How to Get enum item name from its value

Question

I declared a enum type as this   enum WeekEnum   Mon   0  Tue   1  Wed   2  Thu   3  Fri   4  Sat   5  Sun   6       How can I get the item name  Mon  Tue  etc  when I already have the item value  0  1  etc    I already have a function as this  Log Today is  2   enjoy      And now I want the output below      Today is Wed  enjoy

User · Answer

You can t directly  enum in C   are not like Java enums    The usual approach is to create a std  map lt WeekEnum std  string gt    std  map lt WeekEnum std  string gt  m  m Mon     Monday         m Sun     Sunday

User · Answer

On GCC it may look like this   const char  WeekEnumNames             Mon     Mon        Tue     Tue        Wed     Wed        Thu     Thu        Fri     Fri        Sat     Sat        Sun     Sun

User · Answer

If you know the actual enum labels correlated to their values  you can use containers and C  17 s std  string view to quickly access values and their string representations with the     operator while tracking it yourself  std  string view will only allocate memory when created  They can also be designated with static constexpr if you want them available at run-time for more performance savings  This little console app should be fairly fast    include  lt iostream gt   include  lt string view gt   include  lt tuple gt      int main         enum class Weekdays     default behavior starts at 0 and iterates by 1 per entry         Monday Tuesday Wednesday Thursday Friday Saturday Sunday             static constexpr std  string view Monday       Monday       static constexpr std  string view Tuesday      Tuesday       static constexpr std  string view Wednesday    Wednesday       static constexpr std  string view Thursday     Thursday       static constexpr std  string view Friday       Friday       static constexpr std  string view Saturday     Saturday       static constexpr std  string view Sunday       Sunday       static constexpr std  string view opener       enum        static constexpr std  string view closer         is        static constexpr std  string view semi                  std  pair lt Weekdays  std  string view gt  Weekdays List               std  make pair Weekdays  Monday     Monday           std  make pair Weekdays  Tuesday    Tuesday           std  make pair Weekdays  Wednesday  Wednesday           std  make pair Weekdays  Thursday   Thursday           std  make pair Weekdays  Friday     Friday           std  make pair Weekdays  Saturday   Saturday           std  make pair Weekdays  Sunday     Sunday              for  int i 0 i lt sizeof Weekdays List  sizeof Weekdays List 0   i              std  cout lt  lt opener lt  lt i lt  lt closer lt  lt Weekdays List  int i  second lt  lt semi           lt  lt  int Weekdays List  int i  first lt  lt std  endl                return 0      Output    enum 0  is Monday 0 enum 1  is Tuesday 1 enum 2  is Wednesday 2 enum 3  is Thursday 3 enum 4  is Friday 4 enum 5  is Saturday 5 enum 6  is Sunday 6

User · Answer

An enumeration is something of an inverse-array  What I believe you want is this   const char   Week            Sunday    Monday    Tuesday    Wednesday    Thursday    Friday    Saturday         The blank string at the beginning is so that Sunday is 1 instead of 0  cout  lt  lt   Today is    lt  lt  Week 2   lt  lt     enjoy        Or whatever you de like to do with it

User · Answer

No  you have no way to get the  name  from the value in C   because all the symbols are discarded during compilation    You may need this way X Macros

User · Answer

The solution I prefer is to mix arrays and ostream like this   std  ostream amp  operator lt  lt  std  ostream amp  lhs  WeekEnum e        static const std  array lt std  string  7 gt  WEEK STRINGS     Monday    Tuesday    Wednesday    Thursday    Friday    Saturday    Sunday          return os  lt  lt  WEEK STRINGS statuc cast lt WeekEnum gt  e      cout  lt  lt   Today is    lt  lt  WeekEnum  Monday    I also suggest to use enum class instead of Enum

User · Answer

How can I get the item name  Mon  Tue  etc  when I already have the   item value  0  1  etc     On some older C code  quite some time ago   I found code analogous to   std  string weekEnumToStr int n       std  string s  unknown       switch  n          case 0    s    Mon     break     case 1    s    Tue     break     case 2    s    Wed     break     case 3    s    Thu     break     case 4    s    Fri     break     case 5    s    Sat     break     case 6    s    Sun     break          return s      Con  This establishes a  pathological dependency  between the enumeration values and the function    meaning if you change the enum you must change the function to match   I suppose this is true even for a std  map     I vaguely remember we found a utility to generate the function code from the enum code   The enum table length had grown to several hundred     and at some point it is maybe a sound choice to write code to write code       Note -   in an embedded system enhancement effort  my team replaced many tables  100    of null-terminated-strings used to map enum int values to their text strings   The problem with the tables was that a value out of range was often not noticed because many of these tables were gathered into one region of code   memory  such that a value out-of-range reached past the named table end s  and returned a null-terminated-string from some subsequent table     Using the function-with-switch statement also allowed us to add an assert in the default clause of the switch   The asserts found several more coding errors during test  and our asserts were tied into a static-ram-system-log our field techs could search

User · Answer

Here is another neat trick to define enum using X Macro    include  lt iostream gt    define WEEK DAYS   X MON   Monday   true    X TUE   Tuesday   true    X WED   Wednesday   true    X THU   Thursday   true    X FRI   Friday   true    X SAT   Saturday   false    X SUN   Sunday   false    define X day  name  workday  day  enum WeekDay   size t       WEEK DAYS     undef X   define X day  name  workday  name  char const  weekday name           WEEK DAYS     undef X   define X day  name  workday  workday  bool weekday workday         WEEK DAYS     undef X  int main         std  cout  lt  lt   Enum value     lt  lt  WeekDay  THU  lt  lt  std  endl      std  cout  lt  lt   Name string     lt  lt  weekday name WeekDay  THU   lt  lt  std  endl      std  cout  lt  lt  std  boolalpha  lt  lt   Work day     lt  lt  weekday workday WeekDay  THU   lt  lt  std  endl       WeekDay wd   SUN      std  cout  lt  lt   Enum value     lt  lt  wd  lt  lt  std  endl      std  cout  lt  lt   Name string     lt  lt  weekday name wd   lt  lt  std  endl      std  cout  lt  lt  std  boolalpha  lt  lt   Work day     lt  lt  weekday workday wd   lt  lt  std  endl       return 0      Live Demo  https   ideone com bPAVTM  Outputs   Enum value  3 Name string  Thursday Work day  true Enum value  6 Name string  Sunday Work day  false

User · Answer

I have had excellent success with a technique which resembles the X macros pointed to by  RolandXu  We made heavy use of the stringize operator  too  The technique mitigates the maintenance nightmare when you have an application domain where items appear both as strings and as numerical tokens   It comes in particularily handy when machine readable documentation is available so that the macro X      lines can be auto-generated  A new documentation would immediately result in a consistent program update covering the strings  enums and the dictionaries translating between them in both directions   We were dealing with PCL6 tokens    And while the preprocessor code looks pretty ugly  all those technicalities can be hidden in the header files which never have to be touched again  and neither do the source files  Everything is type safe  The only thing that changes is a text file containing all the X      lines  and that is possibly auto generated

User · Answer

You can define an operator that performs the output   std  ostream amp  operator lt  lt  std  ostream amp  lhs  WeekEnum e        switch e        case Monday  lhs  lt  lt   Monday   break         etc           return lhs

[c++] How to Get enum item name from its value

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