Что произойдет, если «сохранить» NSArray или NSMutableArray?

In the source codes

@property(retain) NSString* str;
@sythesize str;
self.str = newStr;

I understand actually following will happen

if( str != newStr ){
     [str release];
     str = [newStr retain]; 
}

So how about the case for NSArray or NSMutableArray ? Seem like it is complex,shallow copy and deep copy should be considered.