Friend class and pointer function

[RyuMaster]

Hi! I need help with understanding this block of code, particullary this line: *getLeftChild() { return this - _child; }

public class UpperNode
{
BOX _box;
int _child;

FORCEINLINE UpperNode *getLeftChild() { return this - _child; }
...

};

Here I have this function:

void
UpperNode::visulization(int level)
{
	if (isLeaf())
		_box.visulization();
	else
		if ((level > 0)) {
			if (level == 1)
				_box.visulization();
			else
			if (getLeftChild())
				getLeftChild()->visulization(level-1);
			if (getRightChild())
				getRightChild()->visulization(level-1);
		}
}

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 reffering 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