[default] Should switch statements always contain a default clause?

In one of my first code reviews (a while back), I was told that it's good practice to include a default clause in all switch statements. I recently remembered this advice but can't remember what the justification was. It sounds fairly odd to me now.

1. Is there a sensible reason for always including a default statement?

2. Is this language dependent? I don't remember what language I was using at the time - maybe this applies to some languages and not to others?

If there is no default case in a switch statement, the behavior can be unpredictable if that case arises at some point of time, which was not predictable at development stage. It is a good practice to include a default case.

switch ( x ){
  case 0 : { - - - -}
  case 1 : { - - - -}
}

/* What happens if case 2 arises and there is a pointer
* initialization to be made in the cases . In such a case ,
* we can end up with a NULL dereference */

Such a practice can result in a bug like NULL dereference, memory leak as well as other types of serious bugs.

For example we assume that each condition initializes a pointer. But if default case is supposed to arise and if we don’t initialize in this case, then there is every possibility of landing up with a null pointer exception. Hence it is suggested to use a default case statement, even though it may be trivial.

Should switch statements always contain a default clause ? No switch cases can exist with out default case, in switch case default case will trigger switch value switch(x) in this case x when not match with any other case values.

I would say it depends on the language, but in C if you're switching on a enum type and you handle every possible value, you're probably better off NOT including a default case. That way, if you add an additional enum tag later and forget to add it to the switch, a competent compiler will give you a warning about the missing case.

If the switch value (switch(variable)) can't reach the default case, then default case is not at all needed. Even if we keep the default case, it is not at all executed. It is dead code.

Should a "switch" statement always include a default clause? No. It should usually include a default.

Including a default clause only makes sense if there's something for it to do, such as assert an error condition or provide a default behavior. Including one "just because" is cargo-cult programming and provides no value. It's the "switch" equivalent of saying that all "if" statements should include an "else".

Here's a trivial example of where it makes no sense:

void PrintSign(int i)
{
    switch (Math.Sign(i))
    {
    case 1:
        Console.Write("positive ");
        break;
    case -1:
        Console.Write("negative ");
        break;
    default: // useless
    }
    Console.Write("integer");
}

This is the equivalent of:

void PrintSign(int i)
{
    int sgn = Math.Sign(i);
    if (sgn == 1)
        Console.Write("positive ");
    else if (sgn == -1)
        Console.Write("negative ");
    else // also useless
    {
    }
    Console.Write("integer");
}

NOT having the default case can actually be beneficial in some situations.

If your switch cases are enums values, by not having a default case, you can get a compiler warning if you are missing any cases. That way, if new enum values are added in the future and you forget to add cases for these values in the switch, you can find out about the problem at compile time. You should still make sure the code takes appropriate action for unhandled values, in case an invalid value was cast to the enum type. So this may work best for simple cases where you can return within the enum case rather than break.

enum SomeEnum
{
    ENUM_1,
    ENUM_2,
    // More ENUM values may be added in future
};

int foo(SomeEnum value)
{
    switch (value)
    {
    case ENUM_1:
        return 1;
    case ENUM_2:
        return 2;
    }
    // handle invalid values here
    return 0;
 }

I disagree with the most voted answer of Vanwaril above.

Any code adds complexity. Also tests and documentation must be done for it. So it is always good if you can program using less code. My opinion is that I use a default clause for non-exhaustive switch statements while I use no default clause for exhaustive switch statements. To be sure that I did that right I use a static code analysis tool. So let's go into the details:

1. Nonexhaustive switch statements: Those should always have a default value. As the name suggests those are statements which do not cover all possible values. This also might not be possible, e.g. a switch statement on an integer value or on a String. Here I would like to use the example of Vanwaril (It should be mentioned that I think he used this example to make a wrong suggestion. I use it here to state the opposite --> Use a default statement):

   switch(keystroke)
   {
     case 'w':
       // move up
     case 'a':
       // move left
     case 's':
       // move down
     case 'd':
       // move right
     default:          
       // cover all other values of the non-exhaustive switch statement
   }
   

   The player could press any other key. Then we could not do anything (this can be shown in the code just by adding a comment to the default case) or it should for example print something on the screen. This case is relevant as it may happen.

