Switch statement must default be the last case

Question

Consider the following switch statement   switch  value       case 1      return 1    default      value           fall-through   case 2      return value   2      This code compiles  but is it valid    defined behavior  for C90 C99  I have never seen code where the default case is not the last case   EDIT  As Jon Cage and KillianDS write  this is really ugly and confusing code and I am well aware of it  I am just interested in the general syntax  is it defined   and the expected output

User · Accepted Answer

The C99 standard is not explicit about this, but taking all facts together, it is perfectly valid.

A case and default label are equivalent to a goto label. See 6.8.1 Labeled statements. Especially interesting is 6.8.1.4, which enables the already mentioned Duff's Device:

Any statement may be preceded by a prefix that declares an identifier as a label name. Labels in themselves do not alter the flow of control, which continues unimpeded across them.

Edit: The code within a switch is nothing special; it is a normal block of code as in an if-statement, with additional jump labels. This explains the fall-through behaviour and why break is necessary.

6.8.4.2.7 even gives an example:

switch (expr) 
{ 
    int i = 4; 
    f(i); 
case 0: 
    i=17; 
    /*falls through into default code */ 
default: 
    printf("%d\n", i); 
}

In the artificial program fragment the object whose identifier is i exists with automatic storage duration (within the block) but is never initialized, and thus if the controlling expression has a nonzero value, the call to the printf function will access an indeterminate value. Similarly, the call to the function f cannot be reached.

The case constants must be unique within a switch statement:

6.8.4.2.3 The expression of each case label shall be an integer constant expression and no two of the case constant expressions in the same switch statement shall have the same value after conversion. There may be at most one default label in a switch statement.

All cases are evaluated, then it jumps to the default label, if given:

6.8.4.2.5 The integer promotions are performed on the controlling expression. The constant expression in each case label is converted to the promoted type of the controlling expression. If a converted value matches that of the promoted controlling expression, control jumps to the statement following the matched case label. Otherwise, if there is a default label, control jumps to the labeled statement. If no converted case constant expression matches and there is no default label, no part of the switch body is executed.

User · Answer

It s valid and very useful in some cases   Consider the following code   switch poll fds  1  1000000       default         here goes the normal case   some events occured    break     case 0         here goes the timeout case    break     case -1          some error occurred  you have to check errno     The point is that the above code is more readable and efficient than cascaded if  You could put default at the end  but it is pointless as it will focus your attention on error cases instead of normal cases  which here is the default case    Actually  it s not such a good example  in poll you know how many events may occur at most  My real point is that there are cases with a defined set of input values where there are  exceptions  and normal cases  If it s better to put exceptions or normal cases at front is a matter of choice   In software field I think of another very usual case  recursions with some terminal values  If you can express it using a switch  default will be the usual value that contains recursive call and distinguished elements  individual cases  the terminal values  There is usually no need to focus on terminal values   Another reason is that the order of the cases may change the compiled code behavior  and that matters for performances  Most compilers will generate compiled assembly code in the same order as the code appears in the switch  That makes the first case very different from the others  all cases except the first one will involve a jump and that will empty processor pipelines  You may understand it like branch predictor defaulting to running the first appearing case in the switch  If a case if much more common that the others then you have very good reasons to put it as the first case   Reading comments it s the specific reason why the original poster asked that question after reading Intel compiler Branch Loop reorganisation about code optimisation   Then it will become some arbitration between code readability and code performance  Probably better to put a comment to explain to future reader why a case appears first

User · Answer

The case statements and the default statement can occur in any order in the switch statement  The default clause is an optional clause that is matched if none of the constants in the case statements can be matched    Good Example  -  switch 5      case 1      echo  1       break    case 2    default      echo  2  default       break    case 3      echo  3       break      Outputs  2 default    very useful if you want your cases to be presented in a logical order in the code  as in  not saying case 1  case 3  case 2 default  and your cases are very long so you do not want to repeat the entire case code at the bottom for the default

