Printing value of pointer in a vector of pointer c++

here is my code:

vector *ptr;
int *tab = new int(20);
ptr->push_back(tab);
cout << *(ptr->at(0)) << endl;

I want to print 20 on the screen, but I got a segmentation fault.
when I use only

vector ptr;

it prints out fine. I get easily the result just by doing :

*ptr.at(0);

But I want to use a pointer not a simple variable.
Can I have some enlightenment?

Thanks