2d push_back doesnt save to vector values

I have a 2d vector which should save x and y coordinates. Everything works as intended except saving this values to vector. What did I do wrong?

void Game::GetShips(Board &b)
{
    vector<vector<int>> shipCors;

    for (int i = 0; i < BOARDSIZE; i++) {

        for (int j = 0; j < BOARDSIZE; j++) {
            if (b.getSpaceValue(i, j) == SHIP) {
                cout << "X :"<< i<<"\n Y:"<<j<<'\n';
                shipCors[i].push_back(j);
              
            }
        }

    }
    cout<< shipCors.size()<<'\n';
 }

You declared an empty vector

vector<vector<int>> shipCors;

So you may not use the subscript operator

shipCors[i].push_back(j);

You could write

for (int i = 0; i < BOARDSIZE; i++) {

    shipCors.resize( shipCors.size() + 1 );
    for (int j = 0; j < BOARDSIZE; j++) {
        if (b.getSpaceValue(i, j) == SHIP) {
            cout << "X :"<< i<<"\n Y:"<<j<<'\n';
            shipCors[i].push_back(j);
          
        }
    }

}

Pay attention to that as you are using the index i then you need to add a "row" of the vector even if the row will be empty after executing the inner for loop.

It will be even better to resize the vector initially before the for loops like

vector<vector<int>> shipCors;
shipCors.resize( BOARDSIZE );

for (int i = 0; i < BOARDSIZE; i++) {

    for (int j = 0; j < BOARDSIZE; j++) {
        if (b.getSpaceValue(i, j) == SHIP) {
            cout << "X :"<< i<<"\n Y:"<<j<<'\n';
            shipCors[i].push_back(j);
          
        }
    }

}

An alternative approach is to have a vector declared like

std::vector<std::pair<int, int>> shipCors;

In this case your loop will look like

for (int i = 0; i < BOARDSIZE; i++) {

    for (int j = 0; j < BOARDSIZE; j++) {
        if (b.getSpaceValue(i, j) == SHIP) {
            cout << "X :"<< i<<"\n Y:"<<j<<'\n';
            shipCors.emplace_back(i, j);
          
        }
    }

}

Or to keep the data sorted you can declare a set like

std::set<std::pair<int, int>> shipCors;