An enumeration is a data type consisting of a set of named values that represent integral constants, known as enumeration constants. An enumeration is also referred to as an enumerated type because you must list (enumerate) each of the values in creating a name for each of them. In addition to providing a way of defining and grouping sets of integral constants, enumerations are useful for variables that have a small number of possible values.
You can declare an enumeration type separately from the definition of variables of that type, as described in Enumeration type definition and Enumeration variable declarations; or you can define an enumeration data type and all variables that have that type in one statement, as described in Enumeration type and variable definitions in a single statement.
An enumeration type definition contains the enum keyword followed by an optional identifier (the enumeration tag) and a brace-enclosed list of enumerators. A comma separates each enumerator in the enumerator list. C99 allows a trailing comma between the last enumerator and the closing brace. XL C++ also supports this feature, for compatibility with C99.
Enumeration definition syntax .-,----------. V | >>-enum--+----------------+--{----enumerator-+--}--;----------->< '-tag_identifier-'
The tag_identifier gives a name to the enumeration type. If you do not provide a tag name, you must put all variable definitions that refer to the enumeration type within the declaration of the type, as described in Enumeration type and variable definitions in a single statement. Similarly, you cannot use a type qualifier with an enumeration definition; type qualifiers placed in front of the enum keyword can only apply to variables that are declared within the type definition.
Enumeration members
The list of enumeration members, or enumerators, provides the data type with a set of values.
Enumeration member declaration syntax >>-identifier--+-------------------------+--------------------->< '-=--enumeration_constant-'
In C, an enumeration
constant is of type int. If a constant expression
is used as an initializer, the value of the expression cannot exceed
the range of int (that is, INT_MIN to INT_MAX as
defined in the header limits.h).
In C++, each enumeration constant has a value that can be
promoted to a signed or unsigned integer value and a distinct type
that does not have to be integral. You can use an enumeration constant
anywhere an integer constant is allowed, or anywhere a value of the
enumeration type is allowed.
The value of an enumeration constant is determined in the following way:
enum grain { oats, wheat, barley, corn, rice };
/* 0 1 2 3 4 */
enum grain { oats=1, wheat, barley, corn, rice };
/* 1 2 3 4 5 */
enum grain { oats, wheat=10, barley, corn=20, rice };
/* 0 10 11 20 21 */
enum status { run, clear=5, suspend, resume, hold=6 };
/* 0 5 6 7 6 */
func()
{
enum score { poor, average, good };
enum rating { below, average, above };
int poor;
}
You must declare the enumeration data type before you can define a variable having that type.
The tag_identifier indicates the previously-defined data type of the enumeration.Enumeration variable declaration syntax .-----------------------------. V | >>---+-------------------------+-+--enum--tag_identifier--------> +-storage_class_specifier-+ '-type_qualifier----------' >--declarator--------------------------------------------------><
The keyword enum is optional in enumeration
variable declarations.
register enum score { poor=1, average, good } rating = good;
C++ also lets you put the storage class immediately
before the declarator list. For example: enum score { poor=1, average, good } register rating = good;
enum score { poor=1, average, good };
register enum score rating = good;
Both examples define the enumeration data type score and the variable rating. rating has the storage class specifier register, the data type enum score, and the initial value good.
enum { Sunday, Monday, Tuesday, Wednesday, Thursday, Friday,
Saturday } weekday;
defines the variable weekday, which can be assigned any of the specified enumeration constants. However, you can not declare any additional enumeration variables using this set of enumeration constants.