What is a typedef enum in Objective-C

Question

I don t think I fundamentally understand what an enum is  and when to use it     For example   typedef enum       kCircle      kRectangle      kOblateSpheroid   ShapeType    What is really being declared here

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The enum  abbreviation of enumeration  is used to enumerate a set of values  enumerators   A value is an abstract thing represented by a symbol  a word   For example  a basic enum could be  enum   xs s m l xl xxl xxxl xxxxl      This enum is called anonymous because you do not have a symbol to name it  But it is still perfectly correct  Just use it like this  enum   xs s m l xl xxl xxxl xxxxl   myGrandMotherDressSize    Ok  The life is beautiful and everything goes well  But one day you need to reuse this enum to define a new variable to store myGrandFatherPantSize  then you write   enum   xs s m l xl xxl xxxl xxxxl   myGrandMotherDressSize  enum   xs s m l xl xxl xxxl xxxxl   myGrandFatherPantSize    But then you have a compiler error  redefinition of enumerator   Actually  the problem is that the compiler is not sure that you first enum and you are second describe the same thing   Then if you want to reuse the same set of enumerators  here xs   xxxxl  in several places you must tag it with a unique name  The second time you use this set you just have to use the tag  But don t forget that this tag does not replace the enum word but just the set of enumerators  Then take care to use enum as usual  Like this      Here the first use of my enum enum sizes   xs s m l xl xxl xxxl xxxxl   myGrandMotherDressSize      here the second use of my enum  It works now  enum sizes myGrandFatherPantSize    you can use it in a parameter definition as well      Observe that here  I still use the enum -  void  buyANewDressToMyGrandMother  enum sizes theSize    You could say that rewriting enum everywhere is not convenient and makes the code looks a bit strange  You are right  A real type would be better   This is the final step of our great progression to the summit  By just adding a typedef let s transform our enum in a real type   Oh the last thing  typedef is not allowed within your class  Then define your type just above    Do it like this      enum definition enum sizes   xs s m l xl xxl xxxl xxxxl    typedef enum sizes size type   interface myClass             size type myGrandMotherDressSize  myGrandFatherPantSize             Remember that the tag is optional  Then since here  in that case  we do not tag the enumerators but just to define a new type  Then we don t really need it anymore      enum definition typedef enum   xs s m l xl xxl xxxl xxxxl   size type    interface myClass   NSObject           size type myGrandMotherDressSize  myGrandFatherPantSize           end   If you are developing in Objective-C with XCode I let you discover some nice macros prefixed with NS ENUM  That should help you to define good enums easily and moreover will help the static analyzer to do some interesting checks for you before to compile   Good Enum

User · Answer

typedef enum   kCircle  kRectangle  kOblateSpheroid   ShapeType    then you can use it like  -     ShapeType shape    and   enum       kCircle      kRectangle      kOblateSpheroid    ShapeType    now you can use it like -  enum ShapeType shape

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An enum declares a set of ordered values - the typedef just adds a handy name to this  The 1st element is 0 etc    typedef enum   Monday 1         WORKDAYS   WORKDAYS today   Monday    The above is just an enumeration of shapeType tags

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enum can reduce many types of  errors  and make the code more manageable   define STATE GOOD 0  define STATE BAD 1  define STATE OTHER 2 int STATE   STATE OTHER   The definition has no constraints  It s simply just a substitution  It is not able to limit all conditions of the state  When the STATE is assigned to 5  the program will be wrong  because there is no matching state  But the compiler is not going to warn STATE   5  So it is better to use like this  typedef enum SampleState       SampleStateGood    0      SampleStateBad      SampleStateOther   SampleState   SampleState state   SampleStateGood

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A user defined type that has the possible values of kCircle  kRectangle  or kOblateSpheroid   The values inside the enum  kCircle  etc  are visible outside the enum  though  It s important to keep that in mind  int i   kCircle  is valid  for example

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The Typedef is a Keyword in C and C    It is used to create new names for basic data types  char  int  float  double  struct  amp  enum    typedef enum       kCircle      kRectangle      kOblateSpheroid   ShapeType    Here it creates enumerated data type ShapeType  amp  we can write new names for enum type ShapeType as given below  ShapeType shape1   ShapeType shape2   ShapeType shape3

