D's Destructors are What Scott Meyers Warned Us About

[Steven Schveighoffer]

On 5/24/18 5:32 AM, Steven Schveighoffer wrote:
> On 5/23/18 7:11 PM, sarn wrote:

>> Here's an example of what I'm talking about:
>> https://github.com/dlang/phobos/blob/master/std/signals.d#L230
>>
>> It's running __dtor manually just to run some code that happens to be 
>> in the destructor.  It's obviously not meant to run any other 
>> destructors (at least, the documentation doesn't say "Use this mixin 
>> in your object and then calling disconnectAll() will destroy 
>> everything in your object.").
> 
> This is a bug. That module is not well-used and has not received a lot 
> of attention, if you look at the development history, almost all changes 
> are global style changes.
> 
>>
>> It's a broken design pattern, but existing code is doing it.  (As I 
>> said, I reviewed a lot of D packages, and I don't have time to file 
>> bug reports or PRs for each one.)
> 
> Understood. I'll file one for you on this one. It's one thing to have 
> some random package using an incorrect pattern, it's another thing to 
> have Phobos doing it.

Wow, so I learned a couple things reading that code.

1. Yes you are right, it's only meant to call the local mixin 
destructor, not any other destructors (it doesn't even call the 
destructor of the class it's mixed into). This "valid" use of __dtor has 
super-huge code smell.
2. I didn't realize that you could mixin extra pieces to a destructor!

I added the bug report here: 
https://issues.dlang.org/show_bug.cgi?id=18903. I outline exactly how to 
fix it, if anyone wants an easy PR opportunity.

-Steve