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17 Enumeration Types

An enumeration type represents a limited set of integer values, each with a name. It is effectively equivalent to a primitive integer type.

Suppose we have a list of possible emotional states to store in an integer variable. We can give names to these alternative values with an enumeration:

enum emotion_state { neutral, happy, sad, worried,
                     calm, nervous };

(Never mind that this is a simplistic way to classify emotional states; it’s just a code example.)

The names inside the enumeration are called enumerators. The enumeration type defines them as constants, and their values are consecutive integers; neutral is 0, happy is 1, sad is 2, and so on. Alternatively, you can specify values for the enumerators explicitly like this:

enum emotion_state { neutral = 2, happy = 5,
                     sad = 20, worried = 10,
                     calm = -5, nervous = -300 };

Each enumerator which does not specify a value gets value zero (if it is at the beginning) or the next consecutive integer.

/* neutral is 0 by default,
   and worried is 21 by default.  */
enum emotion_state { neutral,
                      happy = 5, sad = 20, worried,
                      calm = -5, nervous = -300 };

If an enumerator is obsolete, you can specify that using it should cause a warning, by including an attribute in the enumerator’s declaration. Here is how happy would look with this attribute:

happy __attribute__
        ("impossible under plutocratic rule")))
      = 5,

See Attributes.

You can declare variables with the enumeration type:

enum emotion_state feelings_now;

In the C code itself, this is equivalent to declaring the variable int. (If all the enumeration values are positive, it is equivalent to unsigned int.) However, declaring it with the enumeration type has an advantage in debugging, because GDB knows it should display the current value of the variable using the corresponding name. If the variable’s type is int, GDB can only show the value as a number.

The identifier that follows enum is called a type tag since it distinguishes different enumeration types. Type tags are in a separate name space and belong to scopes like most other names in C. See Type Tags, for explanation.

You can predeclare an enum type tag like a structure or union type tag, like this:

enum foo;

The enum type is incomplete until you finish defining it.

You can optionally include a trailing comma at the end of a list of enumeration values:

enum emotion_state { neutral, happy, sad, worried,
                     calm, nervous, };

This is useful in some macro definitions, since it enables you to assemble the list of enumerators without knowing which one is last. The extra comma does not change the meaning of the enumeration in any way.

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