Situation

I'm creating an ObjectData plugin, let's call the class Parent. Parent has various properties which allow the user to define its behavior.

Then there is another ObjectData, let's call that one Child. Child is supposed to create a list of Nulls. The count of Nulls as well as their Shape property must be defined by Parent where one of the exposed properties of Parent has an effect on the count while the Shape is determined by some logic in Parent.

The crucial part is that both the count and Shape are always set together by Parent. This event must be the trigger for Child to purge its current Nulls and rebuild them from the ground up using the desired Shape. It is not acceptable to create the Nulls at one point and simply changing the Shape later on, both steps must always occur at the same time.

What is the best approach for Parent to trigger this logic in Child?

Idea 1: Message MSG_DESCRIPTION_POSTSETPARAMETER

Child could listen to the MSG_DESCRIPTION_POSTSETPARAMETER in the Message function. This however will be triggered for each changed property separately so I can't exactly trigger the logic at the correct time, which would break the requirement. So I don't think this is the way. At least from what I know about this approach.

The code in Parent would look something like this:

Int32 childNullsCount = getChildNullsCount();
Int32 childNullsShape = getChildNullsShape();

BaseObject* child = BaseObject::Alloc(Ochild);
BaseContainer& childData = child ->GetDataInstanceRef();
childData->SetInt32(CHILD_NULLS_COUNT, childNullsCount);
childData->SetInt32(CHILD_NULLS_SHAPE, childNullsShape);

Idea 2: Custom Message

I could define a new message ID and pass the data together.

Code in Parent:

Int32 childNullsCount = getChildNullsCount();
Int32 childNullsShape = getChildNullsShape();

BaseObject* child = BaseObject::Alloc(Ochild);
// MSG_CHILD_CREATENULLS is defined somewhere else.
child->Message(MSG_CHILD_CREATENULLS, &CreateNullsData(childNullsCount, childNullsShape);

Code in Child:

Bool Child::Message(GeListNode* node, Int32 type, void* data)
{
    switch (type)
    {
    case MSG_CHILD_CREATENULLS:
        CreateNullsData* createData = static_cast<CreateNullsData*>(data);

        createNullsWithShape(createData->Count, createData->Shape);
        break;
    }

    return true;
}

Idea 3: Custom function in Child

Child could expose a function like this:

class Child : public ObjectData
{
public:
  /* ... */
  void CreateNulls(Int32 count, Int32 shape);
}

Parent could then do:

Int32 childNullsCount = getChildNullsCount();
Int32 childNullsShape = getChildNullsShape();

BaseObject* child = BaseObject::Alloc(Ochild);
Child* castedChild = /* what code goes here to cast BaseObject* to Child*? */;
castedChild->CreateNulls(childNullsCount, childNullsShape);

Even if this isn't the way I'd still love to know how to cast BaseObject* to Child*.

Disclaimer

My actual plugin handles a more complex scenario which would obscure the question too much due to all the details so I'm trying to keep this topic very abstract.