Property attributes: strong, weak, copy, assign — when to use which?

Middle PRO

Ownership

• strong — a strong reference; the object lives while the property points to it. Default for object types under ARC. For everything the object owns.
• weak — weak, automatically zeroed on the target's dealloc. For delegate, dataSource, back-references to a parent — to avoid a cycle. Only for objects that support weak references.
• copy — the setter calls copy and stores a snapshot. Mandatory for types with a mutable subclass: NSString, NSArray, NSDictionary, NSSet, NSData — otherwise someone can pass an NSMutableString and change it later. For blocks copy moves the block from the stack to the heap (under ARC strong does the same, but copy is the documented convention).
• assign — plain assignment without retain. For scalars: NSInteger, CGFloat, BOOL, enums, structs. For objects it equals unsafe_unretained — a dangling pointer after dealloc, practically unused.
• unsafe_unretained — the explicit variant of assign for objects.