Re: [Caml-list] Scoped Bound Resource Management just for C++?

[orbitz@ezabel.com]

Do they appear in C++?  I would a dtor takes care of that for you when  
it comes to streams.


On Feb 9, 2011, at 11:52 AM, David Rajchenbach-Teller wrote:

> Actually, in Batteries, we have to go through a number of hoops to  
> be absolutely certain that a stream is flushed before we quit the  
> application.
> We have to interact with:
> - users manually closing the stream;
> - any of the downstream streams being closed for any reason;
> - finalization;
> - the programmer exiting the program with [exit];
> - the user exiting the program with [Ctrl-C];
> - ...
>
> Of course, the same issues appear in C++.
>
> Cheers,
> David
>
> On Feb 9, 2011, at 5:14 PM, Gerd Stolpmann wrote:
>
>> Am Mittwoch, den 09.02.2011, 10:15 -0500 schrieb orbitz@ezabel.com:
>>> Thanks for the answers everyone.
>>>
>>> How does one safely write code in Ocaml that guarantees resources  
>>> will
>>> be freed?  Guillaume mentioned the with-idiom, but even that doesn't
>>> seem entirely safe.
>>
>> You mean C++ is safer in this respect?
>>
>> Come on. Fully automatic memory management as in Ocaml is certainly
>> safer than any semi-automatic scheme. It will find all memory blocks
>> that are not referenced anymore. It's guaranteed. It works even with
>> circular structures (this is not a boy GC).
>>
>> You would use "with" only for cases where non-memory resources are
>> referenced, like file descriptors. And you have to close files in C+ 
>> +,
>> too. If you want to be very careful here, you can even set a  
>> finaliser
>> that emits a warning when you forgot to close a descriptor (but you  
>> have
>> then to remember whether you closed it), like in
>>
>> type managed_fd =
>> { fd : Unix.file_descr;
>>   mutable fd_closed : bool
>> }
>>
>> (* after opening the file: *)
>> let mfd = { fd=fd; fd_closed=false }
>>
>> (* Attach the finaliser: *)
>> let mfd_fin mfd =
>> if not mfd.fd_closed then
>>   prerr_endline "Hey, there is a forgotten file descriptor"
>> Gc.finalise mfd_fin mfd
>>
>> (* Use mfd as in - ensure you always pass mfd around: *)
>> Unix.read mfd.fd ...
>>
>> (* When you close: *)
>> Unix.close mfd.fd;
>> mfd.fd_closed <- true
>>
>> I wouldn't recommend to close fd in mfd_fin, because fd might not  
>> be a
>> simple file, and you can trigger any kind of external activity by
>> closing it.
>>
>> I've written a number of 24/7 server programs in Ocaml now, and I can
>> tell you, resource management is easy. You can usually skip the  
>> "search
>> for memory leaks" step before deploying to production.
>