[libcamera-devel] [PATCH v2 2/5] libcamera: Add a base class to implement the d-pointer design pattern
Jacopo Mondi
jacopo at jmondi.org
Wed Oct 21 12:04:12 CEST 2020
Hi Laurent,
On Tue, Oct 20, 2020 at 04:40:02AM +0300, Laurent Pinchart wrote:
> The d-pointer design patterns helps creating public classes that can be
> extended without breaking their ABI. To facilitate usage of the pattern
> in libcamera, create a base Extensible class with associated macros.
>
> Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> Reviewed-by: Niklas Söderlund <niklas.soderlund at ragnatech.se>
> ---
> Changes since v1:
>
> - Don't hide variable declarations in LIBCAMERA_[DO]_PTR macros
> - Extend documentation
> - Fix typos
> ---
> include/libcamera/extensible.h | 86 +++++++++++++++++++++
> include/libcamera/meson.build | 1 +
> src/libcamera/extensible.cpp | 134 +++++++++++++++++++++++++++++++++
> src/libcamera/meson.build | 1 +
> 4 files changed, 222 insertions(+)
> create mode 100644 include/libcamera/extensible.h
> create mode 100644 src/libcamera/extensible.cpp
>
> diff --git a/include/libcamera/extensible.h b/include/libcamera/extensible.h
> new file mode 100644
> index 000000000000..ea8808ad3e3c
> --- /dev/null
> +++ b/include/libcamera/extensible.h
> @@ -0,0 +1,86 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Copyright (C) 2020, Google Inc.
> + *
> + * extensible.h - Utilities to create extensible public classes with stable ABIs
> + */
> +#ifndef __LIBCAMERA_EXTENSIBLE_H__
> +#define __LIBCAMERA_EXTENSIBLE_H__
> +
> +#include <memory>
> +
> +namespace libcamera {
> +
> +#ifndef __DOXYGEN__
> +#define LIBCAMERA_DECLARE_PRIVATE(klass) \
> +public: \
> + class Private; \
> + friend class Private;
> +
> +#define LIBCAMERA_DECLARE_PUBLIC(klass) \
> + friend class klass;
I am missing why this is needed.
It is meant to be used in klass::Private definition, but being Private
in the klass:: namespace, it shouldn't be necessary, right ?
(In facts, I removed it from Camera::Private and
CameraManager::Private from patches 4/5 and 5/5 and the compiler is
still happy.
> +
> +#define LIBCAMERA_D_PTR(klass) \
> + _d<klass::Private>();
Just <Private> and drop klass from arguments ?
Reading the documentation the first thing I wondered is "why do I need
to pass in the public class name to get a pointer to a private" ?
> +
> +#define LIBCAMERA_O_PTR(klass) \
> + _o<klass>();
I would rather provide two methods and let users call them without
bothering about the 'd' or 'o' name. If a class wants to call the
pointer to its private 'daffyduck' I don't see why we should force it
to be named 'd'.
In public class:
Private *const abcd = _private();
In private class:
ClassName *const asdsad = _public<ClassName>();
without introducing macros that has to be followed and really just
wrap one method call.
> +
> +#else
> +#define LIBCAMERA_DECLARE_PRIVATE(klass)
> +#define LIBCAMERA_DECLARE_PUBLIC(klass)
> +#define LIBCAMERA_D_PTR(klass)
> +#define LIBCAMERA_O_PTR(klass)
> +#endif
I wlso onder if we want macros, if LIBCAMERA_ is needed in the names
(and I won't question the decision to call 'D' pointer a
pointer to the internal Private class. I understand 'O' as it might
recall 'Outer', but 'D' ?)
Macro names with PRIVATE and PUBLIC are more expressive imo, but it
might be just a matter of getting used to it.
> +
> +class Extensible
> +{
> +public:
> + class Private
> + {
> + public:
> + Private(Extensible *o);
> + virtual ~Private();
> +
> +#ifndef __DOXYGEN__
> + template<typename T>
> + const T *_o() const
> + {
> + return static_cast<const T *>(o_);
> + }
> +
> + template<typename T>
> + T *_o()
> + {
> + return static_cast<T *>(o_);
> + }
> +#endif
> +
> + private:
> + Extensible * const o_;
> + };
> +
> + Extensible(Private *d);
> +
> +protected:
> +#ifndef __DOXYGEN__
> + template<typename T>
> + const T *_d() const
> + {
> + return static_cast<const T *>(d_.get());
> + }
> +
> + template<typename T>
> + T *_d()
> + {
> + return static_cast<T*>(d_.get());
> + }
> +#endif
> +
> +private:
> + const std::unique_ptr<Private> d_;
> +};
> +
> +} /* namespace libcamera */
> +
> +#endif /* __LIBCAMERA_EXTENSIBLE_H__ */
> diff --git a/include/libcamera/meson.build b/include/libcamera/meson.build
> index 83bc46729314..15e6b43c9585 100644
> --- a/include/libcamera/meson.build
> +++ b/include/libcamera/meson.build
> @@ -8,6 +8,7 @@ libcamera_public_headers = files([
> 'controls.h',
> 'event_dispatcher.h',
> 'event_notifier.h',
> + 'extensible.h',
> 'file_descriptor.h',
> 'flags.h',
> 'framebuffer_allocator.h',
> diff --git a/src/libcamera/extensible.cpp b/src/libcamera/extensible.cpp
> new file mode 100644
> index 000000000000..1dcb0bf1b12f
> --- /dev/null
> +++ b/src/libcamera/extensible.cpp
> @@ -0,0 +1,134 @@
> +/* SPDX-License-Identifier: LGPL-2.1-or-later */
> +/*
> + * Copyright (C) 2020, Google Inc.
