Visual C++ :: Friend Class And Pointer Function

I need understanding this block of code, particularly this line : *getLeftChild() { return this - _child; }

Code:

public class UpperNode {
BOX _box;
int _child;
FORCEINLINE UpperNode *getLeftChild() { return this - _child; }
...
};

Here I have this function:

Code:
void
UpperNode::visulization(int level) {
if (isLeaf())
_box.visulization();
else
if ((level > 0)) {

[Code] .....

It also makes calls for "getLeftChild()";

But I see that getLeftChild expects function pointer, and I absolutely have no clue where "this" comes from inside function body.

(return this - _child) - "this" has to be integer.

Or, if we gave pointer, and "this" is referring to some UpperNode, then I can't understand to which one, I have no UpperNode array defined or something. So if this functions is actually scaling pointer address, then scaling where to? I could comprehend it, if I had some array of UpperNodes, but not just class. I have UpperNodes array defined in other friendly class, but don't think they are related .....