What is causing my segfault when indexing this 2D vector?

When I am indexing the 2D vector scoremap with the line: int tile = this->scoremap[x][y];, I am getting a segfault. Can anyone explain what I may be doing wrong?

#include "minesweeper.h"
using namespace std;
Minesweeper::Minesweeper() {
// Easy difficulty default values
map_size = 5;
mines = 8;
score = 0;
// Create displaymap & scoremap based on map_size
// displaymap is chars, initialized to quesiton marks (?)
// scoremap is ints, initliazed to zeros (0)
vector<vector<char>> displaymap(map_size, vector<char>(map_size, '?'));
vector<vector<int>> scoremap(map_size, vector<int>(map_size, 0));
};
// Generate mines / score tiles on the scoremap
void Minesweeper::generate_scoremap() {
// First, generate mines on map
int mine_count = this->mines;
// While we have mines to place
while (mine_count != 0) {
// Get a random position on the map
pair<int,int> position = random_position(this->map_size);
int x = position.first;
int y = position.second;
// If it's not already a mine (mines = -1, other tiles = how many mines are touching the tile)
int tile = this->scoremap[x][y];
if (tile != -1) {
tile = -1;
this->scoremap[x][y] = tile;
mine_count--;
}
else {
// find a new spot for this mine
}
}
// Next, set score of each tile that isn't a mine
// Direciton pairs for checking adjacent tiles
vector<std::pair<int, int>> directions = 
{
{-1, -1}, {-1, 0}, {-1, 1},
{0, -1},            {0, 1},
{1, -1},  {1, 0},  {1, 1}
};
// Check each tile and set its score
for (int i = 0; i < this->map_size; i++) {
for (int j = 0; j < this->map_size; j++) {
int tilescore = 0;
// Check to make sure it isn't a mine
if (this->scoremap[i][j] != -1) {
// Check all adjacent cells for mines
for (const auto& dir : directions) {
int newi = i + dir.first;
int newj = j + dir.second;
// If the index is in the map
if (index_valid(newi,newj,this->map_size)) {
// and its a mine, increase tile score
if (this->scoremap[newi][newj] == -1) {
tilescore++;
}
}
}
// Set tile score on scoremap
this->scoremap[i][j] = tilescore;
tilescore = 0;
}
}
}
}

I tried manually creating the 2D vector with the right characters and right size, and it seems to work, but I would like the 2D vector to be generated dynamically according to whatever the map_size variable is (here it's set automatically in the constructor, but later I would like it to be a value from the user, or from picking a specific difficulty level).

Minesweeper's constructor is creating local variables that shadow the member variables of the same names. So, you are not actually putting any data into the member variables that you can then index into later.

Try this instead:

Minesweeper::Minesweeper()
// Easy difficulty default values
map_size = 5;
mines = 8;
score = 0;
// Create displaymap & scoremap based on map_size
// displaymap is chars, initialized to quesiton marks (?)
// scoremap is ints, initliazed to zeros (0)
displaymap.resize(map_size, vector<char>(map_size, '?'));
scoremap.resize(map_size, vector<int>(map_size, 0));
};