Enumerators

The names of the enumerators in a scoped enumeration follow normal scoping rules and are inaccessible outside the scope of the enumeration. The enumerator names in an unscoped enumeration are placed into the same scope as the enumeration itself:

enum color {red, yellow, green};      // unscoped enumeration
enum stoplight {red, yellow, green};  // error: redefines enumerators
enum class peppers {red, yellow, green}; // ok: enumerators are hidden
color eyes = green; // ok: enumerators are in scope for an unscoped enumeration
peppers p = green;  // error: enumerators from peppers are not in scope
                    //     color::green is in scope but has the wrong type
color hair = color::red;   // ok: we can explicitly access the enumerators
peppers p2 = peppers::red; // ok: using red from peppers

By default, enumerator values start at 0 and each enumerator has a value 1 greater than the preceding one. However, we can also supply initializers for one or more enumerators:

enum class intTypes {
    charTyp = 8, shortTyp = 16, intTyp = 16,
    longTyp = 32, long_longTyp = 64
};

As we see with the enumerators for intTyp and shortTyp, an enumerator value need not be unique. When we omit an initializer, the enumerator has a value 1 greater than the preceding enumerator.

Enumerators are const and, if initialized, their initializers must be constant expressions (§ 2.4.4, p. 65). Consequently, each enumerator is itself a constant expression. Because the enumerators are constant expressions, we can use them where a constant expression is required. For example, we can define constexpr variables of enumeration type:

constexpr intTypes charbits = intTypes::charTyp;

Similarly, we can use an enum as the expression in a switch statement and use the value of its enumerators as the case labels (§ 5.3.2, p. 178). For the same reason, we can also use an enumeration type as a nontype template parameter (§ 16.1.1, p. 654). and can initialize class static data members of enumeration type inside the class definition (§ 7.6, p. 302).