libcamera: base: class: Don't pass Extensible pointer to Private constructor

The Extensible and Extensible::Private classes contain pointers to each
other. These pointers are initialized in the respective class's
constructor, by passing a pointer to the other class to each
constructor. This particular construct reduces the flexibility of the
Extensible pattern, as the Private class instance has to be allocated
and constructed in the members initializer list of the Extensible
class's constructor. It is thus impossible to perform any operation on
the Private class between its construction and the construction of the
Extensible class, or to subclass the Private class without subclassing
the Extensible class.

To make the design pattern more flexible, don't pass the pointer to the
Extensible class to the Private class's constructor, but initialize the
pointer manually in the Extensible class's constructor. This requires a
const_cast as the o_ member of the Private class is const.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

2 files changed, 4 insertions, 2 deletions

diff --git a/include/libcamera/base/class.h b/include/libcamera/base/class.h
index 9c7f0f2e..a5946836 100644
--- a/include/libcamera/base/class.h
+++ b/include/libcamera/base/class.h
@@ -64,7 +64,7 @@ public:
 	class Private
 	{
 	public:
-		Private(Extensible *o);
+		Private();
 		virtual ~Private();
 
 #ifndef __DOXYGEN__
@@ -82,6 +82,8 @@ public:
 #endif
 
 	private:
+		/* To initialize o_ from Extensible. */
+		friend class Extensible;
 		Extensible *const o_;
 	};
 
diff --git a/include/libcamera/internal/framebuffer.h b/include/libcamera/internal/framebuffer.h
index a11e895d..8c187adf 100644
--- a/include/libcamera/internal/framebuffer.h
+++ b/include/libcamera/internal/framebuffer.h
@@ -52,7 +52,7 @@ class FrameBuffer::Private : public Extensible::Private
 	LIBCAMERA_DECLARE_PUBLIC(FrameBuffer)
 
 public:
-	Private(FrameBuffer *buffer);
+	Private();
 
 	void setRequest(Request *request) { request_ = request; }