[libcamera-devel] [PATCH 1/3] libcamera: base: class: Expose Extensible private data to other classes

[Hirokazu Honda]

Hi Laurent, thank you for the patch.

On Tue, Jul 6, 2021 at 2:08 PM Umang Jain <umang.jain at ideasonboard.com> wrote:
>
> Hi Laurent,
>
> On 7/5/21 4:58 AM, Laurent Pinchart wrote:
> > Despite sharing the same name, the private data class created by the
> > Extensible design pattern and the C++ private access specifier have
> > different goals. The latter specifies class members private to the
> > class, while the former stores data not visible to the application.
> >
> > There are use cases for accessing the private data class from other
> > classes inside libcamera. Make this possible by exposing public _d()
> > functions in the class deriving from Extensible. This won't allow access
> > to the private data by applications as the definition of the Private
> > class isn't visible outside of libcamera.
> I like this idea.
> > The _d() functions need to be defined as template functions to delay
> > their evaluation, as the static_cast() operator in the Extensible::_d()
> > functions needs the Private class to be fully defined.
> >
> > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
>   Reviewed-by: Umang Jain <umang.jain at ideasonboard.com>

Reviewed-by: Hirokazu Honda <hiroh at chromium.org>
> > ---
> >   include/libcamera/base/class.h | 14 ++++++++++++--
> >   1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/libcamera/base/class.h b/include/libcamera/base/class.h
> > index a07dac057331..8212c3d4a5ae 100644
> > --- a/include/libcamera/base/class.h
> > +++ b/include/libcamera/base/class.h
> > @@ -33,14 +33,24 @@ namespace libcamera {
> >   #define LIBCAMERA_DECLARE_PRIVATE()                                 \
> >   public:                                                                     \
> >       class Private;                                                  \
> > -     friend class Private;
> > +     friend class Private;                                           \
> > +     template <bool B = true>                                        \
> > +     const Private *_d() const                                       \
> > +     {                                                               \
> > +             return Extensible::_d<Private>();                       \
> > +     }                                                               \
> > +     template <bool B = true>                                        \
> > +     Private *_d()                                                   \
> > +     {                                                               \
> > +             return Extensible::_d<Private>();                       \
> > +     }
> >
> >   #define LIBCAMERA_DECLARE_PUBLIC(klass)                                     \
> >       friend class klass;                                             \
> >       using Public = klass;
> >
> >   #define LIBCAMERA_D_PTR()                                           \
> > -     _d<Private>();
> > +     _d();
> >
> >   #define LIBCAMERA_O_PTR()                                           \
> >       _o<Public>();