useful lambda

struct Point {
    int x = 0;
    int y = 0;
};

const Point operator+(const Point& lhs, const Point& rhs) {
    return {lhs.x + rhs.x, lhs.y + rhs.y}; // C++14
//  return Point{lhs.x + rhs.x, lhs.y + rhs.y}; // C++11
}

void bfs(const Matrix<int> a, int N, int M)  {
    const auto inside = [N, M] (const Point& p) {
        return p.x >= 0 && p.y >= 0 && p.x < N && p.y < M;
    };

    // ...
    while (!q.empty()) {
        const Point c = q.top();
        // ...
        for (auto dir : dirs) {
            const Point next_c = c + dir;
            if (inside(next_c) && !visited(next_c)) {
                q.push(next_c);
            }
        }
    }
}