Unreal Engine scene component returning null when attached to actor

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#1

I have a scene component that uses addforce() to create a gravity-esque acceleration that points in the owner’s z axis. When added to an asset that is not based off of a C++ or Blueprint class, the component works as it should, but when attached to an asset that is based off of a C++ or Blueprint class the component returns a nullptr exception. I think that the component is unable to find the owner when attached to class based asset but I am unsure as to why. (I am not completely confident in this hypothesis as I am still inexperienced with Unreal.) Here is my c++ code:

#pragma once

#include "CoreMinimal.h"
#include "Components/SceneComponent.h"
#include "SceneGravity.generated.h"


UCLASS( ClassGroup=(Custom), meta=(BlueprintSpawnableComponent) )
class TESTS_API USceneGravity : public USceneComponent
{
	GENERATED_BODY()

public:	
	// Sets default values for this component's properties
	USceneGravity();

protected:
	// Called when the game starts
	virtual void BeginPlay() override;

public:	
	// Called every frame
	virtual void TickComponent(float DeltaTime, ELevelTick TickType, FActorComponentTickFunction* ThisTickFunction) override;

	UStaticMeshComponent* Mesh;
	UPROPERTY(EditAnywhere)
		float GravStrength = -2000.0f;
};

#include "SceneGravity.h"

// Sets default values for this component's properties
USceneGravity::USceneGravity()
{
	// Set this component to be initialized when the game starts, and to be ticked every frame.  You can turn these features
	// off to improve performance if you don't need them.
	PrimaryComponentTick.bCanEverTick = true;

	// ...
}


// Called when the game starts
void USceneGravity::BeginPlay()
{
	Super::BeginPlay();
	Mesh = Cast<UStaticMeshComponent>(GetOwner()->GetRootComponent());
	// ...
	if (Mesh == nullptr){
		UE_LOG(LogTemp, Warning, TEXT("Gravity references nullptr"));
	}
}


// Called every frame
void USceneGravity::TickComponent(float DeltaTime, ELevelTick TickType, FActorComponentTickFunction* ThisTickFunction)
{
	Super::TickComponent(DeltaTime, TickType, ThisTickFunction);
	const FVector Force = this->GetUpVector();
	if (Mesh != nullptr) {
		/*Exception thrown : read access violation.
		this->Mesh was nullptr.*/
		Mesh->AddForce(Force * Mesh->GetMass() * GravStrength);
	}
	// ...
}

.


#include "SceneGravity.h"

// Sets default values for this component's properties
USceneGravity::USceneGravity()
{
	// Set this component to be initialized when the game starts, and to be ticked every frame.  You can turn these features
	// off to improve performance if you don't need them.
	PrimaryComponentTick.bCanEverTick = true;

	// ...
}


// Called when the game starts
void USceneGravity::BeginPlay()
{
	Super::BeginPlay();
	Mesh = Cast<UStaticMeshComponent>(GetOwner()->GetRootComponent());
	// ...
	if (Mesh == nullptr){
		UE_LOG(LogTemp, Warning, TEXT("Gravity references nullptr"));
	}
}


// Called every frame
void USceneGravity::TickComponent(float DeltaTime, ELevelTick TickType, FActorComponentTickFunction* ThisTickFunction)
{
	Super::TickComponent(DeltaTime, TickType, ThisTickFunction);
	const FVector Force = this->GetUpVector();
	if (Mesh != nullptr) {
		/*Exception thrown : read access violation.
		this->Mesh was nullptr.*/
		Mesh->AddForce(Force * Mesh->GetMass() * GravStrength);
	}
	// ...
}
#2

What do you mean by “non-actor”? A component can only be added to actors that’s why they the base class is called UActorComponent. Judging by your code you mean one without a static mesh?

You would want UPrimitiveComponent if you want it a more general component.

#3

My bad. I meant that assets that weren’t based off of a blueprint or c++ class would work just fine while assets that were based off of c++ and blueprint classes would result in the given exception. Unfortunately, I set out to use this on my player character which is based off of a C++ class. Apologies for misusing the terminology, I can understand how that would be confusing.

#4

Your player character most likely doesn’t have a static mesh component and instead has a skeletal mesh component. So this code is going to result in nullptr being assigned to Mesh

Mesh = Cast<UStaticMeshComponent>(GetOwner()->GetRootComponent());

Both Skeletal and Static Mesh component’s derive from UMeshComponent so you can use that instead.

#5

Both Skeletal and Static Mesh component’s derive from UMeshComponent so you can use that instead.

Unfortunately, that doesn’t seem to work. I am still getting the same exception after using UMeshComponent. I might look for a different solution for my desired mechanic if I can’t find a way to resolve this issue, one other than addforce(), addimpulse(), and SetPhysicsLinearVelocity(). Thanks for making an effort to help, it is much appreciated.

#6

Well if all you need is those 3 functions then that would be a UPrimitiveComponent so using that instead would work with more types.

Also are you sure the root component is what you expect it to be?

#7

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