枚舉只是一種常量命名方式，入下列枚舉表示鏈接狀態(tài)

enum EOCConnectionState:NSInteger{    EOCConnectionStateDisconnected = 1,    EOCConnectionStateConnecting,    EOCConnectionStateConnected,};
編譯器會為每一個枚舉分配一個獨有的編號，從0開始，然而定義沒巨變量的方式卻不太簡潔，如下
enum EOCConnectionState state = EOCConnectionStateConnected;
另一種方式
enum EOCConnectionState{    EOCConnectionStateDisconnected,    EOCConnectionStateConnecting,    EOCConnectionStateConnected,};typedef enum EOCConnectionState EOCConnectionState;這樣使用會簡潔一些
EOCConnectionState state = EOCConnectionStateConnected;c++11標準修訂類枚舉的某些特性，其中一項就是可以指明用何種“底層數(shù)據(jù)類型”來保存枚舉類型的變量。這樣的好處是，可以向前申明枚舉變量。如下
enum EOCConnectionState:NSInteger{    EOCConnectionStateDisconnected = 1,    EOCConnectionStateConnecting,    EOCConnectionStateConnected,};
上述代碼把EOCConnectionStateDisconnected的值設(shè)為1而不是默認分配的0.
還有一種情況應該使用枚舉類型，那就是定義選項的時候，若這些選項可以彼此組合，則更應該如此，和選項之久可以通過“按位或操作符”來組合，例如
enum UIViewAutoresizing{    UIViewAutoresizingNone                 = 0,    UIViewAutoresizingFlexibleLeftMargin   = 1 << 0,    UIViewAutoresizingFlexibleWidth        = 1 << 1,    UIViewAutoresizingFlexibleRightMargin  = 1 << 2,    UIViewAutoresizingFlexibleTopMargin    = 1 << 3,    UIViewAutoresizingFlexibleHeight       = 1 << 4,    UIViewAutoresizingFlexibleBottomMargin = 1 << 5};
Foundation框架中定義了一寫輔助的宏，用這些宏來定義枚舉類型時，也可以指定用于保存枚舉值的底層數(shù)據(jù)類型，這些宏具有向后兼容能力
typedef NS_OPTIONS(NSUInteger, UIViewAutoresizing) {    UIViewAutoresizingNone                 = 0,    UIViewAutoresizingFlexibleLeftMargin   = 1 << 0,    UIViewAutoresizingFlexibleWidth        = 1 << 1,    UIViewAutoresizingFlexibleRightMargin  = 1 << 2,    UIViewAutoresizingFlexibleTopMargin    = 1 << 3,    UIViewAutoresizingFlexibleHeight       = 1 << 4,    UIViewAutoresizingFlexibleBottomMargin = 1 << 5};
typedef NS_ENUM(NSUInteger, UIViewAutoresizing) {    EOCConnectionStateDisconnected ,    EOCConnectionStateConnecting,    EOCConnectionStateConnected,};
從枚舉定義來看，NS_ENUM和NS_OPTIONS本質(zhì)是一樣的，僅僅從字面上來區(qū)分其用途。NS_ENUM是通用情況，NS_OPTIONS一般用來定義具有位移操作或特點的情況。
注：我們總習慣在switch語句中加上default分支，然而，若是枚舉來定義狀態(tài)機，最好不要有default分支，這樣的話，如果稍后又加了一種狀態(tài)，那么編譯器就會發(fā)出浸膏信息，提示新加入的狀態(tài)并未在switch分支中處理