Declaring an enum within a class

Question

In the following code snippet  the Color enum is declared within the Car class in order to limit the scope of the enum and to try not to  pollute  the global namespace   class Car   public      enum Color            RED        BLUE        WHITE           void SetColor  Car  Color color               color   color           Car  Color GetColor   const            return  color        private      Car  Color  color         1  Is this a good way to limit the scope of the Color enum   Or  should I declare it outside of the Car class  but possibly within its own namespace or struct   I just came across this article today  which advocates the latter and discusses some nice points about enums   http   gamesfromwithin com stupid-c-tricks-2-better-enums    2  In this example  when working within the class  is it best to code the enum as Car  Color  or would just Color suffice    I assume the former is better  just in case there is another Color enum declared in the global namespace   That way  at least  we are explicit about the enum to we are referring

User · Answer

Nowadays - using C  11 - you can use enum class for this   enum class Color   RED  BLUE  WHITE      AFAII this does exactly what you want

User · Answer

I prefer following approach  code below   It solves the  namespace pollution  problem  but also it is much more typesafe  you can t assign and even compare two different enumerations  or your enumeration with any other built-in types etc     struct Color       enum Type               Red  Green  Black            Type t       Color Type t    t  t         operator Type    const  return t    private       prevent automatic conversion for any other built-in types such as bool  int  etc    template lt typename T gt      operator T    const       Usage   Color c   Color  Red  switch c       case Color  Red                          break    Color2 c2   Color2  Green  c2   c    error c2   3    error if  c2    Color  Red        error If  c2     error   I create macro to facilitate usage    define DEFINE SIMPLE ENUM EnumName  seq    struct EnumName       enum type                BOOST PP SEQ FOR EACH I DEFINE SIMPLE ENUM VAL  EnumName  seq              type v       EnumName type v    v v          operator type   const  return v     private        template lt typename T gt        operator T    const       define DEFINE SIMPLE ENUM VAL r  data  i  record        BOOST PP TUPLE ELEM 2  0  record    BOOST PP TUPLE ELEM 2  1  record     Usage   DEFINE SIMPLE ENUM Color                 Red  1                  Green  3                    Some references    Herb Sutter  Jum Hyslop  C C   Users Journal  22 5   May 2004 Herb Sutter  David E  Miller  Bjarne Stroustrup Strongly Typed Enums  revision 3   July 2007

User · Answer

If Color is something that is specific to just Cars then that is the way you would limit its scope  If you are going to have another Color enum that other classes use then you might as well make it global  or at least outside Car   It makes no difference  If there is a global one then the local one is still used anyway as it is closer to the current scope  Note that if you define those function outside of the class definition then you ll need to explicitly specify Car  Color in the function s interface

User · Answer

In general  I always put my enums in a struct  I have seen several guidelines including  prefixing    enum Color     Clr Red    Clr Yellow    Clr Blue       Always thought this looked more like C guidelines than C   ones  for one because of the abbreviation and also because of the namespaces in C      So to limit the scope we now have two alternatives    namespaces structs classes   I personally tend to use a struct because it can be used as parameters for template programming while a namespace cannot be manipulated   Examples of manipulation include   template  lt class T gt  size t number              which returns the number of elements of enum inside the struct T

User · Answer

If you are creating a code library  then I would use namespace  However  you can still only have one Color enum inside that namespace  If you need an enum that might use a common name  but might have different constants for different classes  use your approach

[c++] Declaring an enum within a class

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