From: david.brown@hesbynett.no
On 22/10/2025 21:41, Keith Thompson wrote:
> David Brown writes:
>> On 22/10/2025 13:44, Richard Harnden wrote:
>>> On 22/10/2025 10:32, Janis Papanagnou wrote:
>>>> On 22.10.2025 10:56, pozz wrote:
>>>>>> Switch statements without a default case can lead to unexpected
>>>>>> behavior and incomplete handling of all possible cases. When a switch
>>>>>> statement lacks a default case, if a value is encountered that does
>>>>>> not match any of the specified cases, the program will continue
>>>>>> execution without any defined behavior or handling.
>>>>>
>>>>> Maybe I misunderstood that sentence caused by my bad English. I knew
>>>>> that in case the switch value is not present in any case inside the
>>>>> switch, the program continues without doing anything (in the switch) and
>>>>> without any problem.
>>>>>
>>>>> int x = 3;
>>>>> switch(x) {
>>>>> case 1: printf("Hello");break;
>>>>> case 2: printf("World");break;
>>>>> }
>>>>>
>>>>> Will the program execution continue without any defined behaviour?
>>
>> Presumably you meant "without any undefined behaviour" ? The code is
>> fine - if no cases match and there is no default case, execution
>> continues from the end of the switch statement. Like most warnings,
>> this is about a possible bug in the code - not a definite one.
>
> No, pozz meant "without any defined behavior", which is a direct quote
> from the cited document.
>
OK.
> That document is poorly written. The phrase "without any defined
> behavior" strongly implies that the behavior is undefined, which is
> simply wrong.
I guess I owe Pozz an apology for thinking he had made a little typo,
when in fact it was a clear error from the clang-tidy folk! Thanks for
clearing that up.
>
> It says:
>
> When a switch statement lacks a default case, if a value is
> encountered that does not match any of the specified cases, the
> program will continue execution without any defined behavior or
> handling.
>
> It would be more accurate to say:
>
> When a switch statement lacks a default case, if a value is
> encountered that does not match any of the specified cases, the
> switch statement will do nothing and the program will continue
> execution without handling the value.
>
> A warning might be warranted, but the behavior is well defined.
>
> Note that the documentation is for the add-on tool clang-tidy, not for
> the clang compiler.
>
> I've submitted a bug report :
>
> https://github.com/llvm/llvm-project/issues/164699
>
It looks like your patch has been accepted, as far as I could tell.
Very helpful!
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