Array of pointers remaining null despite being directly set - c++

Question

I have a list of 'cell' objects in a 2d array of length [sizeX][sizeY]. These Cell objects contain an array of type *Cell, which should point to each of the given cell's adjacent cells in the format North, East, South, West (Never Eat Soggy Waffles).
This array is called compass, and is defined with a length of 4. After the cells have been initialized (at which time all values of compass are set to nullptr), I have a loop which attempts to fill Cell.compass with pointers to the appropriate nearby cells.
However, I have found that despite all this, in each cell I have found that compass still is full of null values.
In this loop, I also run a function Cell::computeTopology() which populates a vector (which is a property of Cell) of the valid non-null indexes of compass. This is similarly empty.
I have made a breakpoint both before and after this function has been called to the exact same effect. Regardless I will include this function as well. I'm utterly perplexed and some help here would be greatly appreciated.
Problem area:
const int sizeX = SCREEN_WIDTH / SCALE;
const int sizeY = SCREEN_HEIGHT / SCALE;
Cell cells[sizeX][sizeY];
for (int x = 0; x < sizeX; x++){
for (int y = 0; y < sizeY; y++){
cells[x][y].setPos(x, y);
cells[x][y] = Cell();
//cells[x][y].setColor(rand() % 255, rand() % 255, rand() % 255);
}
}
for (int x = 0; x < sizeX; x++) {
for (int y = 0; y < sizeY; y++) {
Cell c = cells[x][y];
if (x - 1 >= 0) {
c.compass[3] = &cells[x - 1][y];
}
if (x + 1 < (SCREEN_WIDTH / SCALE)) {
c.compass[1] = &cells[x + 1][y];
}
if (y - 1 >= 0) {
c.compass[0] = &cells[x][y - 1];
}
if (y + 1 < (SCREEN_HEIGHT / SCALE)) {
c.compass[2] = &cells[x][y + 1];
}
c.computeTopology();
}
}
And the computeTopology() function
void Cell::computeTopology()
{
int i = 0;
for (Cell *c : compass) {
if (c != nullptr) {
notNull.push_back(i);
i++;
}
}
}

Answer 1

Change
Cell c = cells[x][y]; // make a copy
to
Cell& c = cells[x][y];
you want to modify item of array, not copy.
Another issue
cells[x][y].setPos(x, y);
cells[x][y] = Cell(); // ?
you are setting some members of cells[x][y] by calling setPos and after it you are overwriting cells[x][y] by default constructed Cell object. I think the second line in above code should be removed.
Your words
which populates a vector (which is a property of Cell) of the valid
non-null indexes of compass
so i index should be advanced with every iteration of for loop:
int i = 0;
for (Cell *c : compass) {
if (c != nullptr) {
notNull.push_back(i);
}
++i; // moved
}