From: thiago.adams@gmail.com
On 10/23/2025 12:06 PM, David Brown wrote:
> On 23/10/2025 13:03, Thiago Adams wrote:
>> On 10/23/2025 4:12 AM, David Brown wrote:
>>> On 22/10/2025 20:22, Kaz Kylheku wrote:
>>>> 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.
>>>>
>>>
>>> The gcc approach works fine in almost all situations - use "-
>>> Wswitch=error", and add a default case if your switch is not meant to
>>> handle all enumeration values. If the default should do nothing,
>>> it's just "default: // Not all cases need handling". If the default
>>> should never happen, "default: __builtin_unreachable();" or "default:
>>> __builtin_trap();" might be appropriate.
>>>
>>>
>>>
>>
>> But then instead a compiler time error (like I suggest) you leave it
>> for runtime.
>>
>>
>
> As I said - use "-Wswitch=error". That gives you a compile-time error -
> not merely a warning, as you had suggested. But /if/ your switch is not
> meant to handle all cases, which was what Kaz was complaining about,
> then you add a default case. I agree with you that a compile-time error
> is best when possible, which is why that was my suggestion.
>
>
>
I think my sample is not covered by any GCC flag
I will copy past it again:
--------
The problem with this GCC approach is when there are many enumerators
but only a few are used.
For instance :
enum E {A, B, C /*, ...*/, Z};
1)
void f(enum E e)
{
switch (e)
{
//used
case A:
case B:
break;
//NON USED
case C:
...
case Z:
break;
};
}
The problem with (1) is when we have too many
non used enumerators, it is impractical to have a lot of switch cases.
One alternative is to use default for all the non used:
2)
void f(enum E e)
{
switch (e)
{
//used
case A:
case B:
break;
//NON USED (all others)
default:
break;
};
}
The problem with (2) is when we add a new enumerator and
this new enumerations should be used, but there is no warning and it
goes accidentally for default.
Solution?
In C2Y the new keyword _Countof was introduced.
It works returns the number of elements of array. IT IS FOR ARRAY ONLY.
I did an EXTENSION in my compiler where _Countof(enum E) also returns
the number of enumerators.
enum E2 {A, B};
static_assert(_Countof(enum E2) == 2);
(It also could be a new keyword.
static_assert(_EnumCount(enum E2) == 2);)
Having this we can do:
3)
void f(enum E e)
{
switch (e)
{
//used
case A:
case B:
break;
default:
static_assert(_EnumCount(enum E2) == 20);
break;
};
}
Then when adding a new enumerator the programmer will have to review
this code and update to 21 if it is not used, or handle it in a new case.
This is also useful in other scenarios. For instance:
enum E parse_enum_e(const char* s)
{
if (strcmp(s, "A") == 0) return A;
if (strcmp(s, "B") == 0) return B;
if (strcmp(s, "C") == 0) return C;
if (strcmp(s, "D") == 0) return D;
if (strcmp(s, "E") == 0) return E;
if (strcmp(s, "F") == 0) return F;
static_assert(_Countof(enum E) == 6);
return A;
}
If a new enumerator is added we need to include it.
--------
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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