User · Answer

It s valid  but rather nasty  I would suggest it s generally bad to allow fall-throughs as it can lead to some very messy spaghetti code   It s almost certainly better to break these cases up into several switch statements or smaller functions    edit   Tristopia  Your example   Example from UCS-2 to UTF-8 conversion   r is the destination array   wc is the input wchar t    switch utf8 length             Note  code falls through cases          case 3  r 2    0x80    wc  amp  0x3f   wc  gt  gt   6  wc    0x800       case 2  r 1    0x80    wc  amp  0x3f   wc  gt  gt   6  wc    0x0c0       case 1  r 0    wc      would be clearer as to it s intention  I think  if it were written like this   if  utf8 length  gt   1         r 0    wc       if  utf8 length  gt   2                 r 1    0x80    wc  amp  0x3f   wc  gt  gt   6  wc    0x0c0            if  utf8 length    3                         r 2    0x80    wc  amp  0x3f   wc  gt  gt   6  wc    0x800                           edit2   Tristopia  Your second example is probably the cleanest example of a good use for follow-through   for i 0  s i   i          switch s i             case           case            case                d dlen                        fall through         default           d dlen      s i                 but personally I would split the comment recognition into it s own function   bool isComment char charInQuestion           bool charIsComment   false      switch charInQuestion            case           case            case                charIsComment   true       default           charIsComment   false              return charIsComment     for i 0  s i   i          if  isComment s i                   d dlen                       d dlen      s i

User · Answer

yes  this is valid  and under some circumstances it is even useful  Generally  if you don t need it  don t do it

User · Answer

There s no defined order in a switch statement  You may look at the cases as something like a named label  like a goto label  Contrary to what people seem to think here  in the case of value 2 the default label is not jumped to  To illustrate with a classical example  here is Duff s device  which is the poster child of the extremes of switch case in C   send to  from  count  register short  to   from  register count      register n  count 7  8    switch count 8       case 0  do   to    from        case 7       to    from        case 6       to    from        case 5       to    from        case 4       to    from        case 3       to    from        case 2       to    from        case 1       to    from                 while --n gt 0

User · Answer

The default condition can be anyplace within the switch that a case clause can exist   It is not required to be the last clause   I have seen code that put the default as the first clause   The case 2  gets executed normally  even though the default clause is above it   As a test  I put the sample code in a function  called test int value    and ran     printf  0  d n   test 0      printf  1  d n   test 1      printf  2  d n   test 2      printf  3  d n   test 3      printf  4  d n   test 4      The output is   0 2 1 1 2 4 3 8 4 10

User · Answer

There are cases when you are converting ENUM to a string or converting string to enum in case where you are writing reading to from a file   You sometimes need to make one of the values default to cover errors made by manually editing files   switch textureMode    case ModeTiled  default         write to a file  tiled      break   case ModeStretched         write to a file  stretched      break

User · Answer

Chiming in with another example  This can be useful if  default  is an unexpected case  and you want to log the error but also do something sensible  Example from some of my own code     switch  style        default      MSPUB DEBUG MSG   Couldn t match dash style  using solid line  n       case SOLID      return Dash 0  RECT DOT     case DASH SYS          Dash ret shapeLineWidth  dotStyle       ret m dots push back Dot 1  3   shapeLineWidth        return ret           more cases follow

User · Answer

One scenario where I would consider it appropriate to have a  default  located somewhere other than the end of a case statement is in a state machine where an invalid state should reset the machine and proceed as though it were the initial state   For example    switch widget state      default      Fell off the rails--reset and continue        widget state   WIDGET START         Fall through      case WIDGET START              break    case WIDGET WHATEVER              break      an alternative arrangement  if an invalid state should not reset the machine but should be readily identifiable as an invalid state     switch widget state      case WIDGET IDLE      widget ready   0      widget hardware off        break    case WIDGET START              break    case WIDGET WHATEVER              break    default      widget state   WIDGET INVALID STATE         Fall through      case WIDGET INVALID STATE      widget ready   0      widget hardware off            do whatever else is necessary to establish a  safe  condition     Code elsewhere may then check for  widget state    WIDGET INVALID STATE  and provide whatever error-reporting or state-reset behavior seems appropriate   For example  the status-bar code could show an error icon  and the  start widget  menu option which is disabled in most non-idle states could be enabled for WIDGET INVALID STATE as well as WIDGET IDLE

[c] Switch statement: must default be the last case?

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