How to use enums in C

Question

Suppose we have an enum like the following   enum Days  Saturday  Sunday  Tuesday  Wednesday  Thursday  Friday     I want to create an instance of this enum and initialize it with a proper value  so I do   Days day   Days Saturday    Now I want to check my variable or instance with an existing enum value  so I do   if  day    Days Saturday        std  cout  lt  lt   Ok its Saturday       Which gives me a compilation error      error  expected primary-expression before         token   So to be clear  what is the difference between saying   if  day    Days Saturday     Causes compilation error   and  if  day    Saturday       What do these two actually refer to  in that one is OK and one causes a compilation error

User · Answer

You are looking for strongly typed enumerations  a feature available in the C  11 standard  It turns enumerations into classes with scope values   Using your own code example  it is     enum class Days  Saturday  Sunday  Tuesday Wednesday  Thursday  Friday     Days day   Days  Saturday     if  day    Days  Saturday         cout  lt  lt    Today is Saturday     lt  lt  endl          int day2   Days  Sunday     Error  invalid   Using    as accessors to enumerations will fail if targeting a C   standard prior C  11  But some old compilers doesn t supported it  as well some IDEs just override this option  and set a old C   std   If you are using GCC  enable C 11 with -std c  11 or -std gnu11    Be happy

User · Answer

I think your root issue is the use of   instead of     which will use the namespace   Try   enum Days  Saturday  Sunday  Tuesday  Wednesday  Thursday  Friday   Days day   Days  Saturday  if Days  Saturday    day      I like literals before variables          std  cout lt  lt  Ok its Saturday

User · Answer

This will be sufficient to declare your enum variable and compare it   enum Days  Saturday  Sunday  Tuesday  Wednesday  Thursday  Friday   Days day   Saturday  if  day    Saturday        std  cout  lt  lt   Ok its Saturday

User · Answer

You can use a trick to use scopes as you wish  just declare enum in such way   struct Days       enum type            Saturday Sunday Tuesday Wednesday Thursday Friday           Days  type day   Days  Saturday  if  day    Days  Saturday

User · Answer

If we want the strict type safety and scoped enum  using enum class is good in C  11   If we had to work in C  98  we can using the advice given by InitializeSahib San to enable the scoped enum   If we also want the strict type safety  the follow code can implement somthing like enum    include  lt iostream gt  class Color   public      static Color RED                 return Color 0             static Color BLUE                 return Color 1             bool operator   const Color  amp rhs  const               return this- gt value    rhs value            bool operator   const Color  amp rhs  const               return    this    rhs          private      explicit Color int value     value value          int value      int main         Color color   Color  RED        if  color    Color  RED                  std  cout  lt  lt   red   lt  lt  std  endl            return 0      The code is modified from the class Month example in book Effective C   3rd  Item 18

User · Answer

This should not work in C     Days Saturday   Days is not a scope or object that contains members you can access with the dot operator  This syntax is just a C -ism and is not legal in C     Microsoft has long maintained a C   extension that allows you to access the identifiers using the scope operator   enum E   A  B  C     A  E  B     works with Microsoft s extension   But this is non-standard before C  11  In C  03 the identifiers declared in an enum exist only in the same scope as the enum type itself   A  E  B     error in C  03   C  11 makes it legal to qualify enum identifiers with the enum name  and also introduces enum classes  which create a new scope for the identifiers instead of placing them in the surrounding scope   A  E  B     legal in C  11  enum class F   A  B  C     A     error F  B

User · Answer

First of all  make  E  in enum   e  as a lower case   Second  drop type name  Days  in  Days Saturday    Third    buy yourself a good C   book

User · Answer

While C    excluding C  11  has enums  the values in them are  leaked  into the global namespace  If you don t want to have them leaked  and don t NEED to use the enum type   consider the following     class EnumName        public           static int EnumVal1           more definitions         EnumName  EnumVal1    value     if   your value     EnumName  EnumVal1

User · Answer

Enums in C   are like integers masked by the names you give them  when you declare your enum-values  this is not a definition only a hint how it works    But there are two errors in your code    Spell enum all lower case You don t need the Days  before Saturday  If this enum is declared in a class  then use if  day    YourClass  Saturday

User · Answer

Much of this should give you compilation errors       note the lower case enum keyword enum Days   Saturday  Sunday  Monday  Tuesday  Wednesday  Thursday  Friday      Now  Saturday  Sunday  etc  can be used as top-level bare constants and Days can be used as a type   Days day   Saturday       Days Saturday is an error   And similarly later  to test   if  day    Saturday               These enum values are like bare constants - they re un-scoped - with a little extra help from the compiler   unless you re using C  11 enum classes  they aren t encapsulated like object or structure members for instance  and you can t refer to them as members of Days   You ll have what you re looking for with C  11  which introduces an enum class   enum class Days       SUNDAY      MONDAY             etc             if  day    Days  SUNDAY               Note that this C   is a little different from C in a couple of ways  one is that C requires the use of the enum keyword when declaring a variable      day declaration in C  enum Days day   Saturday

User · Answer

If you are still using C  03 and want to use enums  you should be using enums inside a namespace   Eg   namespace Daysofweek  enum Days  Saturday  Sunday  Tuesday Wednesday  Thursday  Friday       You can use the enum outside the namespace like   Daysofweek  Days day   Daysofweek  Saturday   if  day    Daysofweek  Saturday        std  cout lt  lt  Ok its Saturday

User · Answer

This code is wrong   enum Days  Saturday  Sunday  Tuesday  Wednesday  Thursday  Friday   Days day   Days Saturday  if  day    Days Saturday    Because Days is not a scope  nor object  It is a type  And Types themselves don t have members  What you wrote is the equivalent to std  string clear   std  string is a type  so you can t use   on it   You use   on an instance of a class   Unfortunately  enums are magical and so the analogy stops there   Because with a class  you can do std  string  clear to get a pointer to the member function  but in C  03  Days  Sunday is invalid    Which is sad    This is because C   is  somewhat  backwards compatable with C  and C had no namespaces  so enumerations had to be in the global namespace   So the syntax is simply   enum Days  Saturday  Sunday  Tuesday  Wednesday  Thursday  Friday   Days day   Saturday  if  day    Saturday    Fortunately  Mike Seymour observes that this has been addressed in C  11  Change enum to enum class and it gets its own scope  so Days  Sunday is not only valid  but is the only way to access Sunday  Happy days

User · Answer

Rather than using a bunch of if-statements  enums lend themselves well to switch statements  I use some enum switch combinations in the level builder I am building for my game   EDIT  Another thing  I see you want syntax similar to   if day    Days Saturday  etc   You can do this in C     if day    Days  Saturday  etc   Here is a very simple example    EnumAppState h   ifndef ENUMAPPSTATE H  define ENUMAPPSTATE H enum eAppState       STARTUP      EDIT      ZONECREATION      SHUTDOWN      NOCHANGE     endif   Somefile cpp   include  EnumAppState h  eAppState state   eAppState  STARTUP  switch state    case STARTUP        Do stuff     break  case EDIT        Do stuff     break  case ZONECREATION        Do stuff     break  case SHUTDOWN        Do stuff     break  case NOCHANGE        Do stuff     break

User · Answer

Sadly  elements of the enum are  global   You access them by doing day   Saturday  That means that you cannot have enum A   a  b     and enum B   b  a     for they are in conflict

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