Matrix in single piese on memoty (almost)

main.c

#include <stdio.h>
#include <stdlib.h>

#define PTR_SZ sizeof(void*)
#define INT_SZ sizeof(int)

struct monomtx;

int mmtx_get(struct monomtx*, int, int);
void mmtx_set(struct monomtx*, int, int, int);

// consider, that `m` is row count, whereaas, `n` is col count
struct monomtx {
  int** row_ptrs;
  int* data;
  int w, h;
  int (*get)(struct monomtx*, int, int);
  void (*set)(struct monomtx*, int, int, int);
};

// get data from grid
int mmtx_get(struct monomtx* self, int x, int y) {
  int* row_ptr = *(self->row_ptrs + y);
  return *(row_ptr + x * INT_SZ);
}

// get data into grid
void mmtx_set(struct monomtx* self, int x, int y, int data) {
  int* row_ptr = *(self->row_ptrs + y);
  *(row_ptr + x * INT_SZ) = data;
}

typedef struct monomtx monomtx_t;

// `constructor
monomtx_t* mmtx_new(int m, int n) {
  monomtx_t* res = (monomtx_t*)malloc(sizeof(monomtx_t));
  res->h = m;
  res->w = n;
  res->get = &mmtx_get;
  res->set = &mmtx_set;
  res->row_ptrs = (int**)malloc(PTR_SZ * m);
  res->data = (int*)malloc(INT_SZ * n * m);
  for (int i = 0; i < m; ++i) {
    res->row_ptrs[i] = res->data + n * INT_SZ * i;
  }
  return res;
}

// `destructor`
void mmtx_drop(monomtx_t* self) {
  free(self->data);
  for (int i = 0; i < self->h; ++i)
    free(self->row_ptrs[i]);
  free(self->row_ptrs);
  free(self);
}

// usage
int main() {
  monomtx_t* monomatrix = mmtx_new(3, 4);
  monomatrix->set(monomatrix, 0, 0, 40);
  printf("%d\n", monomatrix->get(monomatrix, 1, 0));
  mmtx_drop(monomatrix);
  return 0;
}