Two dimensional array using vector

I want to create 2D array using vector. But, when I do this, I get seg fault. Can anyone please explain what I am doing wrong, and possible solution for this problem.

I made everything public since I dont want to deal with getters and setters now. I want to get the concept of 2D array clear.

#include <iostream>
#include <vector>
using namespace std;

class point
{   
    public:
        point():x(0),y(0){}
        ~point(){}
        point(float xx,float yy):x(xx),y(yy){}
        float x,y;
};

int main()
{
    vector<vector<point> > a; // 2D array
    point p(2,3);
    a[0][0] = p; // error here
    return 0;
}