4.5 Declaring Enumerations

An enumerated type is a user-defined integer type. An enumerated type defines enumeration constants, which are integral constant expressions with values that can be represented as integers. An enumerated type declaration follows this syntax:

enum-specifier:

   enum identifier(opt) { enumerator-list}
   enum identifier

enumerator-list:

   enumerator
   enumerator-list, enumerator

enumerator:

   enumeration-constant
   enumeration-constant = constant_expression

In DEC C, objects of type enum are compatible with objects of type signed int .

The following example shows the declaration of an enumeration type and an enumeration tag:

enum shades
   {
      off, verydim, dim, prettybright, bright
   }  light;

This declaration defines the variable light to be of an enumerated type shades . light can assume any of the enumerated values.

The tag shades is the enumeration tag of the new type. off through bright are the enumeration constants with values 0 through 4. These enumeration constants are constant values and can be used wherever integer constants are valid.

Once a tag is declared, it can be used as a reference to that enumerated type, as in the following declaration, where the variable light1 is an object of the enumerated data type shades :

enum  shades  light1;

An incomplete type declaration of an enumerated type is illegal; for example:

enum e;

An enum tag can have the same spelling as other identifiers in the same program in other name spaces. However, enum constant names share the same name space as variables and functions, so they must have unique names to avoid ambiguity.

Internally, each enumeration constant is associated with an integer constant; the compiler gives the first enumeration constant the value 0 by default, and the remaining enumeration constants are incremented by 1 for each succeeding value. Any enumeration constant can be set to a specific integer constant value. The enumeration constants following such a construct (unless they are also set to specific values) then receive values that are one greater than the previous value. Consider the following example:

enum spectrum
   {
      red, yellow = 4, green, blue, indigo, violet
   }  color2 = yellow;

This declaration gives red , yellow , green , blue , . . . , the values 0, 4, 5, 6, . . . Assigning duplicate values to enumeration constants is permitted.

The value of color2 is an integer (4), not a string such as "red" or "yellow".