参考 YYModel，混合枚举的设计，与 MFC 编程中高八位低八位的思想异曲同工。

YYEncodingType 的第八位是一个普通的枚举，中八位是一套 Options，高八位是另一套 Options。

尽管笔者不赞同这样的设计（易用性差，可维护性低，内封装扩展性差），但这段代码着实令人印象深刻。

typedef NS_OPTIONS(NSUInteger, YYEncodingType) {
    YYEncodingTypeMask       = 0xFF, ///< mask of type value
    YYEncodingTypeUnknown    = 0, ///< unknown
    YYEncodingTypeVoid       = 1, ///< void
    YYEncodingTypeBool       = 2, ///< bool
    YYEncodingTypeInt8       = 3, ///< char / BOOL
    YYEncodingTypeUInt8      = 4, ///< unsigned char
    YYEncodingTypeInt16      = 5, ///< short
    YYEncodingTypeUInt16     = 6, ///< unsigned short
    YYEncodingTypeInt32      = 7, ///< int
    YYEncodingTypeUInt32     = 8, ///< unsigned int
    YYEncodingTypeInt64      = 9, ///< long long
    YYEncodingTypeUInt64     = 10, ///< unsigned long long
    YYEncodingTypeFloat      = 11, ///< float
    YYEncodingTypeDouble     = 12, ///< double
    YYEncodingTypeLongDouble = 13, ///< long double
    YYEncodingTypeObject     = 14, ///< id
    YYEncodingTypeClass      = 15, ///< Class
    YYEncodingTypeSEL        = 16, ///< SEL
    YYEncodingTypeBlock      = 17, ///< block
    YYEncodingTypePointer    = 18, ///< void*
    YYEncodingTypeStruct     = 19, ///< struct
    YYEncodingTypeUnion      = 20, ///< union
    YYEncodingTypeCString    = 21, ///< char*
    YYEncodingTypeCArray     = 22, ///< char[10] (for example)
    
    YYEncodingTypeQualifierMask   = 0xFF00,   ///< mask of qualifier
    YYEncodingTypeQualifierConst  = 1 << 8,  ///< const
    YYEncodingTypeQualifierIn     = 1 << 9,  ///< in
    YYEncodingTypeQualifierInout  = 1 << 10, ///< inout
    YYEncodingTypeQualifierOut    = 1 << 11, ///< out
    YYEncodingTypeQualifierBycopy = 1 << 12, ///< bycopy
    YYEncodingTypeQualifierByref  = 1 << 13, ///< byref
    YYEncodingTypeQualifierOneway = 1 << 14, ///< oneway
    
    YYEncodingTypePropertyMask         = 0xFF0000, ///< mask of property
    YYEncodingTypePropertyReadonly     = 1 << 16, ///< readonly
    YYEncodingTypePropertyCopy         = 1 << 17, ///< copy
    YYEncodingTypePropertyRetain       = 1 << 18, ///< retain
    YYEncodingTypePropertyNonatomic    = 1 << 19, ///< nonatomic
    YYEncodingTypePropertyWeak         = 1 << 20, ///< weak
    YYEncodingTypePropertyCustomGetter = 1 << 21, ///< getter=
    YYEncodingTypePropertyCustomSetter = 1 << 22, ///< setter=
    YYEncodingTypePropertyDynamic      = 1 << 23, ///< @dynamic
};