Why is enum class preferred over plain enum

Question

I heard a few people recommending to use enum classes in C   because of their type safety    But what does that really mean

User · Answer

The basic advantage of using enum class over normal enums is that you may have same enum variables for 2 different enums and still can resolve them(which has been mentioned as type safe by OP)

For eg:

enum class Color1 { red, green, blue };    //this will compile
enum class Color2 { red, green, blue };

enum Color1 { red, green, blue };    //this will not compile 
enum Color2 { red, green, blue };

As for the basic enums, compiler will not be able to distinguish whether red is refering to the type Color1 or Color2 as in hte below statement.

enum Color1 { red, green, blue };   
enum Color2 { red, green, blue };
int x = red;    //Compile time error(which red are you refering to??)

User · Answer

C  11 FAQ mentions below points    conventional enums implicitly convert to int  causing errors when    someone does not want an enumeration to act as an integer   enum color       Red      Green      Yellow     enum class NewColor       Red 1      Green 1      Yellow 1     int main             Implicit conversion is possible     int i   Red           Need enum class name followed by access specifier  Ex  NewColor  Red 1     int j   Red 1     error C2065   Red 1   undeclared identifier          Implicit converison is not possible  Solution Ex  int k    int NewColor  Red 1      int k   NewColor  Red 1     error C2440   initializing   cannot convert from  NewColor  to  int       return 0      conventional enums export their enumerators to the surrounding scope  causing name clashes      Header h  enum vehicle       Car      Bus      Bike      Autorickshow     enum FourWheeler       Car            error C2365   Car   redefinition  previous definition was  enumerator      SmallBus     enum class Editor       vim      eclipes      VisualStudio     enum class CppEditor       eclipes           No error of redefinitions     VisualStudio      No error of redefinitions     QtCreator      The underlying type of an enum cannot be specified  causing confusion  compatibility problems  and makes forward declaration impossible      Header1 h  include  lt iostream gt   using namespace std   enum class Port   unsigned char     Forward declare  class MyClass   public      void PrintPort enum class Port p       void MyClass  PrintPort enum class Port p        cout  lt  lt   int p  lt  lt  endl            Header h enum class Port   unsigned char    Declare enum type explicitly       PORT 1   0x01      PORT 2   0x02      PORT 3   0x04            Source cpp  include  Header1 h   include  Header h   using namespace std  int main         MyClass m      m PrintPort Port  PORT 1        return 0

User · Answer

From Bjarne Stroustrup s C  11 FAQ      The enum classes   new enums    strong enums   address three problems   with traditional C   enumerations          conventional enums implicitly convert to int  causing errors when someone does not want an enumeration to act as an integer    conventional enums export their enumerators to the surrounding scope  causing name clashes    the underlying type of an enum cannot be specified  causing confusion  compatibility problems  and makes forward declaration   impossible          The new enums are  enum class  because they combine aspects of traditional enumerations  names values  with aspects of classes  scoped members and absence of conversions     So  as mentioned by other users  the  strong enums  would make the code safer   The underlying type of a  classic  enum shall be an integer type large enough to fit all the values of the enum  this is usually an int  Also each enumerated type shall be compatible with char or a signed unsigned integer type   This is a wide description of what an enum underlying type must be  so each compiler will take decisions on his own about the underlying type of the classic enum and sometimes the result could be surprising   For example  I ve seen code like this a bunch of times   enum E MY FAVOURITE FRUITS       E APPLE        0x01      E WATERMELON   0x02      E COCONUT      0x04      E STRAWBERRY   0x08      E CHERRY       0x10      E PINEAPPLE    0x20      E BANANA       0x40      E MANGO        0x80      E MY FAVOURITE FRUITS FORCE8   0xFF     Force  8bits  how can you tell       In the code above  some naive coder is thinking that the compiler will store the E MY FAVOURITE FRUITS values into an unsigned 8bit type    but there s no warranty about it  the compiler may choose unsigned char or int or short  any of those types are large enough to fit all the values seen in the enum  Adding the field E MY FAVOURITE FRUITS FORCE8 is a burden and doesn t forces the compiler to make any kind of choice about the underlying type of the enum   If there s some piece of code that rely on the type size and or assumes that E MY FAVOURITE FRUITS would be of some width  e g  serialization routines  this code could behave in some weird ways depending on the compiler thoughts   And to make matters worse  if some workmate adds carelessly a new value to our enum       E DEVIL FRUIT    0x100     New fruit  with value greater than 8bits   The compiler doesn t complain about it  It just resizes the type to fit all the values of the enum  assuming that the compiler were using the smallest type possible  which is an assumption that we cannot do   This simple and careless addition to the enum could subtlety break related code   Since C  11 is possible to specify the underlying type for enum and enum class  thanks rdb  so this issue is neatly addressed   enum class E MY FAVOURITE FRUITS   unsigned char       E APPLE          0x01      E WATERMELON     0x02      E COCONUT        0x04      E STRAWBERRY     0x08      E CHERRY         0x10      E PINEAPPLE      0x20      E BANANA         0x40      E MANGO          0x80      E DEVIL FRUIT    0x100     Warning   constant value truncated      Specifying the underlying type if a field have an expression out of the range of this type the compiler will complain instead of changing the underlying type   I think that this is a good safety improvement   So Why is enum class preferred over plain enum   if we can choose the underlying type for scoped enum class  and unscoped  enum  enums what else makes enum class a better choice     They don t convert implicitly to int  They don t pollute the surrounding namespace  They can be forward-declared

