issue with ivar - ios

Question

I'm subclassing a UIToolbar because I'm gonna reuse it all over my app. The UIToolbar uses a delegate protocol:
//
// UIToolbarCustom.h
//
#import <UIKit/UIKit.h>
#protocol UIToolbarCustomDelegate
#required
- (void)tab:(UIBarButtonItem *)sender;
- (void)ok:(UIBarButtonItem *)sender;
#end
#interface UIToolbarCustom : UIToolbar {
id <UIToolbarCustomDelegate> delegate;
}
#property (strong, nonatomic) id delegate;
#end
The standford iOS development course teacher recommends to explicit declare all the ivars prefixing it with a underscore, like:
#implementation UIToolbarCustom
#synthesize delegate = _delegate;
#end
But, in this specify scenarion it gives me a error:
error: property 'delegate' attempting to use ivar '_delegate'...
The code works just fine if I use:
#synthesize delegate = __delegate; or
#synthesize delegate;
What is going on here? Is there a private instance variable in the UIToolbar class named _delegate?
UPDATE
Thank all you guys for all the clarifications and protips, I'm learning a lot. Turns out that I'm new to iOS development (this is my first app second version, so I'm trying to do it right =p). Following the tips I came out with this new header file:
//
// Toolbar.h
//
#import <UIKit/UIKit.h>
#protocol ToolbarDelegate
#required
- (void)tab:(UIBarButtonItem *)sender;
- (void)ok:(UIBarButtonItem *)sender;
#end
#interface Toolbar : UIToolbar
#property (strong, nonatomic) id delegate;
#end
Notes:
The class prefix was removed.
The delegate declaration was removed (I'm using a ios delegate tutorial code, but the code sample uses a older xcode version, where the declaration is needed).
The synthesize was removed, I also didn't knew that we don't need synthesize our properties anymore.
PS: Obviously the code does not work, because the ivar problem. I'm gonna change its names, so I don't need to synthesize it, not sure about what name to use anyways...

Answer 1

What is going on here? Is there a private instance variable in the UIToolbar class named _delegate?
Yes, that's exactly the problem. You need to come up with a different name for your instance variable. __delegate will work, or you could prefix the name with a 3 letter prefix (see last paragraph).
Do note that you've declared your ivar as delegate, then in the synthesize statement told the compiler to use _delegate. Effectively that means that your delegate ivar isn't being used at all. In any case, if you're writing for iOS (as opposed to 32-bit Mac), like you are, you don't need the explicit instance variable declaration in your subclass's #interface section, because the compiler will automatically create it for you.
Finally, it's bad form to name your own subclass something that begins with 'UI', since the UI prefix is reserved for classes that are part of UIKit. You should use your own 3 letter prefix instead, or else no prefix at all. The problem is that a future version of UIKit could conceivably include a class called "UIToolbarCustom", and your subclass would collide with it.

Answer 2

Since last year, when Xcode 4.3 came out, you don't need to synthesize your properties. It is done for you by the compiler (an ivar is generated, and a leading underscore is added to its name). This means that you also don't need to declare an ivar. If you do, be sure to name it something other than _delegate.
So, all you really need is this line:
#property (strong, nonatomic) id<UIToolbarCustomDelegate> delegate;
UPDATE: please see the Andrew Madsen's answer for the full story. Turns out UIToolbar has its own ivar named _delegate. Who knew!