How to define an enumerated type enum in C

Question

I m not sure what is the proper syntax for using C enums  I have the following code   enum  RANDOM  IMMEDIATE  SEARCH  strategy  strategy   IMMEDIATE    But this does not compile  with the following error   error  conflicting types for    strategy    error  previous declaration of    strategy    was here   What am I doing wrong

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When you say  enum  RANDOM  IMMEDIATE  SEARCH  strategy    you create a single instance variable  called  strategy  of a nameless enum  This is not a very useful thing to do - you need a typedef   typedef enum  RANDOM  IMMEDIATE  SEARCH  StrategyType   StrategyType strategy   IMMEDIATE

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My favorite and only used construction always was   typedef enum MyBestEnum          good enough        GOOD   0         even better        BETTER         divine        BEST      I believe that this will remove your problem you have  Using new type is from my point of view right option

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There seems to be a confusion about the declaration   When strategycomes before  RANDOM  IMMEDIATE  SEARCH  as in the following   enum strategy  RANDOM  IMMEDIATE  SEARCH     you are creating a new type named enum strategy  However  when declaring the variable  you need to use enum strategy itself  You cannot just use strategy  So the following is invalid   enum strategy  RANDOM  IMMEDIATE  SEARCH   strategy a    While  the following is valid  enum strategy  RANDOM  IMMEDIATE  SEARCH    enum strategy queen   RANDOM  enum strategy king   SEARCH  enum strategy pawn 100       When strategy comes after  RANDOM  IMMEDIATE  SEARCH   you are creating an anonymous enum and then declaring strategy to be a variable of that type   So now  you can do something like  enum  RANDOM  IMMEDIATE  SEARCH  strategy  strategy   RANDOM    However  you cannot declare any other variable of type enum  RANDOM  IMMEDIATE  SEARCH  because you have never named it  So the following is invalid  enum  RANDOM  IMMEDIATE  SEARCH  strategy  enum strategy a   RANDOM      You can combine both the definitions too  enum strategy  RANDOM  IMMEDIATE  SEARCH  a  b   a   RANDOM  b   SEARCH  enum strategy c   IMMEDIATE      Typedef as noted before is used for creating a shorter variable declaration   typedef enum  RANDOM  IMMEDIATE  SEARCH  strategy    Now you have told compiler that enum  RANDOM  IMMEDIATE  SEARCH  is synonomous to strategy  So now you can freely use strategy as variable type  You don t need to type enum strategy anymore  The following is valid now  strategy x   RANDOM      You can also combine Typedef along with enum name to get  typedef enum strategyName  RANDOM  IMMEDIATE  SEARCH  strategy    There s not much advantage of using this method apart from the fact that you can now use strategy and enum strategyName interchangeably   typedef enum strategyName  RANDOM  IMMEDIATE  SEARCH  strategy   enum strategyName a   RANDOM  strategy b   SEARCH

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I tried with gcc and come up with for my need I was forced to use the last alternative  to compile with out error    typedef enum state  a   0  b   1  c   2  state   typedef enum state  a   0  b   1  c   2  state   typedef enum state old     New type  alias of the state type  typedef enum state new     New type  alias of the state type   new now       a  old before    b   printf  State   now    d  n   now   printf  Sate before    d  n n   before

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As written  there s nothing wrong with your code   Are you sure you haven t done something like  int strategy      enum  RANDOM  IMMEDIATE  SEARCH  strategy    What lines do the error messages point to   When it says  previous declaration of  strategy  was here   what s  here  and what does it show

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ThoAppelsin in his comment to question posted is right  The code snippet posted in the question it is valid and with no errors  The error you have must be because other bad syntax in any other place of your c source file  enum a b c   defines three symbolic constants  a  b and c  which are integers with values 0 1 and 2 respectively  but when we use enum it is because we don t usually care about the specific integer value  we care more about the meaning of the symbolic constant name   This means you can have this    include  lt stdio h gt  enum  a b c   int main      printf   d n  b     return 0      and this will output 1   This also will be valid    include  lt stdio h gt  enum  a b c   int bb b  int main      printf   d n  bb     return 0      and will output the same as before   If you do this   enum  a b c   enum  a b c     you will have an error  but if you do this   enum alfa a b c   enum alfa    you will not have any error   you can do this   enum  a b c   int aa a    and aa will be an integer variable with value 0  but you can also do this   enum  a b c  aa  a    and will have the same effect  that is  aa being an int with 0 value     you can also do this   enum  a b c  aa  a  aa  7    and aa will be int with value 7   because you cannot repeat symbolic constant definition with the use of enum  as i have said previously  you must use tags if you want to declare int vars with the use of enum   enum tag1  a b c   enum tag1 var1  a  enum tag1 var2  b    the use of typedef it is to safe you from writing each time enum tag1 to define variable  With typedef you can just type Tag1   typedef enum  a b c  Tag1  Tag1 var1  a  Tag1 var2  b    You can also have   typedef enum tag1 a b c Tag1  Tag1 var1  a  enum tag1 var2  b    Last thing to say it is that since we are talking about defined symbolic constants it is better to use capitalized letters when using enum  that is for example   enum  A B C     instead of   enum  a b c

