6.5. Enums
These fall into the category of ‘half baked’. They aren't proper
enumerated types, as in Pascal, and only really serve to help you reduce the
number of #define
statements in your program. They look like
this:
enum e_tag{ a, b, c, d=20, e, f, g=20, h }var;
Just as with structures and unions, the e_tag
is the tag, and
var
is the definition of a variable.
The names declared inside the enumeration are constants with
int
type. Their values are these:
a == 0 b == 1 c == 2 d == 20 e == 21 f == 22 g == 20 h == 21
so you can see that, in the absence of anything to the contrary, the values assigned start at zero and increase. A specific value can be given if you want, when the increase will continue one at a time afterwards; the specific value must be an integral constant (see later) that is representable in an int. It is possible for more than one of the names to have the same value.
The only use for these things is to give a better-scoped version of this:
#define a 0 #define b 1 /* and so on */
It's better scoped because the declaration of enumerations follows the
standard scope rules for C, whereas #define
statements have
file scope.
Not that you are likely to care, but the Standard states that enumeration types are of a type that is compatible with an implementation-defined one of the integral types. So what? For interest's sake here is an illustration:
enum ee{a,b,c}e_var, *ep;
The names a
, b
, and c
all behave as
if they were int
constants when you use them;
e_var
has type enum ee
and ep
is
a pointer to enum ee
. The compatibility requirement means that
(amongst other implications) there will be an integral type whose address
can be assigned to ep
without violating the type-compatibility
requirements for pointers.