Nonatomic will not generate threadsafe routines thru
atomic will generate threadsafe accessors so
atomic variables are threadsafe (can be accessed from multiple threads without botching of data)
copy is required when the object is mutable. Use this if you need the value of the object as it is at this moment, and you don’t want that value to reflect any changes made by other owners of the object. You will need to release the object when you are finished with it because you are retaining the copy.
Assign is somewhat the opposite to
copy. When calling the getter of an
assign property, it returns a reference to the actual data. Typically you use this attribute when you have a property of primitive type (float, int, BOOL…)
retain is required when the attribute is a pointer to a reference counted object that was allocated on the heap. Allocation should look something like:
NSObject* obj = [[NSObject alloc] init]; // ref counted var
The setter generated by
@synthesize will add a reference count to the object when it is copied so the underlying object is not autodestroyed if the original copy goes out of scope.
You will need to release the object when you are finished with it.
retain will increase the reference count and occupy memory in the autorelease pool.
strong is a replacement for the retain attribute, as part of Objective-C Automated Reference Counting (ARC). In non-ARC code it’s just a synonym for retain.
This is a good website to learn about
weak for iOS 5.
weak is similar to
strong except that it won’t increase the reference count by 1. It does not become an owner of that object but just holds a reference to it. If the object’s reference count drops to 0, even though you may still be pointing to it here, it will be deallocated from memory.
The above link contain both Good information regarding Weak and Strong.