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You re trying to declare strategy twice  and that s why you re getting the above error  The following works without any complaints  compiled with gcc -ansi -pedantic -Wall    include  lt stdio h gt   enum   RANDOM  IMMEDIATE  SEARCH   strategy   IMMEDIATE   int main int argc  char   argv       printf  quot strategy   d n quot   strategy        return 0     If instead of the above  the second line were changed to      enum   RANDOM  IMMEDIATE  SEARCH   strategy  strategy   IMMEDIATE       From the warnings  you could easily see your mistake  enums c 5 1  warning  data definition has no type or storage class  enabled by default  enums c 5 1  warning  type defaults to    int    in declaration of    strategy     -Wimplicit-int  enums c 5 1  error  conflicting types for    strategy    enums c 4 36  note  previous declaration of    strategy    was here  So the compiler took strategy   IMMEDIATE for a declaration of a variable called strategy with default type int  but there was already a previous declaration of a variable with this name  However  if you placed the assignment in the main   function  it would be a valid code   include  lt stdio h gt   enum   RANDOM  IMMEDIATE  SEARCH   strategy   IMMEDIATE   int main int argc  char   argv       strategy SEARCH      printf  quot strategy   d n quot   strategy        return 0

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Tarc s answer is the best   Much of the enum discussion is a red herring   Compare this code snippet -  int strategy  strategy   1     void some function void         which gives   error C2501   strategy    missing storage-class or type specifiers error C2086   strategy    redefinition   with this one which compiles with no problem   int strategy  void some function void         strategy   1         The variable strategy needs to be set at declaration or inside a function etc  You cannot write arbitrary software - assignments in particular - at the global scope   The fact that he used enum  RANDOM  IMMEDIATE  SEARCH  instead of int is only relevant to the extent that it has confused people that can t see beyond it  The redefinition error messages in the question show that this is what the author has done wrong   So now you should be able to see why the first of the example below is wrong and the other three are okay   Example 1  WRONG   enum  RANDOM  IMMEDIATE  SEARCH  strategy  strategy   IMMEDIATE  void some function void         Example 2  RIGHT   enum  RANDOM  IMMEDIATE  SEARCH  strategy   IMMEDIATE  void some function void         Example 3  RIGHT   enum  RANDOM  IMMEDIATE  SEARCH  strategy  void some function void         strategy   IMMEDIATE      Example 4  RIGHT   void some function void         enum  RANDOM  IMMEDIATE  SEARCH  strategy      strategy   IMMEDIATE      If you have a working program you should just be able to paste these snippets into your program and see that some compile and some do not

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If you declare the name for enumeration no error will occur   If not declared  you have to use a typedef   enum enum name  RANDOM  IMMEDIATE  SEARCH  strategy  strategy   IMMEDIATE    It will not display an error

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It s worth pointing out that you don t need a typedef  You can just do it like the following  enum strategy   RANDOM  IMMEDIATE  SEARCH    enum strategy my strategy   IMMEDIATE    It s a style question whether you prefer typedef  Without it  if you want to refer to the enumeration type  you need to use enum strategy  With it  you can just say strategy   Both ways have their pro and cons  The one is more wordy  but keeps type identifiers into the tag-namespace where they won t conflict with ordinary identifiers  think of struct stat and the stat function  these don t conflict either   and where you immediately see that it s a type  The other is shorter  but brings type identifiers into the ordinary namespace

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It s worth mentioning that in C   you can use  enum  to define a new type without needing a typedef statement    enum Strategy  RANDOM  IMMEDIATE  SEARCH       Strategy myStrategy   IMMEDIATE    I find this approach a lot more friendly    edit - clarified C   status - I had this in originally  then removed it

