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enumeration declaration

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An enumeration is a distinct type whose value is restricted to one of several explicitly named constants ("enumerators"). The values of the constants are values of an integral type known as the underlying type of the enumeration.

An enumeration is defined by enum-specifier, which appears in decl-specifier-seq of the declaration syntax. The enum-specifier has the following syntax:

enum-key attr(optional) identifier(optional) enum-base(optional) { enumerator-list(optional) } (1)
enum-key attr(optional) identifier enum-base(optional) ; (2) (since C++11)
1) definition of an enumeration type
2) opaque enum declaration: defines the enumeration type but not its enumerators: after this declaration, the type is a complete type and its size is known.
enum-key - one of enum, enum class(since C++11), or enum struct(since C++11)
attr(C++11) - optional sequence of any number of attributes
identifier - the name of the enumeration that's being declared. If present, and if this declaration is a re-declaration, it may be preceded by nested-name-specifier(since C++11): sequence of names and scope-resolution operators ::, ending with scope-resolution operator. The name can be omitted only in unscoped enumeration declarations.
enum-base(C++11) - colon (:), followed by a type-specifier-seq that names an integral type (if it is cv-qualified, qualifications are ignored)
enumerator-list - comma-separated list of enumerator definitions, each of which is either simply an identifier, which becomes the name of the enumerator, or an identifier with an initializer: identifier = constexpr

There are two distinct kinds of enumerations: unscoped enumeration (declared with the enum-key enum and scoped enumeration (declared with the enum-key enum class or enum struct).

Unscoped enumeration

enum name { enumerator = constexpr , enumerator = constexpr , ... } (1)
enum name : type { enumerator = constexpr , enumerator = constexpr , ... } (2) (since C++11)
enum name : type ; (3) (since C++11)
1) declares an unscoped enumeration type whose underlying type is not fixed (in this case, the underlying type is either int or, if not all enumerator values can be represented as int, an implementation-defined larger integral type that can represent all enumerator values. If the enumerator-list is empty, the underlying type is as if the enumeration had a single enumerator with value 0)
2) declares an unscoped enumeration type whose underlying type is fixed
3) opaque enum declaration for an unscoped enumeration must specify the underlying type

Each enumerator becomes a named constant of the enumeration's type (that is, name), visible in the enclosing scope, and can be used whenever constants are required.

enum Color { RED, GREEN, BLUE};
Color r = RED;
switch(r) {
     case RED   : std::cout << "red\n"; break;
     case GREEN : std::cout << "green\n"; break;
     case BLUE  : std::cout << "blue\n"; break;
}

Each enumerator is associated with a value of the underlying type. When initializers are provided in the enumerator-list, the values of enumerators are defined by those initializers. If the first enumerator does not have an initializer, the associated value is zero. For any other enumerator whose definition does not have an initializer, the associated value is the value of the previous enumerator plus one.

enum Foo { A, B, C=10, D, E=1, F, G=F+C};
//A=0, B=1, C=10, D=11, E=1, F=2, G=12

Values of unscoped enumeration type are implicitly-convertible to integral types. If the underlying type is not fixed, the value is convertible first type from the following list able to hold their entire value range: int, unsigned int, long, unsigned long, long long, or unsigned long long. If the underlying type is fixed, the values can be converted to their promoted underlying type.

enum color { red, yellow, green=20, blue };
color col = red;
int n = blue; // n == 21

The name of an unscoped enumeration may be omitted: such declaration only introduces the enumerators into the enclosing scope:

enum { A, B, C=0, D=A+2 }; // defines A = 0, B = 1, C = 0, D = 2

When an unscoped enumeration is a class member, its enumerators may be accessed using class member access operators . and ->:

struct X {
    enum direction { left='l', right='r'};
};
X x;
X* p = &x;
 
int b = X::direction::left;
int a = X::left;
int b = x.left;
int c = p->left;


Scoped enumerations

enum struct|class name { enumerator = constexpr , enumerator = constexpr , ... } (1)
enum struct|class name : type { enumerator = constexpr , enumerator = constexpr , ... } (2) (since C++11)
enum struct|class name : type ; (3) (since C++11)
1) declares a scoped enumeration type whose underlying type is int (the keywords class and struct are exactly equivalent)
2) declares a scoped enumeration type whose underlying type is fixed
3) opaque enum declaration for an unscoped enumeration

Each enumerator becomes a named constant of the enumeration's type (that is, name), which is contained within the scope of the enumeration, and can be accessed using scope resolution operator. There are no implicit conversions from the values of a scoped enumerator to integral types, although static_cast may be used to obtain the numeric value of the enumerator.

enum class Color { RED, GREEN=20, BLUE};
Color r = Color::BLUE;
switch(r) {
    case Color::RED : std::cout << "red\n"; break;
    case Color::GREEN : std::cout << "green\n"; break;
    case Color::BLUE : std::cout << "blue\n"; break;
}
// int n = r; // error: no scoped enum to int conversion
int n = static_cast<int>(r); // OK, n = 21

Example

#include <iostream>
// color may be red (value 0), yellow (value 1), green (value 20), or blue (value 21)
enum color {
    red,
    yellow,
    green = 20,
    blue
};
// altitude may be altitude::high or altitude::low
enum class altitude : char { 
     high='h',
     low='l', // C++11 allows the extra comma
}; 
// the constant d is 0, the constant e is 1, the constant f is 3
enum { d, e, f=e+2 };
int main()
{
    color col = red;
    altitude a;
    a = altitude::low;
 
    std::cout << "red = " << col << " blue = " << blue << '\n'
              << "a = " << static_cast<char>(a) << '\n'
              << "f = " << f << '\n';
}

Output:

red = 0 blue = 21
a = l
f = 3