2. Exhaustive switch statements: Those switch statements cover all possible values, e.g. a switch statement on an enumeration of grade system types. When developing code the first time it is easy to cover all values. However, as we are humans there is a small chance to forget some. Additionally if you add an enum value later such that all switch statements have to be adapted to make them exhaustive again opens the path to error hell. The simple solution is a static code analysis tool. The tool should check all switch statements and check if they are exhaustive or if they have a default value. Here an example for an exhaustive switch statement. First we need an enum:

   public enum GradeSystemType {System1To6, SystemAToD, System0To100}
   

   Then we need a variable of this enum like GradeSystemType type = .... An exhaustive switch statement would then look like this:

   switch(type)
   {
     case GradeSystemType.System1To6:
       // do something
     case GradeSystemType.SystemAToD:
       // do something
     case GradeSystemType.System0To100:
       // do something
   }
   

   So if we extend the GradeSystemType by for example System1To3 the static code analysis tool should detect that there is no default clause and the switch statement is not exhaustive so we are save.

Just one additional thing. If we always use a default clause it might happen that the static code analysis tool is not capable of detecting exhaustive or non-exhaustive switch statements as it always detects the default clause. This is super bad as we will not be informed if we extend the enum by another value and forget to add it to one switch statement.

No.

What if there is no default action, context matters. What if you only care to act on a few values?

Take the example of reading keypresses for a game

switch(a)
{
   case 'w':
     // Move Up
     break;
   case 's':
     // Move Down
     break;
   case 'a':
     // Move Left
     break;
   case 'd':
     // Move Right
     break;
}

Adding:

default: // Do nothing

Is just a waste of time and increases the complexity of the code for no reason.

You should have a default to catch un-expected values coming in.

However, I disagree with the Adrian Smith that your error message for default should be something totally meaningless. There may be an un-handled case you didn't forsee (which is kind of the point) that your user will end up seeing and a message like "unreachable" is entirely pointless and doesn't help anyone in that situation.

Case in point, how many times have you had an utterly meaningless BSOD? Or a fatal exception @ 0x352FBB3C32342?

As far as i see it the answer is 'default' is optional, saying a switch must always contain a default is like saying every 'if-elseif' must contain a 'else'. If there is a logic to be done by default, then the 'default' statement should be there, but otherwise the code could continue executing without doing anything.

The default case may not necessary in the switch used by enum. when switch contained all value, the default case will never execute. So in this case, it is not necessary.

If you know that the switch statement will only ever have a strict defined set of labels or values, just do this to cover the bases, that way you will always get valid outcome.. Just put the default over the label that would programmatically/logically be the best handler for other values.

switch(ResponseValue)
{
    default:
    case No:
        return false;
    case Yes;
        return true;
}

I believe this is quite language specific and for the C++ case a minor point for enum class type. Which appears more safe than traditional C enum. BUT

If you look at the implementation of std::byte its something like:

enum class byte : unsigned char {} ;

Source: https://en.cppreference.com/w/cpp/language/enum

And also consider this:

Otherwise, if T is a enumeration type that is either scoped or unscoped with fixed underlying type, and if the braced-init-list has only one initializer, and if the conversion from the initializer to the underlying type is non-narrowing, and if the initialization is direct-list-initialization, then the enumeration is initialized with the result of converting the initializer to its underlying type.

(since C++17)

Source: https://en.cppreference.com/w/cpp/language/list_initialization

This is an example of enum class representing values that are not defined enumerator. For this reason you cannot place complete trust in enums. Depending on application this might be important.

However, I really like what @Harlan Kassler said in his post and will start using that strategy in some situations myself.

