auto classes and finalizers

[Regan Heath]

On Sun, 09 Apr 2006 18:34:47 -0700, kris <foo at bar.com> wrote:
> Mike Capp wrote:
>> In article <e1c6vl$moj$1 at digitaldaemon.com>, kris says...
>>
>>> I think we can safely put aside the entire-program-as-one-function as  
>>> unrealistic. Given that, and assuming the existance of a dtor implies  
>>> "auto" (and thus raii), how does one manage a "pool" of resources? For  
>>> example, how about a pool of DB connections? Let's assume that they  
>>> need to be correctly closed at some point, and that the pool is likely  
>>> to expand and contract based upon demand over time ...
>>>
>>> So the question is how do those connections, and the pool itself, jive  
>>> with scoped raii? Assuming it doesn't, then one would presumeably  
>>> revert to a manual dispose() pattern with such things?
>>   Two different classes. A ConnectionPool at application scope, e.g. in  
>> main(),
>> and a ConnectionUsage wherever you need one. Both are RAII.  
>> ConnectionPool acts
>> as a factory for ConnectionUsage instances (modulo language  
>> limitations) and
>> adds to the pool as needed; ConnectionUsage just "borrows" an instance  
>> from the
>> pool for the duration of its scope.
>>  cheers
>> Mike
>
> Thanks!
>
> So, when culling the pool (say, on a timeout basis) the cleanup-code for  
> the held resource is not held within the "borrowed" dtor, but in a  
> dispose() method? Otherwise, said dtor would imply raii for the borrowed  
> connection, which would be bogus behaviour for a class instance that is  
> being held onto by the pool? In other words: you'd want to avoid  
> deleting (via raii) the connection object, so you'd have to be careful  
> to not use a dtor in such a case (if we assume dtor means raii).

Unless you add a 'shared' keyword as I described in a previous post. eg.

auto class Connection { //auto required to have dtor
   HANDLE h;
   ~this() { CloseHandle(h); }
}

class ConnectionUsage {
   shared Connection c;
}

ConnectionUsage is not required to be 'auto' because it has no 'auto'  
class members which are not 'shared' resources. Alternately you implement  
reference counting for the Connection class, remove shared, and add 'auto'  
to ConnectionUsage.

Regan