libcamera: Add a base class to implement the d-pointer design pattern

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@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

include/libcamera/extensible.h

@@ -0,0 +1,87 @@
+/* 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;						\
+	using Public = klass;
+
+#define LIBCAMERA_D_PTR()						\
+	_d<Private>();
+
+#define LIBCAMERA_O_PTR()						\
+	_o<Public>();
+
+#else
+#define LIBCAMERA_DECLARE_PRIVATE(klass)
+#define LIBCAMERA_DECLARE_PUBLIC(klass)
+#define LIBCAMERA_D_PTR(klass)
+#define LIBCAMERA_O_PTR(klass)
+#endif
+
+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__ */

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',
     'framebuffer_allocator.h',
     'geometry.h',