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Three things are being declared here  an anonymous enumerated type is declared  ShapeType is being declared a typedef for that anonymous enumeration  and the three names kCircle  kRectangle  and kOblateSpheroid are being declared as integral constants   Let s break that down   In the simplest case  an enumeration can be declared as  enum tagname            This declares an enumeration with the tag tagname   In C and Objective-C  but not C     any references to this must be preceded with the enum keyword   For example   enum tagname x      declare x of type  enum tagname  tagname x      ERROR in C Objective-C  OK in C     In order to avoid having to use the enum keyword everywhere  a typedef can be created   enum tagname          typedef enum tagname tagname      declare  tagname  as a typedef for  enum tagname    This can be simplified into one line   typedef enum tagname         tagname      declare both  enum tagname  and  tagname    And finally  if we don t need to be able to use enum tagname with the enum keyword  we can make the enum anonymous and only declare it with the typedef name   typedef enum         tagname    Now  in this case  we re declaring ShapeType to be a typedef ed name of an anonymous enumeration   ShapeType is really just an integral type  and should only be used to declare variables which hold one of the values listed in the declaration  that is  one of kCircle  kRectangle  and kOblateSpheroid    You can assign a ShapeType variable another value by casting  though  so you have to be careful when reading enum values   Finally  kCircle  kRectangle  and kOblateSpheroid are declared as integral constants in the global namespace   Since no specific values were specified  they get assigned to consecutive integers starting with 0  so kCircle is 0  kRectangle is 1  and kOblateSpheroid is 2

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typedef is useful for redefining the name of an existing variable type  It provides short  amp  meaningful way to call a datatype  e g   typedef unsigned long int TWOWORDS    here  the type unsigned long int is redefined to be of the type TWOWORDS  Thus  we can now declare variables of type unsigned long int by writing   TWOWORDS var1  var2    instead of   unsigned long int var1  var2

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Update for 64-bit Change  According to apple docs about 64-bit changes      Enumerations Are Also Typed   In the LLVM compiler  enumerated types can   define the size of the enumeration  This means that some enumerated   types may also have a size that is larger than you expect  The   solution  as in all the other cases  is to make no assumptions about a   data type   s size  Instead  assign any enumerated values to a variable   with the proper data type   So you have to create enum with type as below syntax if you support for 64-bit   typedef NS ENUM NSUInteger  ShapeType        kCircle      kRectangle      kOblateSpheroid      or   typedef enum ShapeType   NSUInteger      kCircle     kRectangle     kOblateSpheroid   ShapeType    Otherwise  it will lead to warning as Implicit conversion loses integer precision  NSUInteger  aka  unsigned long   to ShapeType  Update for swift-programming   In swift  there s an syntax change   enum ControlButtonID  NSUInteger           case kCircle   kRectangle  kOblateSpheroid

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A typedef allows the programmer to define one Objective-C type as another  For example   typedef int Counter  defines the type Counter to be equivalent to the int type  This drastically improves code readability

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Apple recommends defining enums like this since Xcode 4 4  typedef enum ShapeType   NSUInteger       kCircle      kRectangle      kOblateSpheroid   ShapeType   They also provide a handy macro NS ENUM  typedef NS ENUM NSUInteger  ShapeType        kCircle      kRectangle      kOblateSpheroid     These definitions provide stronger type checking and better code completion  I could not find official documentation of NS ENUM  but you can watch the  quot Modern Objective-C quot  video from WWDC 2012 session here   UPDATE Link to official documentation here

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enum is used to assign value to enum elements which cannot be done in struct  So everytime instead of accessing the complete variable we can do it by the value we assign to the variables in enum  By default it starts with 0 assignment but we can assign it any value and the next variable in enum will be assigned a value the previous value  1

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You can use in the below format  Raw default value starting from 0  so    kCircle is 0    kRectangle is 1  kOblateSpheroid is 2    You can assign your own specific start value   typedef enum   NSUInteger       kCircle     for your value  kCircle   5          kRectangle      kOblateSpheroid   ShapeType   ShapeType circleShape   kCircle  NSLog    lu    unsigned long  circleShape      prints  0

[objective-c] What is a typedef enum in Objective-C?

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