C Enumeration Declarations

An enumeration consists of a set of named integer constants. An enumeration type declaration gives the name of the (optional) enumeration tag and defines the set of named integer identifiers (called the “enumeration set,” “enumerator constants,” “enumerators,” or “members”). A variable with enumeration type stores one of the values of the enumeration set defined by that type.

Variables of enum type can be used in indexing expressions and as operands of all arithmetic and relational operators. Enumerations provide an alternative to the #define preprocessor directive with the advantages that the values can be generated for you and obey normal scoping rules.

In ANSI C, the expressions that define the value of an enumerator constant always have int type; thus, the storage associated with an enumeration variable is the storage required for a single int value. An enumeration constant or a value of enumerated type can be used anywhere the C language permits an integer expression.

Syntax

enum-specifier :

enum identifier _opt { enumerator-list }
enum identifier

The optional identifier names the enumeration type defined by enumerator-list. This identifier is often called the “tag” of the enumeration specified by the list. A type specifier of the form

enum identifier { enumerator-list }

declares identifier to be the tag of the enumeration specified by the enumerator-list nonterminal. The enumerator-list defines the “enumerator content.” The enumerator-list is described in detail below.

If the declaration of a tag is visible, subsequent declarations that use the tag but omit enumerator-list specify the previously declared enumerated type. The tag must refer to a defined enumeration type, and that enumeration type must be in current scope. Since the enumeration type is defined elsewhere, the enumerator-list does not appear in this declaration. Declarations of types derived from enumerations and typedef declarations for enumeration types can use the enumeration tag before the enumeration type is defined.

Syntax

enumerator-list :

enumerator
enumerator-list , enumerator

enumerator :

enumeration-constant
enumeration-constant = constant-expression

enumeration-constant :

identifier

Each enumeration-constant in an enumeration-list names a value of the enumeration set. By default, the first enumeration-constant is associated with the value 0. The next enumeration-constant in the list is associated with the value of ( constant-expression + 1 ), unless you explicitly associate it with another value. The name of an enumeration-constant is equivalent to its value.

You can use enumeration-constant = constant-expression to override the default sequence of values. Thus, if enumeration-constant = constant-expression appears in the enumerator-list, the enumeration-constant is associated with the value given by constant-expression. The constant-expression must have int type and can be negative.

The following rules apply to the members of an enumeration set:

• An enumeration set can contain duplicate constant values. For example, you could associate the value 0 with two different identifiers, perhaps named null and zero, in the same set.
  
   
• The identifiers in the enumeration list must be distinct from other identifiers in the same scope with the same visibility, including ordinary variable names and identifiers in other enumeration lists.
  
   
• Enumeration tags obey the normal scoping rules. They must be distinct from other enumeration, structure, and union tags with the same visibility.

Examples

These examples illustrate enumeration declarations:

enum DAY            /* Defines an enumeration type    */
{
    saturday,       /* Names day and declares a       */
    sunday = 0,     /* variable named workday with    */ 
    monday,         /* that type                      */
    tuesday,
    wednesday,      /* wednesday is associated with 3 */
    thursday,
    friday
} workday;

The value 0 is associated with saturday by default. The identifier sunday is explicitly set to 0. The remaining identifiers are given the values 1 through 5 by default.

In this example, a value from the set DAY is assigned to the variable today.

enum DAY today = wednesday;

Note that the name of the enumeration constant is used to assign the value. Since the DAY enumeration type was previously declared, only the enumeration tag DAY is necessary.

To explicitly assign an integer value to a variable of an enumerated data type, use a type cast: 

workday = ( enum DAY ) ( day_value - 1 );

This cast is recommended in C but is not required.

enum BOOLEAN  /* Declares an enumeration data type called BOOLEAN */
{
    false,     /* false = 0, true = 1 */
    true 
}; 

enum BOOLEAN end_flag, match_flag; /* Two variables of type BOOLEAN */

This declaration can also be specified as

enum BOOLEAN { false, true } end_flag, match_flag;\

or as

enum BOOLEAN { false, true } end_flag;
enum BOOLEAN match_flag;

An example that uses these variables might look like this:

if ( match_flag == false )
    {
     .
     .   /* statement */ 
     .
    }
    end_flag = true;

Unnamed enumerator data types can also be declared. The name of the data type is omitted, but variables can be declared. The variable response is a variable of the type defined:

enum { yes, no } response;