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Date: -- (:)
From: Jon Harrop <jon@f...>
Subject: Re: [Caml-list] Re: exception safety / RAII ?
On Thursday 10 March 2005 14:33, Stefan Monnier wrote:
> > However, provided you don't need to make any guarantees about when the
> > file is  closed during the running of the program, I still think that
> > implicitly closing a file (or deallocating an external resource) via the
> > GC is not  sloppy coding. Indeed, this facility can be very useful and
> > can eliminate an important class of run-time errors.
>
> Can you give examples where it's useful?  I don't consider "not needing to
> call `close' on line NNNN of my code" to be useful.

OpenGL display lists from earlier in this thread.

> Can you describe the "important class of runtime errors" that would be
> supposedly eliminated?

In that case, undefined rendering behaviour when a display list is executed 
after it has been incorrectly destroyed. This could also wreck the OpenGL 
state and break rendering for the rest of the frame or even for the rest of 
the program.

Unless you go to great lengths to catch such errors in all cases (which could 
require arbitrarily complicated tests, slowing down your program) such 
mistakes could make an OCaml program segfault with more fragile external 
resources.

> > In the case of files, yes.  More generally, this can be applied to all
> > sorts of external resources where that is not true.
>
> Might be: there's no way we can generalize.  I'm talking about files here.
> Finalizers are great, but they shouldn't be used for files.

Let me try another file-specific example then: Programs which save output at 
some point during their execution. From the users point of view, there is no 
"visible" difference between having the program close the file as soon as 
writing is complete and leaving it up to the GC to close the file a short 
time afterwards (and before it completes). I have many such programs.

> I'd also argue that they generally shouldn't be used for any "external
> resource".  Though I wouldn't be surprised if there's a counter example
> somewhere where I'd agree that finalizers are good solution for the
> management of some particular kind of external resource in some
> particular circumstance.

Even if the external resource is memory?

Generally, I'd always consider exploiting the GC because of the safety it 
provides...

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
Objective CAML for Scientists
http://www.ffconsultancy.com/products/ocaml_for_scientists