From: 643-408-1753@kylheku.com
On 2025-10-22, Thiago Adams wrote:
> On 10/22/2025 8:44 AM, Richard Harnden wrote:
> ....
>>> Your program fragment is well defined.
>>>
>>> What the poster certainly tried to express was that in case you
>>> haven't implemented a complete list of all possible cases and
>>> also not provided a 'default' to catch all non-specified cases,
>>> then you might get in troubles with your program, probably by
>>> possible oversights, future extensions, new data, and whatnot.
>>>
>>> Personally I have the habit to always define a default branch,
>>> and even if that default is impossible to reach you'll find an
>>> error message (like "internal error with unexpected value...")
>>> generated at that place.
>>>
>> Use an enum, and the compiler will warn you ...
>>
>> $ cat x.c
>> #include
>>
>> enum x {A, B, C};
>>
>> int main(void)
>> {
>> enum x x = C;
>>
>> switch (x)
>> {
>> case A:
>> printf("A\n");
>> break;
>>
>> case B:
>> printf("B\n");
>> break;
>> }
>>
>> return 0;
>> }
>>
>> $ gcc -Wall x.c
>> x.c: In function ‘main’:
>> x.c:9:9: warning: enumeration value ‘C’ not handled in switch [-Wswitch]
>> 9 | switch (x)
>> | ^~~~~~
>>
>>
>
> The problem with this GCC approach is when there are many enumerators
> but only a few are used.
The problem with the C and GCC approach is that there is no
one-size-fits all solution.
Some switches are intended to be exhaustive, such that
missing a case is a bug.
Some are not.
You need an "eswitch" for the exhaustively handled enumerations, and
switch for the others.
GCC can turn on diagnostics over ranges of a file with pragma
and there is also _Pragram, but it's all too clumsy.
In Common Lisp we have case vs ecase:
[1]> (case 3 (0 "zero") (1 "one") (2 "two"))
NIL
[2]> (case 0 (0 "zero") (1 "one") (2 "two"))
"zero"
[3]> (ecase 3 (0 "zero") (1 "one") (2 "two"))
*** - The value of 3 must be one of 0, 1, 2
The value is: 3
If falling through without hitting a case would be wrong, you use ecase;
problem solved.
--
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