Why are instance variables defined in the header file in Objective-C

Question

I can understand defining the functions in the @interface of the header file, but why the instance variables? Shouldn\'t the instance variables be private, only accessible

Answer 1

Note that under the Objective C 2.0 new "Modern Runtime" (availabe in iPhone applications and 64-bit Mac OS X 10.5 applications) you do not need to specify the ivars, you can specify the properties and then use @synthesize to generate the ivars.

This is because in the Modern Runtime, ivars have a global indirection symbol that holds the offset for the ivar. This also resolves the fragile base class issue, allowing ivars to be reordered and added without requiring recompiling of subclasses (deleting or renaming ivars may still cause link errors).

However you still have to list properties in the main interface, so there does not seem to be any way to entirely hide private ivars which is unfortunate. You cannot, for example, use a property and @synthesize in a category.