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What is the C++ way of calling 'Add Spline Mesh Component'?

Development Discussion C++ Gameplay Programming
#1

I am trying to recreate the blueprint spline track example in C++. In the Blueprint there is a ‘Add Spline Mesh Component’ call and I am not sure how I would translate this to C++.

I tried


void Foo::OnConstruction(const FTransform& Transform)
{
  int32 SplinePoints = Spline->GetNumSplinePoints();
  for (int32 i = 0; i < SplinePoints; ++i){
    FVector Location, Tangent;
    Spline->GetLocalLocationAndTangentAtSplinePoint(i, Location, Tangent);
    USplineMeshComponent *s = CreateDefaultSubobject<USplineMeshComponent>(TEXT(""));
    ...
  }
}

but this crashes the editor. I just want to spawn a SplineMeshComponent in the construction script, how would I do that?

#2

I think the correct function would be ConstructObject. Unfortunately this also crashes :frowning:


void ASurfMeshGenerator::OnConstruction(const FTransform& Transform)
{
  int32 SplinePoints = Spline->GetNumSplinePoints();
  for (int32 i = 0; i < SplinePoints; ++i){
    int32 next_index = (i + 1) % SplinePoints;
    FVector Location, Tangent, LocationNext,TangentNext;
    Spline->GetLocalLocationAndTangentAtSplinePoint(i, Location, Tangent);
    Spline->GetLocalLocationAndTangentAtSplinePoint(next_index, LocationNext, TangentNext);
    auto *s = ConstructObject<USplineMeshComponent>(USplineMeshComponent::StaticClass(),GetTransientPackage());
    s->SetStaticMesh(Mesh);
    s->SetStartAndEnd(Location, Tangent, LocationNext,TangentNext);
 }
}

Does Ue4 also crash that often for you? Maybe it’s better to just use blueprints for editor scripting.

#3

I also tried


void ASurfMeshGenerator::OnConstruction(const FTransform& Transform)
{
  int32 SplinePoints = Spline->GetNumSplinePoints() - 1;
  for (int32 i = 0; i < SplinePoints; ++i){
    FVector Location, Tangent, LocationNext, TangentNext;
    Spline->GetLocalLocationAndTangentAtSplinePoint(i, Location, Tangent);
    Spline->GetLocalLocationAndTangentAtSplinePoint(i + 1, LocationNext, TangentNext);
    auto *s = ConstructObject<USplineMeshComponent>(USplineMeshComponent::StaticClass(), GetOwner());
    s->SetStaticMesh(Mesh);
    s->SetStartAndEnd(Location, Tangent, LocationNext,TangentNext);
  }
}

but s-&gt;SetStartAndEnd(Location, Tangent, LocationNext,TangentNext); results also in an editor crash. Is ConstructObject also wrong?

#4

Okay it seems that I have missued the API.


void AFoo::OnConstruction(const FTransform& Transform)
{
    USplineMeshComponent *s = ConstructObject<USplineMeshComponent>(USplineMeshComponent::StaticClass());
    FVector v(10,10,10);
    s->SetStaticMesh(Mesh);
    s->SetStartAndEnd(v, v, v,v);
}

The following code throws the error


bool UWorld::AreActorsInitialized() const
{
	return PersistentLevel && PersistentLevel->Actors.Num() && bActorsInitialized;
}

Where PersistentLevel “is unable to read memory”. Is it incorrect to use ConstructObject in OnConstruction?


s->SetStartAndEnd(v, v, v,v);

Is the line that is “responsible” because it calls


void USplineMeshComponent::MarkSplineParamsDirty()
{
	MarkRenderStateDirty();

#if WITH_EDITOR
	if (!GetWorld()->AreActorsInitialized())
	{
		RecreateCollision();
	}
#endif // WITH_EDITOR
}

Any help is greatly appreciated.

#5

I followed the guide here:

https://answers.unrealengine.com/questions/174108/can-you-give-me-an-example-of-splinemeshcomponent-1.html

I think the line causing your issue is


USplineMeshComponent *s = ConstructObject<USplineMeshComponent>(USplineMeshComponent::StaticClass());

When it should be


USplineMeshComponent *s = ConstructObject<USplineMeshComponent>(USplineMeshComponent::StaticClass(), this);

Using the guide I was able to get the meshes to appear, but the problem I am having is that the guide uses


bCreatedByConstructionScript

which is now deprecated. Even if I try to use this value, it seems that my meshes are not deleted when construction is rerun, so I am spawning hundreds of meshes each time I move my spline around. Can anyone show me how to properly destroy meshes which were created during construction?

#6

I would be greatly interested in this as well!

#7

Instead of bCreatedByConstructionScript, you should now set the CreationMethod property to EComponentCreationMethod::UserConstructionScript.

Epic should update their documentation faster.

#8

This has been changed now to NewObject<UObjectType>(InParent, Class, Name, Flags). ConstructObject was deprecated.