User · Answer

Enumerations are used to represent a set of integer values   The class keyword after the enum specifies that the enumeration is strongly typed and its enumerators are scoped  This way enum classes prevents accidental misuse of constants   For Example   enum class Animal Dog  Cat  Tiger   enum class Pets Dog  Parrot     Here we can not mix Animal and Pets values   Animal a   Dog           Error  which DOG      Animal a   Pets  Dog     Pets  Dog is not an Animal

User · Answer

C   has two kinds of enum   enum classes Plain enums  Here are a couple of examples on how to declare them   enum class Color   red  green  blue       enum class  enum Animal   dog  cat  bird  human       plain enum   What is the difference between the two   enum classes - enumerator names are local to the enum and their values do not implicitly convert to other types  like another enum or int   Plain enums - where enumerator names are in the same scope as the enum and their values implicitly convert to integers and other types   Example  enum Color   red  green  blue                          plain enum  enum Card   red card  green card  yellow card          another plain enum  enum class Animal   dog  deer  cat  bird  human        enum class enum class Mammal   kangaroo  deer  human              another enum class  void fun             examples of bad use of plain enums      Color color   Color  red      Card card   Card  green card       int num   color        no problem      if  color    Card  red card     no problem  bad          cout  lt  lt   quot bad quot   lt  lt  endl       if  card    Color  green       no problem  bad          cout  lt  lt   quot bad quot   lt  lt  endl          examples of good use of enum classes  safe      Animal a   Animal  deer      Mammal m   Mammal  deer       int num2   a       error     if  m    a             error  good          cout  lt  lt   quot bad quot   lt  lt  endl       if  a    Mammal  deer     error  good          cout  lt  lt   quot bad quot   lt  lt  endl      Conclusion  enum classes should be preferred because they cause fewer surprises that could potentially lead to bugs

User · Answer

Because  as said in other answers  class enum are not implicitly convertible to int bool  it also helps to avoid buggy code like   enum MyEnum     Value1    Value2         if  var    Value1    Value2     Should be  var    Value2  no error warning

User · Answer

One thing that hasn t been explicitly mentioned - the scope feature gives you an option to have the same name for an enum and class method  For instance   class Test   public        these call ProcessCommand   internally    void TakeSnapshot       void RestoreSnapshot    private     enum class Command    wouldn t be possible without  class               TakeSnapshot          RestoreSnapshot          void ProcessCommand Command cmd      signal the other thread or whatever

User · Answer

do not implicitly convert to int can choose which type underlie ENUM namespace to avoid polluting happen Compared with normal class  can be declared forward  but do not have methods

User · Answer

It s worth noting  on top of these other answers  that C  20 solves one of the problems that enum class has  verbosity  Imagining a hypothetical enum class  Color   void foo Color c    switch  c        case Color  Red           case Color  Green           case Color  Blue              etc         This is verbose compared to the plain enum variation  where the names are in the global scope and therefore don t need to be prefixed with Color      However  in C  20 we can use using enum to introduce all of the names in an enum to the current scope  solving the problem   void foo Color c    using enum Color    switch  c        case Red           case Green           case Blue              etc         So now  there is no reason not to use enum class

[c++] Why is enum class preferred over plain enum?

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