Expression is not assignable

Question

I have an object (a custom UIView) in which I have this property in the public API:
@property (nonatomic) CGPoint theOrigin;
then, in the implementation file I have:
@synthesize theOrigin = _theOrigin;
also in the implementation file I have the method:
-(void)setDefaultOrigin {
    _theOrigin.x = self.bounds.origin.x + self.bounds.size.width/2;
    _theOrigin.y = self.bounds.origin.y + self.bounds.size.height/2;
which doesn't generate any compiler complaints but if I try to code the above method
like this:
-(void)setDefaultOrigin {
    self.theOrigin.x = self.bounds.origin.x + self.bounds.size.width/2;
    self.theOrigin.y = self.bounds.origin.y + self.bounds.size.height/2;
I get a complaint on both lines of the method stating that the "Expression is not assignable"
and it won't allow me to build. I am not understanding why I can't code it the second way.

Answer 1

self.theOrigin.x assumes x is a property of theOrigin, which it is not.  It's a field of a structure.  _theOrigin.x references the supporting ivar directly and correctly sets the x field of the structure.