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C enum stuff q  enum stuff  a  b -4  c  d -2  e  f -3  g  s   Declaration which acts as a tentative definition of a signed integer s with complete type and declaration which acts as a tentative definition of signed integer q with incomplete type in the scope  which resolves to the complete type in the scope because the type definition is present anywhere in the scope   like any tentative definition  the identifiers q and s can be redeclared with the incomplete or complete version of the same type int or enum stuff multiple times but only defined once in the scope i e  int q   3  and can only be redefined in a subscope  and only usable after the definition   Also you can only use the complete type of enum stuff once in the scope because it acts as a type definition  A compiler enumeration type definition for enum stuff is also made present at file scope  usable before and below  as well as a forward type declaration  the type enum stuff can have multiple declarations but only one definition completion in the scope and can be redefined in a subscope   It also acts as a compiler directive to substitute a with rvalue 0  b with -4  c with 5  d with -2  e with -3  f with -1 and g with -2 in the current scope  The enumeration constants now apply after the definition until the next redefinition in a different enum which cannot be on the same scope level  typedef enum bool  false  true  bool     this is the same as  enum bool  false  true   typedef enum bool bool     or enum bool  false  true   typedef unsigned int bool     remember though  bool is an alias for  Bool if you include stdbool h     and casting to a bool is the same as the    operator   The tag namespace shared by enum  struct and union is separate and must be prefixed by the type keyword  enum  struct or union  in C i e  after enum a  a  b  enum a c must be used and not a c  Because the tag namespace is separate to the identifier namespace  enum a  a  b is allowed but enum a  a  b  b is not because the constants are in the same namespace as the variable identifiers  the identifier namespace  typedef enum a  a b  b is also not allowed because typedef-names are part of the identifier namespace  The type of enum bool and the constants follow the following pattern in C   -------------- ----- ----- -----      enum bool    a 1  b  a   c 3      -------------- ----- ----- -----    unsigned int   int   int   int      -------------- ----- ----- -----    -------------- ----- ----- -----      enum bool    a 1   b -2  c 3      -------------- ----- ----- -----         int       int   int   int      -------------- ----- ----- -----    -------------- ----- --------------- -----      enum bool    a 1  b  - 0x80000000  c 2    -------------- ----- --------------- -----    unsigned int   int    unsigned int   int    -------------- ----- --------------- -----    -------------- ----- --------------- -----      enum bool    a 1  b  - 2147483648  c 2    -------------- ----- --------------- -----    unsigned int   int    unsigned int   int    -------------- ----- --------------- -----    ----------- ----- --------------- ------    enum bool   a 1  b  - 0x80000000  c -2    ----------- ----- --------------- ------       long     int        long        int    ----------- ----- --------------- ------    ----------- ----- --------------- ------    enum bool   a 1   b 2147483648    c -2    ----------- ----- --------------- ------       long     int        long        int    ----------- ----- --------------- ------    ----------- ----- --------------- ------    enum bool   a 1   b -2147483648   c -2    ----------- ----- --------------- ------       int      int        int         int    ----------- ----- --------------- ------    --------------- ----- --------------- -----      enum bool     a 1   b 99999999999   c 1    --------------- ----- --------------- -----    unsigned long   int   unsigned long   int    --------------- ----- --------------- -----    ----------- ----- --------------- ------    enum bool   a 1   b 99999999999   c -1    ----------- ----- --------------- ------       long     int        long        int    ----------- ----- --------------- ------    This compiles fine in C   include  lt stdio h gt  enum c j  enum c f  m  p  typedef int d  typedef int c  enum c j  enum m  n    int main       enum c j    enum d l     enum d q     enum m y     printf  quot  llu quot   j      C   In C    enums can have a type enum Bool  bool  True  False  Bool  enum Bool  bool  True  False  maybe  Bool    error  In this situation  the constants and the identifier all have the same type  bool  and an error will occur if a number cannot be represented by that type  Maybe   2  which isn t a bool  Also  True  False and Bool cannot be lower case otherwise they will clash with language keywords  An enum also cannot have a pointer type  The rules for enums are different in C     include  lt iostream gt  c j    not allowed  unknown type name c before enum c f  p  line enum c j    not allowed  forward declaration of enum type not allowed and variable can have an incomplete type but not when it s still a forward declaration in C   unlike C enum c f  m  p  typedef int d  typedef int c     not allowed in C   as it clashes with enum c  but if just int c were used then the below usages of c j  would have to be enum c j   enum  c j  enum m  n    int main        enum  c j    enum d l     not allowed in same scope as typedef but allowed here    d q    m y    simple type specifier not allowed  need elaborated type specifier enum m to refer to enum m here   p v     not allowed  need enum p to refer to enum p   std  cout  lt  lt  j     Enums variables in C   are no longer just unsigned integers etc  they re also of enum type and can only be assigned constants in the enum  This can however be cast away   include  lt stdio h gt  enum a  l  c  enum d  f    int main       c 0     not allowed    c l    c  a 1    c  enum a 4    printf  quot  llu quot   c     4    Enum classes enum struct is identical to enum class  include  lt stdio h gt  enum class a  b  c  int main       printf  quot  llu quot   a  b lt 1      not allowed   printf  quot  llu quot    int a  b lt 1      printf  quot  llu quot   a  b lt  a 1      printf  quot  llu quot   a  b lt  enum a 1     printf  quot  llu quot   a  b lt  enum class a 1      not allowed    printf  quot  llu quot   b lt  enum a 1     not allowed    The scope resolution operator can still be used for non-scoped enums   include  lt stdio h gt  enum a  bool  l  w    int main       enum a  bool  w  l  f    printf  quot  llu quot     a  w      But because w cannot be defined as something else in the scope  there is no difference between   w and   a  w

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Declaring an enum variable is done like this   enum strategy  RANDOM  IMMEDIATE  SEARCH   enum strategy my strategy   IMMEDIATE    However  you can use a typedef to shorten the variable declarations  like so   typedef enum  RANDOM  IMMEDIATE  SEARCH  strategy  strategy my strategy   IMMEDIATE    Having a naming convention to distinguish between types and variables is a good idea   typedef enum  RANDOM  IMMEDIATE  SEARCH  strategy type  strategy type my strategy   IMMEDIATE

[c] How to define an enumerated type (enum) in C?

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