Enumerations

An enumerated type is a distinct type whose value is a value of its underlying type (see below), which includes the values of explicitly named constants (enumeration constants).

Syntax

Enumerated type is declared using the following enumeration specifier as the type specifier in the declaration grammar:


`enum` identifier(optional) `{` enumerator-list }

where enumerator-list is a comma-separated list (with trailing comma permitted) (since C99) of enumerators, each of which has the form:


enumerator	(1)

enumerator `=` constant-expression	(2)

where

identifier, enumerator	-	identifiers that are introduced by this declaration
constant-expression	-	integer constant expression whose value is representable as a value of type int

As with struct or union, a declaration that introduced an enumerated type and one or more enumeration constants may also declare one or more objects of that type or type derived from it.

enum color_t {RED, GREEN, BLUE}, c = RED, *cp = &c;
 // introduces the type enum color_t
 // the integer constants RED, GREEN, BLUE 
 // the object c of type enum color_t
 // the object cp of type pointer to enum color_t

Explanation

Each enumerator that appears in the body of an enumeration specifier becomes an integer constant with type int in the enclosing scope and can be used whenever integer constants are required (e.g. as a case label or as a non-VLA array size).

enum color_t { RED, GREEN, BLUE} r = RED;
switch(r) {
     case RED   : puts("red"); break;
     case GREEN : puts("green"); break;
     case BLUE  : puts("blue"); break;
}

If enumerator is followed by = constant-expression, its value is the value of that constant expression. If enumerator is not followed by = constant-expression, its value is the value one greater than the value of the previous enumerator in the same enumeration. The value of the first enumerator (if it does not use = constant-expression) is zero.

enum Foo { A, B, C=10, D, E=1, F, G=F+C};
//A=0, B=1, C=10, D=11, E=1, F=2, G=12

The identifier itself, if used, becomes the name of the enumerated type in the tags name space and requires the use of the keyword enum (unless typedef'd into the ordinary name space).

enum color_t { RED, GREEN, BLUE};
enum color_t r = RED; // OK
// color_t x = GREEN: // Error: color_t is not in ordinary name space
typedef enum color_t color;
color x = GREEN; // OK

Each enumerated type is compatible with one of: char, a signed integer type, or an unsigned integer type. It is implementation-defined which type is compatible with any given enumerated type, but whatever it is, it must be capable of representing all enumerator values of that enumeration.

Enumerated types are integer types, and as such can be used anywhere other integer types can, including in implicit conversions and arithmetic operators.

enum { ONE = 1, TWO } e;
long n = ONE; // promotion
double d = ONE; // conversion
e = 1.2; // conversion, e is now ONE
e = e + 1; // e is now TWO

Notes

Unlike struct or union, there are no forward-declared enums in C:

enum Color; // Error: no forward-declarations for enums in C
enum Color { RED, GREEN, BLUE};

Enumerations permit the declaration of named constants in a more convenient and structured fashion than does #define; they are visible in the debugger, obey scope rules, and participate in the type system.

#define TEN 10
struct S { int x : TEN; }; // OK

or

enum { TEN = 10 };
struct S { int x : TEN; }; // also OK

References

C11 standard (ISO/IEC 9899:2011):

6.2.5/16 Types (p: 41)

6.7.2.2 Enumeration specifiers (p: 117-118)

C99 standard (ISO/IEC 9899:1999):

6.2.5/16 Types (p: 35)

6.7.2.2 Enumeration specifiers (p: 105-106)

C89/C90 standard (ISO/IEC 9899:1990):

3.1.2.5 Types

3.5.2.2 Enumeration specifiers

Keywords

enum

Language
headers
Type support
Dynamic memory management
Error handling
Program utilities
Variadic function support
Date and time utilities
Strings library
Algorithms
Numerics
Input/output support
Localization support
Thread support (C11)
Atomic operations (C11)
Technical Specifications

ASCII chart
Basic concepts
translation phases
comment
identifier
scope
lifetime
name space
type
arithmetic types
object
main function
memory model
Keywords
Preprocessor
Statements
Expressions
Initialization
Declarations
Functions
Miscellaneous
history of C
Technical Specifications