Enumerations
An enumeration is a data type that consists 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.
Enumeration type definition
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.
Enumeration definition syntax
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.
Elaborated type specifier
Elaborated type specifier syntax
The elaborated type specifier refers to a previously declared enumeration. The x is a variable that has the type tag_identifier.
enum
keyword
can be used to refer to scoped or unscoped enumerations during variable
declaration or definition. For example:// a scoped enumeration
enum class color { red, white, black, yellow };
// an unscoped enumeration
enum letter {A, B, C, D};
// valid, regular type name usage
color pic1 = color :: white;
// valid, elaborated type usage
enum color pic2 = color :: red;
enum class
or enum
struct
in the elaborated type specifier. For example:enum class color pic3 = color :: black; // invalid
enum letter let1 = letter :: A; // valid
Enumeration members
The list of enumeration members, or enumerators, provides the data type with a set of values.
Enumeration member declaration syntax
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
). Otherwise,
the condition is tolerated, a diagnostic message is issued, but the
value of the enumeration constant is undefined.
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:
- An equal sign (
=
) and a constant expression after the enumeration constant gives an explicit value to the enumeration constant. The enumeration constant represents the value of the constant expression. - If no explicit value is assigned to the first enumerator, then it takes the value 0 (zero).
- Enumeration constants with no explicitly assigned values receive the integer value that is one greater than the value represented by the previous enumeration constant.
oats
, wheat
, barley
, corn
,
and rice
as enumeration constants. The number under
each constant shows the integer value. 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 */
suspend
and hold
have
the same integer value. enum status { run, clear=5, suspend, resume, hold=6 };
/* 0 5 6 7 6 */
average
and poor
cause
compiler errors: func()
{
enum score { poor, average, good };
enum rating { below, average, above };
int poor;
}
Enumeration variable declarations
You must declare the enumeration data type before you can define a variable having that type.
Enumeration variable declaration syntax
The keyword
enum
is optional in enumeration
variable declarations.
Enumeration type and variable definitions in a single statement
register enum score { poor=1, average, good } rating = good;
![C++ only](ngcpp_begin.gif)
enum score { poor=1, average, good } register rating = good;
![C++ only](ngcpp_end.gif)
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 cannot declare any additional enumeration variables using
this set of enumeration constants.