namespaces for enum types - best practices

xtofl_6610 0
Viewed 96668

Often, one needs several enumerated types together. Sometimes, one has a name clash. Two solutions to this come to mind: use a namespace, or use 'larger' enum element names. Still, the namespace solution has two possible implementations: a dummy class with nested enum, or a full blown namespace.

I'm looking for pros and cons of all three approaches.

Example:

// oft seen hand-crafted name clash solution
enum eColors { cRed, cColorBlue, cGreen, cYellow, cColorsEnd };
enum eFeelings { cAngry, cFeelingBlue, cHappy, cFeelingsEnd };
void setPenColor( const eColors c ) {
    switch (c) {
        default: assert(false);
        break; case cRed: //...
        break; case cColorBlue: //...
        //...
    }
 }


// (ab)using a class as a namespace
class Colors { enum e { cRed, cBlue, cGreen, cYellow, cEnd }; };
class Feelings { enum e { cAngry, cBlue, cHappy, cEnd }; };
void setPenColor( const Colors::e c ) {
    switch (c) {
        default: assert(false);
        break; case Colors::cRed: //...
        break; case Colors::cBlue: //...
        //...
    }
 }


 // a real namespace?
 namespace Colors { enum e { cRed, cBlue, cGreen, cYellow, cEnd }; };
 namespace Feelings { enum e { cAngry, cBlue, cHappy, cEnd }; };
 void setPenColor( const Colors::e c ) {
    switch (c) {
        default: assert(false);
        break; case Colors::cRed: //...
        break; case Colors::cBlue: //...
        //...
    }
  }
enums scope nested c++
8 Answers

Original C++03 answer:

The benefit from a namespace (over a class) is that you can use using declarations when you want.

The problem with using a namespace is that namespaces can be expanded elsewhere in the code. In a large project, you would not be guaranteed that two distinct enums don't both think they are called eFeelings

For simpler-looking code, I use a struct, as you presumably want the contents to be public.

If you're doing any of these practices, you are ahead of the curve and probably don't need to scrutinize this further.

Newer, C++11 advice:

If you are using C++11 or later, enum class will implicitly scope the enum values within the enum's name.

With enum class you will lose implicit conversions and comparisons to integer types, but in practice that may help you discover ambiguous or buggy code.

user_162870

FYI In C++0x there is a new syntax for cases like what you mentioned (see C++0x wiki page)

enum class eColors { ... };
enum class eFeelings { ... };
jmihalicza_590190

I would definitely avoid using a class for this; use a namespace instead. The question boils down to whether to use a namespace or to use unique ids for the enum values. Personally, I'd use a namespace so that my ids could be shorter and hopefully more self-explanatory. Then application code could use a 'using namespace' directive and make everything more readable.

From your example above:

using namespace Colors;

void setPenColor( const e c ) {
    switch (c) {
        default: assert(false);
        break; case cRed: //...
        break; case cBlue: //...
        //...
    }
}
user_116770

An difference between using a class or a namespace is that the class cannot be reopened like a namespace can. This avoids the possibility that the namespace might be abused in the future, but there is also the problem that you cannot add to the set of enumerations either.

A possible benefit for using a class, is that they can be used as template type arguments, which is not the case for namespaces:

class Colors {
public:
  enum TYPE {
    Red,
    Green,
    Blue
  };
};

template <typename T> void foo (T t) {
  typedef typename T::TYPE EnumType;
  // ...
}

Personally, I'm not a fan of using, and I prefer the fully qualified names, so I don't really see that as a plus for namespaces. However, this is probably not the most important decision that you'll make in your project!

Richard_Corden_116980

Since enums are scoped to their enclosing scope, it's probably best to wrap them in something to avoid polluting the global namespace and to help avoid name collisions. I prefer a namespace to class simply because namespace feels like a bag of holding, whereas class feels like a robust object (cf. the struct vs. class debate). A possible benefit to a namespace is that it can be extended later - useful if you're dealing with third-party code that you cannot modify.

This is all moot of course when we get enum classes with C++0x.

Kristo_468210
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