Subview disappears with auto layout - objective-c

Question

I just added a UIView as subview to the main UIView on interface builder (basic single view application).
Without setting any constraints, my subview disappears.
subview's frame = (0 0; 320 0);
Why is that?
If I try to add some constraints like for trailing space, leading space, top space and bottom space to be fixed, still my view disappears.
How can I solve this?
Thank you.
Just to clarify thing a little I created a test project (single view application), and added 2 subview to the main view like in the image. I didn't change any default constraint.
And you can see the error in the log of the image.
Logs:
**2013-01-19 17:16:02.435 Test[8871:c07] Unable to simultaneously satisfy constraints.
Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints)
(
"<NSAutoresizingMaskLayoutConstraint:0x106178c0 h=--- v=--- V:[UIWindow:0x9917040(480)]>",
"<NSLayoutConstraint:0x106159e0 UIView:0x991a5a0.bottom == UIWindow:0x9917040.bottom>",
"<NSLayoutConstraint:0x991ab00 V:|-(518)-[BottomView:0x9919c90] (Names: '|':UIView:0x991a5a0 )>",
"<NSLayoutConstraint:0x10615960 V:|-(20)-[UIView:0x991a5a0] (Names: '|':UIWindow:0x9917040 )>",
"<NSLayoutConstraint:0x991aa80 BottomView:0x9919c90.bottom == UIView:0x991a5a0.bottom>"
)
Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x991aa80 BottomView:0x9919c90.bottom == UIView:0x991a5a0.bottom>
Break on objc_exception_throw to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.**
Constraints:
Also here is the result on the simulator:

Answer 1

It is good practice to understand those logs but if you are going to use Autolayout you are going to have to read up on this. Everyone says it is simply but I personally have not found it simple.
Apples Programming Guide For AutoLayout
Please read this guide especially the debugging section.
As a very very general rule if you are going to add a view then you need to turn off autoresizingmasks (Spring and struts) for the view. Add the view as a subview and give it 2 or 3 constraints. In your case above you would give it a constaint that it should have a left or leading space to superview of 0. A top space to superview of 0 and a width of 320.
EDIT; Here is an example of adding a view; note you do not need to create a frame. The constraints may be a little strange. The first puts the view in the centre of the superview. The second gives it a width of 200. The next method is the vertical constraint which puts the view at the bottom and makes it 2 high.
UIView *sView = [[UIView alloc] init];
[sView setTranslatesAutoresizingMaskIntoConstraints:NO];
[superView addSubview:sView];
[superView addConstraint:[NSLayoutConstraint constraintWithItem:sView
attribute:NSLayoutAttributeCenterX
relatedBy:NSLayoutRelationEqual
toItem:superView
attribute:NSLayoutAttributeCenterX
multiplier:1.0
constant:0]];
[superView addConstraint:[NSLayoutConstraint constraintWithItem:sView
attribute:NSLayoutAttributeWidth
relatedBy:NSLayoutRelationEqual
toItem:Nil
attribute:NSLayoutAttributeWidth
multiplier:1.0
constant:200]];
[superView addConstraints:[NSLayoutConstraint
constraintsWithVisualFormat:#"V:[sView(2)]|"
options:0
metrics:nil
views:NSDictionaryOfVariableBindings(sView)]];