Justification

Defensive programming dictates that multiple choice constructs have a guard (condition), to trap "impossible" errors. Since the default action is a "catch-all" it aids the reader to enumerate the possible special cases before the general case.

Exception: If a case clause is present for each of the possible values of an enum type, a default clause is not required.

Example 1

#define CCBB_THE_CHOICE 1

int trigger;

trigger = 2;
switch (trigger)
{
case CCBB_THE_CHOICE:
   break;
} /* WRONG: Default clause is missing */

Example 2

#define CCBB_THE_CHOICE 1

int trigger;
...
switch (trigger)
{
case CCBB_THE_CHOICE:
   break;
default: /* RIGHT */
   /* any statement */
   break;
}

Example 3

enum Colors {Red, Blue, Green} color;

color = Red;
switch (color)
{
case Red:
   break;
case Blue:
   break;
case Green:
   break;
} /* RIGHT: no default clause required because all enum values are covered. */