libcamera: camera: Handle camera objects through shared pointers

The Camera class is explicitly reference-counted to manage the lifetime
of camera objects. Replace this open-coded implementation with usage of
the std::shared_ptr<> class.

This API change prevents pipeline handlers from subclassing the Camera
class. This isn't deemed to be an issue. Mark the class final to make
this explicit.

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

include/libcamera/camera.h

@@ -7,22 +7,25 @@
 #ifndef __LIBCAMERA_CAMERA_H__
 #define __LIBCAMERA_CAMERA_H__
 
+#include <memory>
 #include <string>
 
 namespace libcamera {
 
-class Camera
+class Camera final
 {
 public:
-	Camera(const std::string &name);
+	static std::shared_ptr<Camera> create(const std::string &name);
+
+	Camera(const Camera &) = delete;
+	void operator=(const Camera &) = delete;
 
 	const std::string &name() const;
-	void get();
-	void put();
 
 private:
-	virtual ~Camera() { };
+	explicit Camera(const std::string &name);
-	int ref_;
+	~Camera();
+
 	std::string name_;
 };
 

include/libcamera/camera_manager.h

@@ -24,10 +24,10 @@ public:
 	int start();
 	void stop();
 
-	const std::vector<Camera *> &cameras() const { return cameras_; }
+	const std::vector<std::shared_ptr<Camera>> &cameras() const { return cameras_; }
-	Camera *get(const std::string &name);
+	std::shared_ptr<Camera> get(const std::string &name);
 
-	void addCamera(Camera *camera);
+	void addCamera(std::shared_ptr<Camera> camera);
 
 	static CameraManager *instance();
 
@@ -42,7 +42,7 @@ private:
 
 	std::unique_ptr<DeviceEnumerator> enumerator_;
 	std::vector<PipelineHandler *> pipes_;
-	std::vector<Camera *> cameras_;
+	std::vector<std::shared_ptr<Camera>> cameras_;
 
 	std::unique_ptr<EventDispatcher> dispatcher_;
 };

src/libcamera/camera.cpp

@@ -41,19 +41,36 @@ namespace libcamera {
  * streams from a single image source. It provides the main interface to
  * configuring and controlling the device, and capturing image streams. It is
  * the central object exposed by libcamera.
+ *
+ * To support the central nature of Camera objects, libcamera manages the
+ * lifetime of camera instances with std::shared_ptr<>. Instances shall be
+ * created with the create() function which returns a shared pointer. The
+ * Camera constructors and destructor are private, to prevent instances from
+ * being constructed and destroyed manually.
  */
 
 /**
- * \brief Construct a named camera device
+ * \brief Create a camera instance
+ * \param[in] name The name of the camera device
  *
- * \param[in] name The name to set on the camera device
+ * The caller is responsible for guaranteeing unicity of the camera name.
  *
- * The caller is responsible for guaranteeing unicity of the camera
+ * \return A shared pointer to the newly created camera object
- * device name.
  */
-Camera::Camera(const std::string &name)
+std::shared_ptr<Camera> Camera::create(const std::string &name)
-	: ref_(1), name_(name)
 {
+	struct Allocator : std::allocator<Camera> {
+		void construct(void *p, const std::string &name)
+		{
+			::new(p) Camera(name);
+		}
+		void destroy(Camera *p)
+		{
+			p->~Camera();
+		}
+	};
+
+	return std::allocate_shared<Camera>(Allocator(), name);
 }
 
 /**
@@ -66,24 +83,13 @@ const std::string &Camera::name() const
 	return name_;
 }
 
-/**
+Camera::Camera(const std::string &name)
- * \brief Acquire a reference to the camera
+	: name_(name)
- */
-void Camera::get()
 {
-	ref_++;
 }
 
-/**
+Camera::~Camera()
- * \brief Release a reference to the camera
- *
- * When the last reference is released the camera device is deleted. Callers
- * shall not access the camera device after calling this function.
- */
-void Camera::put()
 {
-	if (--ref_ == 0)
-		delete this;
 }
 
 } /* namespace libcamera */

src/libcamera/camera_manager.cpp

@@ -40,9 +40,11 @@ namespace libcamera {
  * This will enumerate all the cameras present in the system, which can then be
  * listed with list() and retrieved with get().
  *
- * Cameras are reference-counted, and shall be returned to the camera manager
+ * Cameras are shared through std::shared_ptr<>, ensuring that a camera will
- * with Camera::put() after being used. Once all cameras have been returned to
+ * stay valid until the last reference is released without requiring any special
- * the manager, it can be stopped with stop().
+ * action from the application. Once the application has released all the
+ * references it held to cameras, the camera manager can be stopped with
+ * stop().
  *
  * \todo Add ability to add and remove media devices based on hot-(un)plug
  * events coming from the device enumerator.
@@ -148,15 +150,13 @@ void CameraManager::stop()
  * \param[in] name Name of camera to get
  *
  * Before calling this function the caller is responsible for ensuring that
- * the camera manger is running. A camera fetched this way shall be
+ * the camera manger is running.
- * released by the user with the put() method of the Camera object once
- * it is done using the camera.
  *
- * \return Pointer to Camera object or nullptr if camera not found
+ * \return Shared pointer to Camera object or nullptr if camera not found
  */
-Camera *CameraManager::get(const std::string &name)
+std::shared_ptr<Camera> CameraManager::get(const std::string &name)
 {
-	for (Camera *camera : cameras_) {
+	for (std::shared_ptr<Camera> camera : cameras_) {
 		if (camera->name() == name)
 			return camera;
 	}
@@ -172,9 +172,9 @@ Camera *CameraManager::get(const std::string &name)
  * handle with the camera manager. Registered cameras are immediately made
  * available to the system.
  */
-void CameraManager::addCamera(Camera *camera)
+void CameraManager::addCamera(std::shared_ptr<Camera> camera)
 {
-	for (Camera *c : cameras_) {
+	for (std::shared_ptr<Camera> c : cameras_) {
 		if (c->name() == camera->name()) {
 			LOG(Warning) << "Registering camera with duplicate name '"
 				     << camera->name() << "'";
@@ -182,7 +182,7 @@ void CameraManager::addCamera(Camera *camera)
 		}
 	}
 
-	cameras_.push_back(camera);
+	cameras_.push_back(std::move(camera));
 }
 
 /**

src/libcamera/pipeline/vimc.cpp

@@ -64,8 +64,8 @@ bool PipeHandlerVimc::match(CameraManager *manager, DeviceEnumerator *enumerator
 	 * will be chosen depends on how the Camera
 	 * object is modeled.
 	 */
-	Camera *camera = new Camera("Dummy VIMC Camera");
+	std::shared_ptr<Camera> camera = Camera::create("Dummy VIMC Camera");
-	manager->addCamera(camera);
+	manager->addCamera(std::move(camera));
 
 	return true;
 }

test/list-cameras.cpp

@@ -30,7 +30,7 @@ protected:
 	{
 		unsigned int count = 0;
 
-		for (Camera *camera : cm->cameras()) {
+		for (const std::shared_ptr<Camera> &camera : cm->cameras()) {
 			cout << "- " << camera->name() << endl;
 			count++;
 		}