How does ContactFilter2D actually work?

So, I'm trying to use 'Rigidbody2D.IsTouching()' to check if my rigidbody is touching anything in the scene, and in the docs it says to do that I need to pass in a ContactFilter2D. However, it does not actually say how to properly make a ContactFilter2D. Right now, my code looks like this:

 ContactFilter2D cf;
 cf.NoFilter();
 if (rb.IsTouching(cf)) {
    // Do Something.
 }

This is not working, and I understand why. When I declared 'cf' I didn't assign any data to it, so it can't be used, even by its methods. The problem is, ContactFilter2D is a type that contains a whole host of stuff, and I have no idea how to properly "Declare" something like that. It doesn't seem possible.