Just an example of unsafe enum class:

enum class Numbers : unsigned
{
    One = 1u,
    Two = 2u
};

int main()
{
    Numbers zero{ 0u };
    return 0;
}

It is an optional coding 'convention'. Depending on the use is whether or not it is needed. I personally believe that if you do not need it it shouldn't be there. Why include something that won't be used or reached by the user?

If the case possibilities are limited (i.e. a Boolean) then the default clause is redundant!

Depends on how the switch in particular language works, however in most languages when no case is matched, the execution falls through the switch statement without warning. Imagine you expected some set of values and handled them in switch, however you get another value in the input. Nothing happens and you don't know nothing happened. If you caught the case in default, you would know there was something wrong.

Atleast it is not mandatory in Java. According to JLS, it says atmost one default case can be present. Which means no default case is acceptable . It at times also depends on the context that you are using the switch statement. For example in Java, the following switch block does not require default case

private static void switch1(String name) {
    switch (name) {
    case "Monday":
        System.out.println("Monday");
        break;
    case "Tuesday":
        System.out.println("Tuesday");
        break;
    }
}

But in the following method which expects to return a String, default case comes handy to avoid compilation errors

    private static String switch2(String name) {
    switch (name) {
    case "Monday":
        System.out.println("Monday");
        return name;

    case "Tuesday":
        System.out.println("Tuesday");
        return name;

    default:
        return name;
    }
}

though you can avoid compilation error for the above method without having default case by just having a return statement at the end, but providing default case makes it more readable.

I would always use a default clause, no matter what language you are working in.

Things can and do go wrong. Values will not be what you expect, and so on.

Not wanting to include a default clause implies you are confident that you know the set of possible values. If you believe you know the set of possible values then, if the value is outside this set of possible values, you'd want to be informed of it - it's certainly an error.

That's the reason why you should always use a default clause and throw an error, for example in Java:

switch (myVar) {
   case 1: ......; break;
   case 2: ......; break;
   default: throw new RuntimeException("unreachable");
}

There's no reason to include more information than just the "unreachable" string; if it actually happens, you're going to need to look at the source and the values of the variables etc anyway, and the exception stacktrace will include that line number, so no need to waste your time writing more text into the exception message.

In my company, we write software for the Avionics and Defense market, and we always include a default statement, because ALL cases in a switch statement must be explicitly handled (even if it is just a comment saying 'Do nothing'). We cannot afford the software just to misbehave or simply crash on unexpected (or even what we think impossible) values.

It can be discussed that a default case is not always necessary, but by always requiring it, it is easily checked by our code analyzers.

Having a default clause when it's not really needed is Defensive programming This usually leads to code that is overly complex because of too much error handling code. This error handling and detection code harms the readability of the code, makes maintenance harder, and eventually leads to more bugs than it solves.

So I believe that if the default shouldn't be reached - you don't have to add it.

Note that "shouldn't be reached" means that if it reached it's a bug in the software - you do need to test values that may contain unwanted values because of user input, etc.

Some (outdated) guidelines say so, such as MISRA C:

The requirement for a final default clause is defensive programming. This clause shall either take appropriate action or contain a suitable comment as to why no action is taken.

That advice is outdated because it is not based on currently relevant criteria. The glaring omission being what Harlan Kassler said:

Leaving out the default case enables the compiler to optionally warn or fail when it sees an unhandled case. Static verifiability is after all better than any dynamic check, and therefore not a worthy sacrifice for when you need the dynamic check as well.

As Harlan also demonstrated, the functional equivalent of a default case can be recreated after the switch. Which is trivial when each case is an early return.

The typical need for a dynamic check is input handling, in a wide sense. If a value comes from outside the program's control, it can't be trusted.

This is also where Misra takes the standpoint of extreme defensive programming, whereby as long as an invalid value is physically representable, it must be checked for, no matter if the program is provably correct. Which makes sense if the software needs to be as reliable as possible in the presence of hardware errors. But as Ophir Yoktan said, most software are better off not "handling" bugs. The latter practice is sometimes called offensive programming.