> + *
> + * extensible.cpp - Utilities to create extensible public classes with stable ABIs
> + */
> +
> +#include <libcamera/extensible.h>
> +
> +/**
> + * \file extensible.h
> + * \brief Utilities to create extensible public classes with stable ABIs
> + */
> +
> +namespace libcamera {
> +
> +/**
> + * \def LIBCAMERA_DECLARE_PRIVATE
> + * \brief Declare private data for a public class
> + * \param klass The public class name
> + *
> + * The LIBCAMERA_DECLARE_PRIVATE() macro plumbs the infrastructure necessary to
> + * make a class manage its private data through a d-pointer. It shall be used at
> + * the very top of the class definition, with the public class name passed as
> + * the \a klass parameter.
> + */
> +
> +/**
> + * \def LIBCAMERA_DECLARE_PUBLIC
> + * \brief Declare public data for a private class
> + * \param klass The public class name
> + *
> + * The LIBCAMERA_DECLARE_PUBLIC() macro is the counterpart of
> + * LIBCAMERA_DECLARE_PRIVATE() to be used in the private data class. It shall be
> + * used at the very top of the private class definition, with the public class
> + * name passed as the \a klass parameter.
> + */
> +
> +/**
> + * \def LIBCAMERA_D_PTR(klass)
> + * \brief Retrieve the private data pointer
> + * \param[in] klass The public class name
> + *
> + * This macro can be used in any member function of a class that inherits,
> + * directly or indirectly, from the Extensible class, to create a local
> + * variable named 'd' that points to the class' private data instance.
> + */
> +
> +/**
> + * \def LIBCAMERA_O_PTR(klass)
> + * \brief Retrieve the public instance corresponding to the private data
> + * \param[in] klass The public class name
> + *
> + * This macro is the counterpart of LIBCAMERA_D_PTR() for private data classes.
> + * It can be used in any member function of the private data class to create a
> + * local variable named 'o' that points to the public class instance
> + * corresponding to the private data.
The two macros do not "create a local variable named ['o'|'d'] anymore
> + */
> +
> +/**
> + * \class Extensible
> + * \brief Base class to manage private data through a d-pointer
> + *
> + * The Extensible class provides a base class to implement the
> + * <a href="https://wiki.qt.io/D-Pointer">d-pointer</a> design pattern (also
> + * known as <a href="https://en.wikipedia.org/wiki/Opaque_pointer">opaque pointer</a>
> + * or <a href="https://en.cppreference.com/w/cpp/language/pimpl">pImpl idiom</a>).
> + * It helps creating public classes that can be extended without breaking their
> + * ABI. Such classes store their private data in a separate private data object,
> + * referenced by a pointer in the public class (hence the name d-pointer).
> + *
> + * Classes that follow this design pattern are referred herein as extensible
> + * classes. To be extensible, a class PublicClass shall:
> + *
> + * - inherit from the Extensible class or from another extensible class
> + * - invoke the LIBCAMERA_DECLARE_PRIVATE() macro at the very top of the class
> + * definition
> + * - define a private data class named PublicClass::Private that inherits from
> + * the Private data class of the base class
> + * - invoke the LIBCAMERA_DECLARE_PUBLIC() macro at the very top of the Private
> + * data class definition
> + * - pass a pointer to a newly allocated Private data object to the constructor
> + * of the base class
> + *
> + * Additionally, if the PublicClass is not final, it shall expose one or more
> + * constructors that takes a pointer to a Private data instance, to be used by
> + * derived classes.
> + *
> + * The Private class is fully opaque to users of the libcamera public API.
> + * Internally, it can be kept private to the implementation of PublicClass, or
> + * be exposed to other classes. In the latter case, the members of the Private
> + * class need to be qualified with appropriate access specifiers. The
> + * PublicClass and Private classes always have full access to each other's
> + * protected and private members.
> + */
> +
> +/**
> + * \brief Construct an instance of an Extensible class
> + * \param[in] d Pointer to the private data instance
> + */
> +Extensible::Extensible(Extensible::Private *d)
> + : d_(d)
> +{
> +}
I wonder if we could avoid hainvg in the extensible derived classes
: Extensible(new Private(...))
by making Extensible accept a template argument pack that can be
forwarded to the construction of d_.
I just wonder, I'm not proposing to try it :)
> +
> +/**
> + * \var Extensible::d_
> + * \brief Pointer to the private data instance
> + */
> +
> +/**
> + * \class Extensible::Private
> + * \brief Base class for private data managed through a d-pointer
> + */
> +
> +/**
> + * \brief Construct an instance of an Extensible class private data
> + * \param[in] o Pointer to the public class object
> + */
> +Extensible::Private::Private(Extensible *o)
> + : o_(o)
> +{
> +}
> +
> +Extensible::Private::~Private()
Why doesn't doxygen complain ?
Thanks
j
> +{
> +}
> +
> +/**
> + * \var Extensible::Private::o_
> + * \brief Pointer to the public class object
> + */
> +
> +} /* namespace libcamera */
> diff --git a/src/libcamera/meson.build b/src/libcamera/meson.build
> index 47ddb4014a61..b9f6457433f9 100644
> --- a/src/libcamera/meson.build
> +++ b/src/libcamera/meson.build
> @@ -17,6 +17,7 @@ libcamera_sources = files([
> 'event_dispatcher.cpp',
> 'event_dispatcher_poll.cpp',
> 'event_notifier.cpp',
> + 'extensible.cpp',
> 'file.cpp',
> 'file_descriptor.cpp',
> 'flags.cpp',
> --
> Regards,
>
> Laurent Pinchart
>
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