[Dlang-study] [lifetime] Few root decisions to take on RC classes

Andrei Alexandrescu andrei at erdani.com
Thu Jan 14 11:39:52 PST 2016

Happy New Year in 2016 and beyond to everyone following this list! Well 
we hope to get ownership done this year :o).

I was reviewing older exchanges, in particular the one quoted below 
regarding exceptions providing messages as lazy/transferrable sequences 
of characters.

Droppable is definitely interesting, but let me make a simple point: a 
low-tech reference counted string would fit the bill perfectly well. 

* Want lazy computation? Compute the string upon the first call to 
msg(), then memoize it.

* Want static allocation? Provide a means to encode that in the 
ref-counted string, e.g. by setting the reference count to uint.max etc.

I understand it's old tech that's not exciting, but (a) we need to 
figure out safe reference counted types anyway and (b) reference counted 
strings are well understood and very successful across the industry.

For fixing the issues with reference counting and safety, the silence in 
this group as of late suggests there are no new ideas and new work on 
this. So I suggest we proceed by refining http://wiki.dlang.org/DIP77.



On 11/06/2015 07:45 AM, Marc Sch├╝tz wrote:
> On Friday, 6 November 2015 at 02:44:39 UTC, Michel Fortin wrote:
>> But how do you prevent the payload from being leaked in a way that it
>> outlives the droppable? For instance, someone might want to log the
>> exception message and put it into a queue for a separate logging
>> thread to process. You're giving raw access to the string, so it's
>> very easy mess things up if you are not careful.
> Well, that requires borrowing/sealed references:
> struct Droppable(T) {
>      private T payload;
>      void delegate(ref T object) dropper;
>      ref T borrow() return { return payload; }
>      alias borrow this;
>      @disabled this(this);
>      ~this() {
>          if(dropper) dropper(payload);
>      }
> }
> But that's a separate topic. My point was that this is a mechanism to
> abstract away from specific ownership schemes without making the
> ownership part of the type (i.e. `Exception` can be used as-is no matter
> whether the message if garbage collected or refcounted or static).
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