Difference between declaring 2d vectors by two ways

Question

What is difference between these two ways of declaring 2d vectors:

vector<vector<int>> b(n+n-1);

and

vector<vector<int>> b(n+n-1, vector<int>(n+n-1));

Answer 1

vector<vector<int>> b(n+n-1);

Allocate a vector b with n+n-1 elements and initialize the elements with default value (a vector with zero elements).

vector<vector<int>> b(n+n-1, vector<int>(n+n-1));

Allocate a vector b with n+n-1 elements and initialize the elements with the specified value (a vector with n+n-1 elemements).