How can I count the items in an enum

Question

This question came to my mind  when I had something like  enum Folders  FA  FB  FC     and wanted to create an array of containers for each folder   ContainerClass m containers 3        m containers FA            etc     Using maps it s much more elegant to use  std  map lt Folders  ContainerClass  gt  m containers    But to come back to my original question  What if I do not want to hard-code the array size  is there a way to figure out how many items are in Folders   Without relying on e g  FC being the last item in the list which would allow something like ContainerClass m containers FC 1  if I m not mistaken

User · Answer

Here is the best way to do it in compilation time. I have used the arg_var count answer from here.

#define PP_NARG(...) \
     PP_NARG_(__VA_ARGS__,PP_RSEQ_N())
#define PP_NARG_(...) \
         PP_ARG_N(__VA_ARGS__)

#define PP_ARG_N( \
          _1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
         _11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
         _21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
         _31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
         _41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
         _51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
         _61,_62,_63,N,...) N
#define PP_RSEQ_N() \
         63,62,61,60,                   \
         59,58,57,56,55,54,53,52,51,50, \
         49,48,47,46,45,44,43,42,41,40, \
         39,38,37,36,35,34,33,32,31,30, \
         29,28,27,26,25,24,23,22,21,20, \
         19,18,17,16,15,14,13,12,11,10, \
         9,8,7,6,5,4,3,2,1,0

#define TypedEnum(Name, ...)                                      \
struct Name {                                                     \
    enum {                                                        \
        __VA_ARGS__                                               \
    };                                                            \
    static const uint32_t Name##_MAX = PP_NARG(__VA_ARGS__);      \
}

#define Enum(Name, ...) TypedEnum(Name, __VA_ARGS__)

To declare an enum:

Enum(TestEnum, 
Enum_1= 0,
Enum_2= 1,
Enum_3= 2,
Enum_4= 4,
Enum_5= 8,
Enum_6= 16,
Enum_7= 32);

the max will be available here:

int array [TestEnum::TestEnum_MAX];
for(uint32_t fIdx = 0; fIdx < TestEnum::TestEnum_MAX; fIdx++)
{
     array [fIdx] = 0;
}

User · Answer

I like to use enums as arguments to my functions   It s an easy means to provide a fixed list of  options   The trouble with the top voted answer here is that using that  a client can specify an  invalid option    As a spin off  I recommend doing essentially the same thing  but use a constant int outside of the enum to define the count of them   enum foobar   foo  bar  baz  quz    const int FOOBAR NR ITEMS 4    It s not pleasant  but it s a clean solution if you don t change the enum without updating the constant

User · Answer

There s not really a good way to do this  usually you see an extra item in the enum  i e   enum foobar  foo  bar  baz  quz  FOOBAR NR ITEMS     So then you can do   int fuz FOOBAR NR ITEMS     Still not very nice though   But of course you do realize that just the number of items in an enum is not safe  given e g   enum foobar  foo  bar   5  baz  quz   20     the number of items would be 4  but the integer values of the enum values would be way out of the array index range  Using enum values for array indexing is not safe  you should consider other options   edit  as requested  made the special entry stick out more

User · Answer

How about traits  in an STL fashion  For instance   enum Foo       Bar      Baz      write an  std  numeric limits lt enum Foo gt   max     specialization  possibly constexpr if you use c  11   Then  in your test code provide any static assertions to maintain the constraints that std  numeric limits  max     last item

User · Answer

Add a entry  at the end of your enum  called Folders MAX or something similar and use this value when initializing your arrays   ContainerClass  m containers Folders MAX

User · Answer

For C    there are various type-safe enum techniques available  and some of those  such as the proposed-but-never-submitted Boost Enum  include support for getting the size of a enum   The simplest approach  which works in C as well as C    is to adopt a convention of declaring a    MAX value for each of your enum types   enum Folders   FA  FB  FC  Folders MAX   FC    ContainerClass  m containers Folders MAX   1        m containers FA            etc    Edit  Regarding   FA  FB  FC  Folders MAX   FC  versus  FA  FB  FC  Folders MAX   I prefer setting the    MAX value to the last legal value of the enum for a few reasons    The constant s name is technically more accurate  since Folders MAX gives the maximum possible enum value   Personally  I feel like Folders MAX   FC stands out from other entries out a bit more  making it a bit harder to accidentally add enum values without updating the max value  a problem Martin York referenced   GCC includes helpful warnings like  enumeration value not included in switch  for code such as the following   Letting Folders MAX    FC   1 breaks those warnings  since you end up with a bunch of    MAX enumeration values that should never be included in switch     switch  folder       case FA         case FB            Oops  forgot FC

User · Answer

I really do not see any way to really get to the number of values in an enumeration in C    Any of the before mention solution work as long as you do not define the value of your enumerations if you define you value that you might run into situations where you either create arrays too big or too small  enum example  test1   -2  test2   -1  test3   0  test4   1  test5   2     in this about examples the result would create a array of 3 items when you need an array of 5 items  enum example2  test1   test2   test3   test4   test5   301     in this about examples the result would create a array of 301 items when you need an array of 5 items  The best way to solve this problem in the general case would be to iterate through your enumerations but that is not in the standard yet as far as I know

[c++] (How) can I count the items in an enum?

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