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

                                    .-,----------.         
                                    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.

Elaborated type specifier

Elaborated type specifier syntax

>>-enum--tag_identifier--x-------------------------------------><

The elaborated type specifier refers to a previously declared enumeration. The x is a variable that has the type tag_identifier.

// 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 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

>>-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). 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:

1. 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.
2. If no explicit value is assigned to the first enumerator, then it takes the value 0 (zero).
3. Enumeration constants with no explicitly assigned values receive the integer value that is one greater than the value represented by the previous enumeration constant.

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;
    }

Enumeration variable declarations

You must declare the enumeration data type before you can define a variable having that type.

Enumeration variable declaration syntax

   .-----------------------------.                         
   V                             |                         
>>---+-------------------------+-+--enum--tag_identifier-------->
     +-storage_class_specifier-+                           
     '-type_qualifier----------'                           

>--declarator--------------------------------------------------><

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++ 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 cannot declare any additional enumeration variables using this set of enumeration constants.