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C Mechanics
Enumerations

Enumerations

An enumeration is a user-defined data type whose underlying type is restricted to a range of explicitly named constants (enumeration constants).

For example, the following enumeration defines a new type called color_t whose underlying type is int and whose values are RED, GREEN, and BLUE:

#include <stdio.h>
 
enum color_t {
    RED,
    GREEN,
    BLUE
};
 
int main(void) {
    enum color_t color = RED;
 
    printf("%d\n", color); // 0
 
    return 0;
}

Enumerations as Integers

By default, the first enumeration constant is assigned the value 0, the second enumeration constant is assigned the value 1, and so on.

We can change the value of an enumeration constant by explicitly assigning it a value:

#include <stdio.h>
 
enum color_t {
    RED = 1,
    GREEN = 3,
    BLUE = 4
};
 
int main(void) {
    enum color_t color = RED;
    enum color_t color2 = GREEN;
 
    printf("%d\n", color); // 1
    printf("%d\n", color2++); // 3
    printf("%d\n", color2); // 4
 
    return 0;
}

With typedef

We can use the typedef keyword to create a new name for an enumeration type:

// we use an anonymous enum (without a tag):
typedef enum {
    RED,
    GREEN,
    BLUE
} color_t;
 
/* same as:
 
enum color_t {
    RED,
    GREEN,
    BLUE
} color_t;
 
*/
 
int main(void) {
    color_t color = RED;
 
    /* same as (if declared with a tag):
 
    enum color_t color = RED;
 
    */
 
    return 0;
}

With struct

We can use an enumeration as a member of a struct:

#include <stdio.h>
 
typedef enum {
    RED,
    GREEN,
    BLUE
} color_t;
 
typedef struct {
    int x;
    int y;
    color_t color;
} point_t;
 
int main(void) {
    point_t point = { .x = 1, .y = 2, .color = RED };
 
    return 0;
}
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