Note in the following example that the ivars use underscore prefix (which makes them easy to discern from other, non-ivar variables):

@interface MyClass : NSObject {
@private
// Private ivar needs getter/setter to be accessible
UITextField *_textField;
}
// Note that the property name does NOT have the underscore prefix
@property (nonatomic, retain) IBOutlet UITextField *textField;


This shows how the @synthesize tag is the “bridge” that connects the ivar to the property (i.e., automatic method generation):

@implementation
...
// Generate textField: and setTextField: methods for _textField ivar
@synthesize textField=_textField;
...
@end


As another sidenote (similar to Ahruman’s), it’s interesting to note that the IBAction “type qualifier,” like IBOutlet, is also pre-processor macro defined by UIKit in UINibDeclarations.h:

#ifndef IBAction
#define IBAction void
#endif


This means that - (IBAction)someMethod:(id)sender; gets turned into - (void)someMethod:(id)sender;.

This points at a wart in Objective-C: the ivars of a class are (generally) implementation details, but are part of the @interface block. This is because, in the traditional Obj-C runtimes, ivars were accessed as fixed offsets from the self pointer, meaning that subclass implementations had to know about the ivar layouts of their superclasses which thus had to be public.

This is fixed in the new non-fragile runtime, as used in 64-bit Mac OS X apps and all iPhone apps. When targeting the new runtime, you don’t need the ivar declaration (as long as you don’t access the ivar directly), and just need the @property declaration in the @interface block and the @synthesize directive in the @implementation block (or an explicit implementation not using an ivar, or @dynamic).

The wart still exists, since you can’t declare ivars in the @implementation block in the non-fragile runtime except for synthesized ivars. Feel free to dupe my bug on the issue.

As a side note, IBOutlet isn’t really ignored by the parser; it’s an empty preprocessor macro, so it’s removed before anything Obj-C-specific sees it.