In C, where the origin seems to be, the code block of the switch statement is not a special construct. It is a normal block of code, just as a block under an if statement.
switch ()
{
}
if ()
{
}
case and default are jump labels inside this block, specifically related to switch. They are handled just as normal jump labels for goto. There is one specific rule that is important here: Jump labels can be nearly everywhere in the code, without interrupting the code flow.
As a normal code block, it doesn't need to be a compound statement. The labels are optional, too. These are valid switch statements in C:
switch (a)
case 1: Foo();
switch (a)
Foo();
switch (a)
{
Foo();
}
The C standard itself gives this as an example (6.8.4.2):
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.
Furthermore, default is a jump label, too, and thus can be anywhere, without the need to be the last case.
This also explains Duff's Device:
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 > 0);
}
Why the fall-through? Because in the normal code flow in a normal block of code, fall-through to the next statement is expected, just as you would expect it in an if code block.
if (a == b)
{
Foo();
/* "Fall-through" to Bar expected here. */
Bar();
}
switch (a)
{
case 1:
Foo();
/* Automatic break would violate expected code execution semantics. */
case 2:
Bar();
}
My guess is that the reason for this was ease of implementation. You don't need special code for parsing and compiling a switch block, caring for special rules. You just parse it like any other code and only have to care for the labels and the jump selection.
An interesting follow-up question from all this is if the following nested statements print "Done." or not.
int a = 10;
switch (a)
{
switch (a)
{
case 10: printf("Done.\n");
}
}
The C standard cares for this (6.8.4.2.4):
A case or default label is accessible only within the closest enclosing switch statement.
CASEstatement equivalently to a giant if/elseif block.case 'a': case 'A': case 'b': case 'B'but mostly because I cannot docase in [ 'a', 'A', 'b', 'B' ]. A slightly better question is, in my current preferred language (C#), the break is mandatory, and there is no implicit fall-though; forgettingbreakis a syntax error... :\case TOKEN_A: /*set flag*/; case TOKEN_B: /*consume token*/; break; case TOKEN_C: /*...*/breakis present anywhere” is a significantly simpler rule to implement than “Don’t emit a jump iffallthroughis present in aswitch”.