expose a 2d array immutably from a c struct

Question

I read this: https://stackoverflow.com/questions/46209696/how-to-use-2d-arrays-inside-a-c-struct

I did something like that in order to protect the internals of a "graph_t" structure. is there a more conventional way of doing it?

1st method in graph.h

typedef struct graph_t graph_t;
const bool (*graph_get_adjacency_matrix(graph_t *graph))[MAX_NODES];

in graph.c

struct graph_t
{
  int num_nodes;

  int existing_nodes[MAX_NODES];
  bool adjacency_matrix[MAX_NODES][MAX_NODES];
};

const bool (*graph_get_adjacency_matrix(graph_t *graph))[MAX_NODES]
{
  return (const bool (*)[MAX_NODES])graph->adjacency_matrix;
}

in test.c

  const bool (*adj)[MAX_NODES] = graph_get_adjacency_matrix(g);
  bool value = adj[i][j];

2nd method in graph.h

typedef struct graph_t graph_t;
const bool (*graph_get_adjacency_matrix(graph_t *graph))[MAX_NODES];

in graph.c

struct graph_t
{
  int num_nodes;

  int existing_nodes[MAX_NODES];
  bool *adjacency_matrix[MAX_NODES];
};

const bool *const *graph_get_adjacency_matrix(graph_t *graph)
{
  return (const bool *const *)graph->adjacency_matrix;
}

in test.c

  const bool *const *adj = graph_get_adjacency_matrix(g);
  bool value = adj[i][j];

I tried to encapsulate a structure representing a graph.