I don't think this is casting, but what is it?

Question

This is enclosed in a for loop:

v[i] = new (&vv[i]) vertex(pts[i],i);

• vertex is a struct
• pts is a point*
• v is a vertex**
• vv is a vertex*

What does the (&vv[i]) part do?

Answer 1

It looks like placement new. It's the same as an ordinary new statement, but instead of actually allocating memory, it uses memory already available and which is pointed to by the expression inside the parentheses.

In your case it uses the memory in vv[i] to create the new vertex object, then returns a pointer to that (i.e. &vv[i]) and it's assigned to v[i].

See e.g. this reference for more details.