libcamera: camera_manager: Make the class a singleton

There can only be a single camera manager instance in the application.
Creating it as a singleton helps avoiding mistakes. It also allows the
camera manager to be used as a storage of global data, such as the
future event dispatcher.

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

src/libcamera/camera_manager.cpp

@@ -161,4 +161,19 @@ Camera *CameraManager::get(const std::string &name)
 	return nullptr;
 }
 
+/**
+ * \brief Retrieve the camera manager instance
+ *
+ * The CameraManager is a singleton and can't be constructed manually. This
+ * function shall instead be used to retrieve the single global instance of the
+ * manager.
+ *
+ * \return The camera manager instance
+ */
+CameraManager *CameraManager::instance()
+{
+	static CameraManager manager;
+	return &manager;
+}
+
 } /* namespace libcamera */