[PATCH] PR14253: 'warn_unused' attribute for unused warning for non-POD types

[Lubos Lunak]

On Tuesday 16 of July 2013, Richard Smith wrote:
> On Tue, Jul 16, 2013 at 10:45 AM, David Blaikie <dblaikie at gmail.com> wrote:
> > I know we'd previously discussed the idea of being able to warn about
> > unused variables of non-trivial types - you'd gone so far as to
> > suggest the idea of a positive attribute (annotate the classes that
> > have side effects, rather than those that don't, since the former
> > seems like the exception rather than the rule).
>
> Right. We usually try to stay on the side of no-false-positives, but in
> this case diligent users would find themselves annotating nearly all
> classes, and that doesn't seem like a good tradeoff.

 I'd rather worry about those that won't be dilligent.

> Also, we identified 
> that there are at least three different kinds of class here:
>
>  1) Those where the ctor and/or the dtor has external, permanent
> side-effects (probably very rare)
>  2) Those where the ctor or dtor have external, temporary side-effects, but
> running one then the other is essentially equivalent to running neither
> (for instance, many RAII objects follow this pattern)
>  3) Those where the ctor and dtor have no external side-effects
>
> Consider:
>
> {
>   my_lock(my_mutex); // A
>   my_lock ml(my_mutex); // B
>   my_string("foo"); // C
>   my_string ms("foo"); // D
> }
>
> We never warn for a class that is in category (1). my_lock would be in
> category (2), so we warn on A but not B. my_string would be in category
> (3), so we warn on both C and D.
>
> Which of these does the warn_unused attribute model?

 It doesn't model anything. It prevents disabling of the unused-variable 
warning for whatever type that has the attribute.

-- 
 Lubos Lunak
 l.lunak at suse.cz