Weird copy assignment code

Code:

---

    vector& operator=(const vector& a){
        double *p=new double[a.sz];
        copy(a.elem, a.elem+a.sz, elem);
        delete[] elem;
        elem=p;
        sz=a.sz;
        return *this;
    }

---

In chapter 17 were trying to replicate std vector.
Why does it delete elem after it copies to elem? I dont get it

Exception safe assignment – Vorbrodt's C++ Blog

You delete after you copy so that if the copy assignment throws an exception, no change occurs or affects your original variable.

EDIT:

Looked at John's reply and realised I didn't look at your code correctly. If everything was written right, like John mentions to change elem to p in the copy, then you can consider my response valid.

The error is that it's copying to elem instead of p. Change it to p.

BTW, "chapter 17" is not some kind of universal reference. There's a lot of books out there!

Thx it works now;)
author made a typo