"Cannot access memory at address" on array of vectors - c++

Question

I am trying to make a Graph in C++, and I have almost no code, but I get some weird error. If I just run code, I get Process finished with exit code 0, but if I take a look at debugger (specifically, it I try to check my graph object), I see Cannot access memory at address 0x....
I am new to C++, so I cannot really get what has given me this error. Also, I almost don't have code yet & these few lines I took from my previous program which worked without this problem.
Anyway, I have a vertex class:
#ifndef P2_VERTEX_H
#define P2_VERTEX_H
class vertex {
private:
int id;
public:
explicit vertex(int id) { this->id = id; }
int get_id() { return id; }
};
#endif //P2_VERTEX_H
And then a graph header:
#ifndef P2_GRAPH_H
#define P2_GRAPH_H
#include <vector>
#include "vertex.h"
class graph {
private:
int N; // number of vertices
int M; // number of edges
std::vector<vertex*> vertices; // vector of vertices
std::vector<vertex*> *adj; // ARRAY of vectors of vertices
public:
graph(int n_vert);
graph(bool from_file, const char *file_name);
};
with implementation of graph:
#include "graph.h"
#include <iostream>
#include <fstream>
#include <sstream>
graph::graph(int n_vert) {
N = n_vert;
}
And I instantiate graph as:
#include "graph.h"
int main() {
graph g = graph(4);
return 0;
}
Specifically, I get this mistake if I uncomment std::vector<vertex*> *adj; in a graph header. While I realise that this is probably not the perfect way of storing adjacency list, I fail to see why it gives me an error I mentioned. Especially since I used it before, just instead of std::vector<vertex*> I had std::vector<edge*> where edge was some struct. I tried also to have std::vector<vertex> isntead of std::vector<vertex*> but I have the same error.
Upd:
If I initialize adj in constructor:
adj = new std::vector<vertex*>[N];
I get Duplicate variable object name in debugger after reaching this line.

Answer 1

The issue is that you never initialized adj so it will point to a random location in memory.
You have to initialize the pointer to nullptr to get rid of it.
For example, in the constructor initialization list:
graph::graph(int n_vert) : N(n_vert), adj(nullptr)
{}
By the way you forgot to initialize other field members.