addEdge(int u, int v)
{
    adj[u].push_back(v);
}

int main()
{
    return 0;
}