Should switch statements always contain a default clause

Question

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    Is there a sensible reason for always including a default statement  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

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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

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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

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Some  outdated  guidelines say so  such as MISRA C      The requirement for a final default clause is defensive programming  This clause shall either take appropriate action or contain a suitable comment as to why no action is taken    That advice is outdated because it is not based on currently relevant criteria  The glaring omission being what Harlan Kassler said   Leaving out the default case enables the compiler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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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Switch cases should almost always have a default case   Reasons to use a default  1 To  catch  an unexpected value  switch type        case 1            something     case 2            something else     default             unknown type  based on the language             there should probably be some error-handling            here  maybe an exception     2  To handle  default  actions  where the cases are for special behavior   You see this a LOT in menu-driven programs and bash shell scripts  You might also see this when a variable is declared outside the switch-case but not initialized  and each case initializes it to something different  Here the default needs to initialize it too so that down the line code that accesses the variable doesn t raise an error   3  To show someone reading your code that you ve covered that case   variable    variable     value     1   2  switch variable        case 1             something     case 2             something else     default             will NOT execute because of the line preceding the switch      This was an over-simplified example  but the point is that someone reading the code shouldn t wonder why variable cannot be something other than 1 or 2     The only case I can think of to NOT use default is when the switch is checking something where its rather obvious every other alternative can be happily ignored  switch keystroke        case  w              move up     case  a              move left     case  s              move down     case  d              move right        no default really required here

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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

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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    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  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

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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

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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

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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

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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

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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

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

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