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

[Laurent Pinchart]

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.

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>
---
 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>();
-- 
Regards,

Laurent Pinchart