iOS - Position UIView in center of superview using Auto Layout programmatically

How do you programmatically set a UIView to be in the center of its superview using Auto Layout?

UIButton* viewObj = [UIButton buttonWithType:UIButtonTypeRoundedRect];
[viewObj setTranslatesAutoresizingMaskIntoConstraints:NO];
[viewObj setBackgroundColor:[UIColor greenColor]];
[self.view addSubview:viewObj];

NSLayoutConstraint* cn = [NSLayoutConstraint constraintWithItem:viewObj
                                                      attribute:NSLayoutAttributeCenterX
                                                      relatedBy:NSLayoutRelationEqual
                                                         toItem:self.view
                                                      attribute:NSLayoutAttributeCenterX 
                                                     multiplier:1.0
                                                       constant:0];
[self.view addConstraint:cn];
cn = [NSLayoutConstraint constraintWithItem:viewObj 
                                  attribute:NSLayoutAttributeCenterY
                                  relatedBy:NSLayoutRelationEqual 
                                     toItem:self.view
                                  attribute:NSLayoutAttributeCenterY
                                 multiplier:1.0
                                   constant:0];
[self.view addConstraint:cn];

The above code works for me for UIButton, but I'm having trouble replacing the first line with something that works for a UIView.

I've tried

UIView* viewObj = [UIView alloc] initWithFrame:CGRectZero];

but the view does not show up in simulator.

Any suggestions?

Thanks!