typedef NS_ENUM vs typedef enum

Question

On the Adopting Modern Objective-C guide, Apple recommends using the NS_ENUM macro instead of enum. I've also read an explanation from NSHipster about NS_ENUM and NS_OPTIONS.

Maybe I've missed something but I don't quite understand what is the difference between the following two snippets and if any why is NS_ENUM the recommended way to go (except maybe, for backwards compatibility with older compilers)

// typedef enum
typedef enum {
    SizeWidth,
    SizeHeight
}Size;

// typedef NS_ENUM
typedef NS_ENUM(NSInteger, Size) {
    SizeWidth,
    SizeHeight
};

Answer 1

First, NS_ENUM uses a new feature of the C language where you can specify the underlying type for an enum. In this case, the underlying type for the enum is NSInteger (in plain C it would be whatever the compiler decides, char, short, or even a 24 bit integer if the compiler feels like it).

Second, the compiler specifically recognises the NS_ENUM macro, so it knows that you have an enum with values that shouldn't be combined like flags, the debugger knows what's going on, and the enum can be translated to Swift automatically.