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

[David Rajchenbach-Teller <David.Teller@univ-orleans.fr>]

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.