Declaring ivars in superclass or @synthesize in subclass?

Question

When I just declare a @property in a superclass without declaring ivars, subclass it and try to implement getter using the superclasses ivar (_propertyName) in subclass, xcode invokes an error stating Use of undeclared identifier '_propertyName'.

What is the solution conforming to best programming practices?

Should I @synthesize propertyName = _propertyName in the @implementation of the subclass or

@interface SuperClass : AnotherClass
{
    Type *_propertyName;
}

@property Type *propertyName;

@end

EDIT:
I do understand the automatic "synthesis" of the properties' accessor methods and creation of "underbar ivars" by the compiler.
The ivar is accessible from the implementation of the SuperClass without any @synthesize or declaration of ivars in the interface or implementation section.

Further clarification of my case: Disclaimer: Contents stolen block of code from Alfie Hanssen

@interface SuperViewController : UIViewController
@property (nonatomic, strong) UITableView * tableView; // ivar _tableView is automatically @synthesized
@end

#import "SuperViewController.h"

@interface SubViewController : SuperViewController
// Empty
@end

@implementation SubViewController

- (void)viewDidLoad
{
    NSLog(@"tableView: %@", self.tableView); // this is perfectly OK
}

// ************* This causes problem **************
- (UITableView *) tableView {
    if (!_tableView) {    // Xcode error: Use of undeclared identifier '_propertyName'
        _tableView = [[SubclassOfUITableView alloc] init];
    }
    return _tableView;
}
// ************************************************


@end

Answer 1

If you want to use an ivar in both the superclass and the subclass, you have to declare it in the interface of the superclass, because it's the only file that's included in both implementations. Otherwise, you're just trying to guess what could be in the implementation of the superclass and xcode will not play game.

The above is true whether there is a property that use that ivar or not.

Now if you have a property in the superclass and you write in the subclass implementation :

@synthesize propertyName = _propertyName

You're just saying that you're ditching whatever implementation of that property was in the superclass and you want the standard setter and getter generated by xcode, working on an ivar named _propertyname. Maybe the superclass works the same way, maybe not.

Answer 2

This should work just fine. self.tableView and _tableView should both be accessible from SubViewController.

@interface SuperViewController : UIViewController
@property (nonatomic, strong) UITableView * tableView; // ivar _tableView is automatically @synthesized
@end

#import "SuperViewController.h"

@interface SubViewController : SuperViewController
// Empty
@end

@implementation SubViewController

- (void)viewDidLoad
{
    NSLog(@"tableView: %@", self.tableView);
}

@end

Edit: Okay now I understand you're question better. Check out this question. Seems like a duplicate and seems like it answers your question. I.e. looks like you'll have to explicitly define the instance variable.

Answer 3

Apparently the automatic synthesis of the ivar doesn't work in this case. The following code fails with the compiler error "use of undeclared identifier _x", but only for the use in JFNBclass. Uncomment the ivar declaration, and the compiler error goes away. Default synthesis doesn't seem to create _x that is visible to the inheriting class.

JFNAclass.h

#import <Foundation/Foundation.h>

@interface JFNAclass : NSObject {
  // NSNumber *_x;
}

@property NSNumber *x;
- (NSNumber *) xTest;

@end

JFNAclass.m

#import "JFNAclass.h"

@implementation JFNAclass

- (NSNumber *) xTest
{
  return _x;
}

@end

JFNBclass.h

#import <Foundation/Foundation.h>
#import "JFNAclass.h"

@interface JFNBclass : JFNAclass

- (void) printIt;
- (void) setIt;

@end

JFNBclass.m

#import "JFNBclass.h"

@implementation JFNBclass

- (void) setIt
{
    _x = [[NSNumber alloc] initWithInt:2];
}

- (void) printIt
{
  NSLog(@"_x is %@", _x);
}

@end

Answer 4

Ivars created by @synthesize are @private. (This is true for both explicit @synthesize statements and automatic synthesis.)

Your class SuperViewController does have an ivar _tableView, but it is @private so your subclass can't use it. If you need an ivar that subclasses can use, you'll need to declare that ivar explicitly and make it something other than @private. (You'd also want to use @synthesize tableView=_tableView so the property uses your declared ivar.)

Answer 5

I wouldn't access ivar at all. You want to override getter, so just call super.

- (UITableView *)tableView {
    UITableView *tableView = [super tableView];
    if ( ! tableView) {
        tableView = [[SubclassOfUITableView alloc] init];
        self.tableView = tableView;
    }
    return tableView;
}