Using class variables

Development Programming & Scripting
1

What’s the proper way to use class variables and actually create objects from the variable?

My specific problem appears to be that I’m trying to create an object in a constructor; NewObject() gives the error “NewObject can’t be used to create default subobjects (inside of UObject derived class constructor) as it produces inconsistent object names. Use ObjectInitializer.CreateDefaultSuobject<> instead.”. However, CreateDefaultSuobject is not an option because the class is passed in through the template, meaning it must be a specific type, and I can’t use a class for dynamic object creation.

What I’m trying to accomplish with this is streamlining the mesh components. Right now I have mesh component declared in the abstract class Weapon, however since it’s abstract, Weapon itself will not have a mesh. What I want to do is basically, in Weapon, “Create a mesh specified by a variable, attach to root, set properties on that mesh”. It looks like if I can’t do this, I would need to copy/paste this code into every single subclass that I make.

I’m still new to this so it’s possible that my approach is completely off. Any assistance would be appreciated. Thanks.

2

Yeah, I had this same problem and couldn’t find a way to do this in the constructor.
I ended up creating a virtual function (which can be overridden by subclasses) which was called from OnConstruction().

OnConstruction() doesn’t technically form part of the construction routine (it’s execution doesn’t originate from the constructor - it is called later by some other Unreal-specific code, similar to BeginPlay()).

I do have a question for you:

Is the component you’re trying to create always e.g. a SkeletalMeshComponent?
If so, you can always still create the component in the constructor, but use the OnConstruction() approach to set the actual mesh (i.e. MeshComponent->SetSkeletalMesh(yourmesh). You can use ObjectFinder to find the mesh from a text reference.

I also wrote a fairly detailed forum post about my findings here if you’re interested in some more details.

3

The component is always the same class, i.e. UStaticMeshComponent. I considered that, but I don’t think it will work for the same reason I can’t just set it while NULL.

It’d be something like:

Mesh = GetSomePlaceholderMesh();
Mesh->bCastDynamicShadow = false;
Mesh->AttachParent = RootComponent;
//...
Mesh = GetTheRealMesh();

I’d like to be able to assign variables on the mesh without setting it, but since it’s an actual object, GetTheRealMesh() would just “overwrite” any changes I had made.

4

If you do it in the constructor, yes. The reason is that you have no control over when parent constructors are called, it always goes:

  • Execute Base Class Constructor from start to finish
  • Execute First Child Class Constructor from start to finish
  • Execute Second Child Class Constructor from start to finish
  • Etc

When you do it in a virtual function (which is not part of the formal construction routine, like OnConstruction() or BeginPlay()), you do have control over when Superclass functions are called. E.g.

In YourParentClass.cpp:

void ParentClass::OnConstruction()
{
	Super::OnConstruction();
	CustomInit();
}

void ParentClass::CustomInit()
{
	// Get the mesh to use
	UStaticMeshComponent* meshToUse = GetMesh();
	
	// Set mesh
	Mesh->SetStaticMesh(meshToUse);
	
	// Set attributes
	SetMeshAttributes();
}

UStaticMeshComponent* ParentClass::GetMesh()
{
	return someDummyMesh;
}

void ParentClass::SetMeshAttributes()
{
	Mesh->bCastDynamicShadow = false;
	...
}

Now, in YourChildClass.cpp:

void ChildClass::SetMeshAttributes()
{
    // Not calling Super here!
	Mesh->bCastDynamicShadow = true;
	...
}
	
UStaticMeshComponent* ChildClass::GetMesh()
{
	// You can also use a class reference here, as the child class value of such a property would be in effect at this point in time.
	return theRealMesh;
}

Or am I missing this entirely?

5

I think this would work, but wouldn’t reflect the changes fully in the editor, which is what I was hoping to accomplish by putting it in the constructor in the first place.

I’m just going to assume that I’m doing this wrong, and try to find a completely different approach for this.

Thanks.