Question

I’m looking at some sample code and I’m puzzled over the lack of declaration of a specific ivar. Hoping someone can help me understand this better:

typedef NSUInteger (^NumberOfItemsInSection)(ViewClass *viewClass, NSUInteger section);

// class declaration    
@interface SampleScrollView : UIScrollView
@property (nonatomic, copy) NumberOfItemsInSection itemsSectionBlock;
@end

// class implementation
@implementation SampleScrollView

@synthesize itemsSectionBlock = _itemsSectionBlock;

- (void)setItemsSectionBlock:(NumberOfItemsInSection)itemsSectionBlock
{
    // _itemsSectionBlock is not declared any where in the class 
    // How does the compiler not complain?

    _itemsSectionBlock = [itemsSectionBlock copy];
    [self reloadData];
}

@end

The instance variable, “_itemsSectionBlock”, is not declared any where and it can just be used in the property’s setter override. How does that work?

Answer 1

It’s part of the modern runtime, and cuts down on the duplication of code – declaring iVars and then declaring properties for those iVars.

It’s handled for you by the @synthesize

The modern runtime lets you do other things that you thought you couldn’t do before. For example, you can now declare iVars in the .m file as part of a class extension, which reduces the amount of information you expose in your public interface.

Update

The modern LLVM 4 compiler even lets you do away with the @sytnthesize line. If you declare a property it will auto-synthesize for you and it will even create a backing store with a leading underscore.