Question

I read some tutorials here about properties ,but i still have some doubts to clarify, is there a difference between

@interface MyClass : NSObject {    
}
@property(nonatomic,retain) NSString *temp;
@end

AND

@interface MyClass : NSObject {
    NSString *temp;
}
@property(nonatomic,retain) NSString *temp;
@end

Answer 1

The difference is that in the first version, the compiler will automatically create an instance variable (IIRC, it will be named _temp but I don’t know for sure). This is only supported on iOS and Mac 64 bit.

In the second example, you provide the variable.

There’s actually a way to tell the compiler which variable to use for the property, which I use a lot:

@interface MyClass : NSObject {
    NSString *temp_;
}
@property(nonatomic,retain) NSString *temp;
@end

@implementation MyClass
@synthesize temp = temp_;
@end

This way the variable and the property have different names and you can’t confuse them (e.g. by forgetting to prefix self.).

Minor side-note: it’s often desirable to use copy instead of retain for NSString *, since you might assign an NSMutableString * to the property. Now if you would change that mutable string unexpected things might happen.