ProcessMulticastDelegate throws exception: read access violation

Development Programming & Scripting
1

Hi,

I’m having a read access exception thrown by the ProcessMulticastDelegate. (See image attached to see where)

My Source code:

#pragma once

#include "Components/ActorComponent.h"
#include "Components/LaserComponent.h"
#include "LaserReceiverComponent.generated.h"

DECLARE_DYNAMIC_MULTICAST_DELEGATE_OneParam(FOnRemoveLaserDelegate, ULaserComponent*, LaserComponent);
DECLARE_DYNAMIC_MULTICAST_DELEGATE_OneParam(FOnAddLaserDelegate, ULaserComponent*, LaserComponent);

UCLASS(ClassGroup = (Custom), meta = (BlueprintSpawnableComponent))
class MYPROJECT_API ULaserReceiverComponent : public UActorComponent
{
	GENERATED_BODY()

public:

	UPROPERTY(Category= MyCategory, BlueprintAssignable)
	FOnRemoveLaserDelegate OnRemoveLaser;

	UPROPERTY(Category = MyCategory, BlueprintAssignable)
	FOnAddLaserDelegate OnAddLaser;

	ULaserReceiverComponent();

	virtual void BeginPlay() override;

	virtual void TickComponent(float DeltaTime, ELevelTick TickType, FActorComponentTickFunction* ThisTickFunction) override;

	void AddLaser(ULaserComponent* Laser);

	void RemoveLaser(ULaserComponent* Laser);
};

CPP

#include "MyProject.h"
#include "LaserReceiverComponent.h"



ULaserReceiverComponent::ULaserReceiverComponent() :Super()
{
	PrimaryComponentTick.bCanEverTick = true;
	bWantsBeginPlay = true;
}

void ULaserReceiverComponent::BeginPlay()
{
	Super::BeginPlay();
}

void ULaserReceiverComponent::TickComponent(float DeltaTime, ELevelTick TickType, FActorComponentTickFunction* ThisTickFunction)
{
	Super::TickComponent(DeltaTime, TickType, ThisTickFunction);
}

void ULaserReceiverComponent::AddLaser(ULaserComponent* Laser)
{
	OnAddLaser.Broadcast(Laser);
}

void ULaserReceiverComponent::RemoveLaser(ULaserComponent* Laser)
{
	OnRemoveLaser.Broadcast(Laser);
}

CALLER CPP

void ULaserComponent::SetNewReceiver(ULaserReceiverComponent* NewReceiver)
{
	if (NewReceiver == Receiver)
	{
		return;
	}

	if (Receiver)
	{
		Receiver->RemoveLaser(this);
	}
	if (NewReceiver)
	{
		Receiver->AddLaser(this);
	}
	
	Receiver = NewReceiver;
}

126273-1.png
126273-1.png1869×1434 110 KB

126275-2.jpg
126275-2.jpg1434×1100 163 KB

2

Hi,

You’ve called AddLaser on Receiver, yet it was NewReceiver you tested for null.

Steve

3

Hi Steve,

Thanks for your answer. I feel dumb, now :slight_smile:

I still don’t understand why it throws the exception in ProcessMulticastDelegate and ULaserReceiverComponent::AddLaser is able to execute having a null Object.

My main expertise was Java before I started learning UE4.

Thanks very much for you answer.

Cheers,
J

4

C++ isn’t like Java. Dereferencing a null pointer doesn’t throw an exception, it causes undefined behaviour. This can cause it to anything at all, including throw a hardware exception (which it does in your case, eventually, halfway through that function) or even have no visible effects at all.

http://en.cppreference.com/w/cpp/language/ub

“Examples of undefined behavior are … null pointer dereference… and the compiled program is not required to do anything meaningful.”

Hope this helps,

Steve

5

Thanks Steve,

It did ring the bell once you explained it. Very useful link. It’s been 15 years since I studied C++ at uni, and I have a Java mindset that I need to re-educate.

Thanks again.