Declaring an enum within a class

1. 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).

2. 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.

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<typename T>
    operator T () const;
};

Usage:

Color c = Color::Red;
switch(c)
{
   case Color::Red:
     //некоторый код
   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<typename T> \
    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:

1. Herb Sutter, Jum Hyslop, C/C++ Users Journal, 22(5), May 2004
2. Herb Sutter, David E. Miller, Bjarne Stroustrup Strongly Typed Enums (revision 3), July 2007

Nowadays - using C++11 - you can use enum class for this:

enum class Color { RED, BLUE, WHITE };

AFAII this does exactly what you want.

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 <class T>
size_t number() { /**/ }

which returns the number of elements of enum inside the struct T :)