Struct "inheritance"

[Vidar Wahlberg]

Good day.

I know "inheritance" is a misleading word as there's no such thing when 
it comes to structs, but I couldn't think of a better description for 
this problem:

Let's say I got a struct for a location on a 2-dimensional plane:
struct Point {
   int x;
   int y;
}
Further I also need to represent a location in a 3-dimensional space:
struct Coordinate {
   int x;
   int y;
   int z;
}
If these were classes instead I could simply make Coordinate inherit 
from Point and only add "int z;". This would also make the thing I'm 
trying to achieve much easier; Consider I have a method that does 
something nifty with "x" and "y", I'd like this method to handle both 
"Point" and "Coordinate", if they were classes it would be fairly simple:
int somethingNifty(Point p) {
   return p.x + p.y;
}


So why not just use classes? I've understood it as there may be a 
performance gain by using structs over classes, and in my program Point 
and Coordinate are used heavily.

Obviously I'm fairly new to both D and structs, I have plenty Java and 
some C++ experience (I've avoided using structs in C++) and I have "The 
D Programming Language" book by Andrei which I'm happy to look up in, 
although I've failed to find an answer to this question in the book, in 
this newsgroup and on the net.
I would greatly appreciate if someone could give me a nudge the the 
right direction.