[RFC PATCH v2 02/16] apps: common: event_loop: Disable copy/move

[Barnabás Pőcze]

Hi


2025. január 22., szerda 10:03 keltezéssel, Jacopo Mondi <jacopo.mondi at ideasonboard.com> írta:

> Hi Barnabás
> 
> On Tue, Jan 14, 2025 at 06:21:56PM +0000, Barnabás Pőcze wrote:
> > The compiler generated functions are not appropriate, so
> > delete the copy/move constructor/assignment to avoid
> > potential issues.
> >
> > Signed-off-by: Barnabás Pőcze <pobrn at protonmail.com>
> 
> The class has this member
> 	std::list<std::unique_ptr<Event>> events_;
> 
> Is there a copy constructor generated for the class even if
> unique_ptr<> do not provide one ?

That list would indeed cause the copy constructor/assignment to be deleted,
however, move constructor/assignment would still be generated, which would
be incorrect since the owning `event_base` pointer is stored in a raw pointer.
However, the class has an `std::mutex` member, so there are in fact no copy/move
constructors/assignment operators defined by default.

So this change does not really change much, my bad, this can be probably be ignored.
Nonetheless, I have run into at least one situation in libcamera where a move
constructor was defined by the compiler but semantically it was wrong. Specifically,
it was `EventNotifier`; I am wondering if `Object` should disable copy/move altogether.

In any case, I'd support it if libcamera would enforce the *rule of 0/5* more strictly.
( https://en.cppreference.com/w/cpp/language/rule_of_three )


Regards,
Barnabás Pőcze

> 
> > ---
> >  src/apps/common/event_loop.h | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/src/apps/common/event_loop.h b/src/apps/common/event_loop.h
> > index d7d012c76..4e8dd0a46 100644
> > --- a/src/apps/common/event_loop.h
> > +++ b/src/apps/common/event_loop.h
> > @@ -13,6 +13,8 @@
> >  #include <memory>
> >  #include <mutex>
> >
> > +#include <libcamera/base/class.h>
> > +
> >  #include <event2/util.h>
> >
> >  struct event_base;
> > @@ -43,8 +45,11 @@ public:
> >  			   std::function<void()> &&handler);
> >
> >  private:
> > +	LIBCAMERA_DISABLE_COPY_AND_MOVE(EventLoop)
> > +
> >  	struct Event {
> >  		Event(std::function<void()> &&callback);
> > +		LIBCAMERA_DISABLE_COPY_AND_MOVE(Event)
> >  		~Event();
> >
> >  		static void dispatch(int fd, short events, void *arg);
> > --
> > 2.48.0
> >
> >
>