Question

What is the difference between following code fragments?

No. 1

@interface HistoryJsonBean : NSObject
{
}

@property (nonatomic,retain) NSString * unit;

No. 2

@interface HistoryJsonBean : NSObject
{
    NSString * unit;
}

@property (nonatomic,retain) NSString * unit;

I am a newbie, is there really a difference between above two codes?

Answer 1

The first syntax is relying on a feature of modern Objective C runtime systems to create the corresponding variable from a property definition. So, the second syntax is just Objective C 2.0 syntax used with older runtime systems.

Have also a look at this post from S.O.

EDIT:

The variable in the latter case is an ivar.

The property definition was originally just a short-way to define accessor methods for its ivar (with the proper memory management). Newer runtime systems relax the need to declare the ivar, and you can go by with just the property definition (which will also imply the allocation of memory to store the value associated to the property).

From the interface point of view, on a new runtime system there is no difference between the two syntaxes you mentioned. Now, if the runtime system handles them is two different ways, this could only be said by inspecting the implementation in the runtime systems source code. My guess is that there is no difference at that level either.