How does NSArray differ from Swift Array?

Middle PRO

Main differences

• Semantics. Array is a struct, a value type with copy-on-write; NSArray is a class, a reference type. Immutability for NSArray is at the class level (NSArray vs NSMutableArray), for Array at the variable level (let vs var).
• Typing. Array<Element> is generic and holds any type, including structs and enums. NSArray holds objects only (AnyObject); Swift structs get boxed when bridged (Int → NSNumber, String → NSString, your own struct → _SwiftValue).
• Thread safety. Neither is thread-safe under mutation, but Array as a value type is safe to read from several threads (no shared mutable state); NSMutableArray is a shared object.
• API. NSArray has Foundation methods (firstObject, objectAtIndex, KVC/KVO, arrayController), Array has the Collection protocols (map, filter, lazy, slices).