[PATCH v1] libcamera: base: bound_method: Simplify `invokePack()`

[Kieran Bingham]

Quoting Laurent Pinchart (2025-03-31 17:21:58)
> Hi Barnabás,
> 
> Thank you for the patch.
> 
> On Mon, Mar 31, 2025 at 06:03:01PM +0200, Barnabás Pőcze wrote:
> > Use `if constexpr` instead of SFINAE to handle return values of type `void`.
> > 
> > Signed-off-by: Barnabás Pőcze <barnabas.pocze at ideasonboard.com>
> > ---
> >  include/libcamera/base/bound_method.h | 17 ++++++-----------
> >  1 file changed, 6 insertions(+), 11 deletions(-)
> > 
> > diff --git a/include/libcamera/base/bound_method.h b/include/libcamera/base/bound_method.h
> > index dd3488eeb..aad75c02a 100644
> > --- a/include/libcamera/base/bound_method.h
> > +++ b/include/libcamera/base/bound_method.h
> > @@ -98,21 +98,16 @@ public:
> >       using PackType = BoundMethodPack<R, Args...>;
> >  
> >  private:
> > -     template<std::size_t... I, typename T = R>
> > -     std::enable_if_t<!std::is_void<T>::value, void>
> > +     template<std::size_t... I>
> > +     void
> >       invokePack(BoundMethodPackBase *pack, std::index_sequence<I...>)
> 
> You can write
> 
>         template<std::size_t... I>
>         void invokePack(BoundMethodPackBase *pack, std::index_sequence<I...>)
> 
> >       {
> >               PackType *args = static_cast<PackType *>(pack);
> > -             args->ret_ = invoke(std::get<I>(args->args_)...);
> > -     }
> >  
> > -     template<std::size_t... I, typename T = R>
> > -     std::enable_if_t<std::is_void<T>::value, void>
> > -     invokePack(BoundMethodPackBase *pack, std::index_sequence<I...>)
> > -     {
> > -             /* args is effectively unused when the sequence I is empty. */
> > -             PackType *args [[gnu::unused]] = static_cast<PackType *>(pack);
> > -             invoke(std::get<I>(args->args_)...);
> > +             if constexpr (!std::is_void_v<R>)
> > +                     args->ret_ = invoke(std::get<I>(args->args_)...);
> > +             else
> > +                     invoke(std::get<I>(args->args_)...);
> 
> I'm surprised that this works. I didn't know that within a templated
> entity the discarded statement is not instantiated. It's interesting.
> 
> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

No chance of me putting a reviewed by tag on template magic - it would
be a lie ;-)

But if Laurent's happy with this - and the CI passes, I'm fine to merge
it...

Acked-by: Kieran Bingham <kieran.bingham at ideasonboard.com>

> 
> >       }
> >  
> >  public:
> 
> -- 
> Regards,
> 
> Laurent Pinchart