libcamera: bound_method: Propagate method return value

Propagate the return value of the bound method all the way to the caller
of activate(). The value is stored in the arguments pack for indirect
invocation.

As C++ doesn't allow instantiating a variable of type void, we need to
specialize the template class BoundMethodPack for methods returning
void. This in turn requires template specialization for the
BoundMethodArgs class in order to store the return value in the pack,
and for the BoundMemberMethod class to extract the return value from the
pack.

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

src/libcamera/object.cpp

@@ -143,7 +143,7 @@ void Object::message(Message *msg)
 }
 
 /**
- * \fn void Object::invokeMethod()
+ * \fn R Object::invokeMethod()
  * \brief Invoke a method asynchronously on an Object instance
  * \param[in] func The object method to invoke
  * \param[in] type Connection type for method invocation
@@ -156,6 +156,10 @@ void Object::message(Message *msg)
  * Arguments \a args passed by value or reference are copied, while pointers
  * are passed untouched. The caller shall ensure that any pointer argument
  * remains valid until the method is invoked.
+ *
+ * \return For connection types ConnectionTypeDirect and
+ * ConnectionTypeBlocking, return the return value of the invoked method. For
+ * connection type ConnectionTypeQueued, return a default-constructed R value.
  */
 
 /**