The Issue With Default in Switch Statements with Enums

Reading the coding standards at a company I recently joined revealed to me the issue with default label within the switch statement and why it’s prohibitted when its being used to enumerate through an enum. default label is convenient to handle any edge cases and it’s often used to handle errors. However, when working with enums, it is often the case that the prpogrammer intends to handle all possible values in the enum. To catch this mishap, programmers would enable -Wswitch or -Werror=switch to their compiler. For instance, let’s suppose I have an enum named Suit to represent the different suits in a deck of cards.

enum Suit {
  Diamonds,
  Hearts,
  Clubs,
  Spades
};

Let’s suppose I forget to enumerate through Spades:

switch(suit) {
  case Diamonds:
    printf("Diamonds\n");
    break;
  case Hearts:
    printf("Hearts\n");
    break;
  case Clubs:
    printf("Clubs\n");
    break;
}

Then I’ll get the following warning:

$ LC_MESSAGES=C gcc -Wswitch /tmp/test.c
/tmp/test.c: In function ‘main’:
/tmp/test.c:12:3: warning: enumeration value ‘Spades’ not handled in switch [-Wswitch]
   12 |   switch(suit) {
      |   ^~~~~~

Note: LC_MESSAGES=C is just to instruct GCC to default to traditional C English language behavior since my system is in French

Based on GCC Documentation on Warning Options:

-Wswitch
  Warn whenever a switch statement has an index of enumerated type and lacks a 
  case for one or more of the named codes of that enumeration. 
  (The presence of a default label prevents this warning.) 
  case labels outside the enumeration range also provoke warnings when this 
  option is used. This warning is enabled by -Wall.

Based on the documentation, we should no longer see the warning anymore if we add a default label:

switch(suit) {
    case Diamonds:
      printf("Diamonds\n");
      break;
    case Hearts:
      printf("Hearts\n");
      break;
    case Clubs:
      printf("Clubs\n");
      break;
    default:
}

And as expected, we see no warnings:

$ LC_MESSAGES=C gcc  -Wswitch /tmp/test.c
$

However, I notice a similar warning option in the documentation which will catch this misbehavior even with the default label:

-Wswitch-enum
    Warn whenever a switch statement has an index of enumerated type and lacks 
    a case for one or more of the named codes of that enumeration. case labels 
    outside the enumeration range also provoke warnings when this option is used.

So regardless if we have a default label or not:

$ LC_MESSAGES=C gcc  -Wswitch-enum /tmp/test.c
/tmp/test.c: In function ‘main’:
/tmp/test.c:12:3: warning: enumeration value ‘Spades’ not handled in switch [-Wswitch-enum]
   12 |   switch(suit) {
      |   ^~~~~~

On a side note, -Wall will not catch this misbehavior if a default is present:

$ LC_MESSAGES=C gcc  -Wall /tmp/test.c
$

This is because -Wall enables most warnings but not all warnings. Based on the documentation, we see that -Wall enables -Wswitch instead of Wswitch-enum.

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