Class destructors - clarify what is safe

[Ali Çehreli]

On 2/14/26 4:46 PM, H. S. Teoh wrote:
> On Sat, Feb 14, 2026 at 10:03:17PM +0000, Paul Backus via Digitalmars-d-learn wrote:
>> On Saturday, 14 February 2026 at 18:42:21 UTC, H. S. Teoh wrote:
>>>> As we can explicitly call destroy() on a class instance multiple
>>>> times, do we need to be "careful" of only closing a file once,
>>>> rather than closing a file multiple times, if that can cause
>>>> issues.
>>>
>>> Calling .destroy on a class object more than once may trigger UB.
>>
>> This is technically true, in the sense that calling any function could
>> conceivably trigger UB, but destructors *should* be idempotent, and
>> any destructor marked as @safe *must* be idempotent.
> 
> True.  So any dtor that closes a file should set its handle to a null
> value so that any subsequent calls will be a no-op.
> 
> 
> --T

The way I've learned to see it for decades now, initially from the point 
of view of C++, calling the destructor more than once should be a mistake.

The reason is, when the destructor completes, the object is considered 
to be dead. Calling any non-static function on it should be considered a 
programming error.

I know structs are different because they are set to their initial state 
after certain operations (e.g. I think destroy()) but in that case the 
object is still an object, just morphed to its .init value.

What do others think?

Ali