Referencing parent components

Development Programming & Scripting C++
1

I’d like to make it so that, if a UPrimitiveComponent that simulates physics has an AttachParent that also simulates physics, the constraint configured for the child is tied to the parent, rather than tied to world space.
I believe that the change can be made locally in BodyInstance.cpp:



		DOFConstraint->PriAxis2 = Normal;
		DOFConstraint->SecAxis2 = Sec;
		DOFConstraint->Pos2 = TM.GetLocation();

/* BEGIN NEW CODE */
		
		FBodyInstance *parentBody = nullptr;
		UPrimitiveComponent *parentComponent = OwnerComponent.Get();
		if (parentComponent != nullptr) {
			parentComponent = Cast<UPrimitiveComponent>(parentComponent->GetAttachParent());
		}
		if (parentComponent != nullptr) {
			//	TODO: parentComponent is gotten from a weak pointer; how long will
			//	this body pointer live?
			parentBody = parentComponent->GetBodyInstance();
		}

/* END NEW CODE */

		// Create constraint instance based on DOF
		DOFConstraint->InitConstraint(this, parentBody, 1.f, OwnerComponent.Get());

However, I’m concerned that getting a pointer to a bodystate in the parent is dangerous, because the component pointer is a weak pointer!
Also, the BodyState is an inline struct in the parent component.

What do I need to do to make sure that dangling references to the parent aren’t kept around?
My initial idea is to recreate the physics state inside UPrimitiveComponent::OnAttachmentChanged(), but I don’t know if that will be enough, and I also don’t know if that will be unnecessarily heavy-handed:



void UPrimitiveComponent::OnAttachmentChanged()
{
	UWorld* World = GetWorld();
	if (World && World->Scene)
	{
		World->Scene->UpdatePrimitiveAttachment(this);
	}
	/* We only need to recreate physics state if we had a physical parent component 
	   that was used for the constraint. Moving physically simulated component attachment 
	   should be rare enough that optimizing for that shouldn't be necessary. */
	RecreatePhysicsState();
}

So, advice and pointers appreciated!

2

Oddly, adding the parent component’s body to the joint doesn’t seem to be enough to cause the physics bodies to link together.
When I create a small chain that is parented, I can see the bodies of each object get linked through the call to InitConstraint(), but the bodies still stay pinned in world space.
I traced through all the way down to the PhysX library calls, and it seems like it’s doing the right thing, but obviously I’m missing something or getting something wrong.

3

Progress! Further questions in this thread: Allowing hierarchical armatures of primitive components in a single actor (patch) - C++ Gameplay Programming - Unreal Engine Forums