Property attributes

There are many attributes for property as follows:

• atomic:
  • Is default behavior
  • will ensure the present process is completed by the cpu, before another process access the variable
  • not fast, as it ensures the process is completed entirely
• nonatomic:
  • Is NOT default behavior
  • faster (for synthesized code, ie for variable created using @property, @synthesize )
  • not thread safe
  • may result in unexpected behavior, when two different process access the same variable at the same time
• 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. For attributes whose
  type is an immutable value class that conforms to the NSCopying protocol,
  you almost always should specify copy in
  your @property declaration.

• retain: is
  required when the attribute is a pointer to an object. The setter generated by@synthesize will
  retain (aka add a retain count to) the object. You will need to release the object when you are finished with it. By using retain it will increase the retain count and occupy memory in 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.

• 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.

• 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...)

A reference: http://www.raywenderlich.com/5677/beginning-arc-in-ios-5-part-1