c 2b 2b initialize vector of vector with size

1vector<vector<int>> v(10, vector<int>(10));

1vector<vector<vector<double>>> f(3, vector<vector<double>>(4, vector<double>(5)));
2

1// create a vector with 20 integer elements
2std::vector<int> arr(20);
3
4for(int x = 0; x < 20; ++x)
5   arr[x] = x;

1#include <vector>
2
3auto n = 20
4// create a vector with n=20 integer elements
5std::vector<int> arr(n);