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Creating Gameplay in Unreal Engine

Complex coding examples in Unreal Engine with side-by-side C#, C++, and Blueprint examples.

Creating Gameplay in Unreal Engine
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This page explains Unreal Engine (UE) gameplay programming concepts for Unity users. The explanations below assume you are familiar with Unity C# and want to learn UE C++ and Blueprint.

The examples below cover some common gameplay programming use cases in Unity C# and how to implement the same functionality in UE.

Instantiating GameObject / Spawning Actor

In Unity, you use the Instantiate function to create new instances of objects. This function takes any UnityEngine.Object type (GameObject, MonoBehaviour, etc.), and makes a copy of it.

C# 
public GameObject EnemyPrefab;
public Vector3 SpawnPosition;
public Quaternion SpawnRotation;

void Start()
{
	GameObject NewGO = (GameObject)Instantiate(EnemyPrefab, SpawnPosition, SpawnRotation);
	NewGO.name = "MyNewGameObject";
}

UE has two different functions to instantiate objects:

  • NewObject creates new UObject types.

  • SpawnActor spawns AActor types.

UObjects and NewObject

Subclassing UObject in UE is similar to subclassing ScriptableObject in Unity. These are useful for gameplay classes that do not need to spawn into the world or have attached components as Actors do.

In Unity, if you created a subclass of ScriptableObject, you can instantiate it like this:

C# 
MyScriptableObject NewSO = ScriptableObject.CreateInstance<MyScriptableObject>();

In UE, if you create a subclass of UObject, you can instantiate it like this:

C++ 
UMyObject* NewObj = NewObject<UMyObject>();

AActors and SpawnActor

You can spawn Actors using the SpawnActor method on a World (UWorld in C++) object. Some UObjects and all Actors provide a GetWorld method to get the World object.

In the example below, we use those methods with an existing Actor's spawn parameters to emulate the functionality of Unity's Instantiate method.

Example

Below is the example AActor subclass, AMyActor. The default constructor initializes the int32 and USphereComponent*.

Note the use of the CreateDefaultSubobject function. This function creates Components and assigns default properties to them. Sub-objects created with this function act as a default template, so you can modify them in a subclass or Blueprint.

C++ 
UCLASS()
class AMyActor : public AActor
{
	GENERATED_BODY()

	UPROPERTY()
	int32 MyIntProp;

	UPROPERTY()
	USphereComponent* MyCollisionComp;

This creates a clone of a AMyActor, including all member variables, UPROPERTYs, and Components.

C++ 
AMyActor* CreateCloneOfMyActor(AMyActor* ExistingActor, FVector SpawnLocation, FRotator SpawnRotation)
{
	UWorld* World = ExistingActor->GetWorld();
	FActorSpawnParameters SpawnParams;
	SpawnParams.Template = ExistingActor;
	World->SpawnActor<AMyActor>(ExistingActor->GetClass(), SpawnLocation, SpawnRotation, SpawnParams);
}

Casting from One Type to Another

In this case, we get a component we know we have, then cast it to a specific type and conditionally do something.

Unity

C# 
Collider collider = gameObject.GetComponent<Collider>;
SphereCollider sphereCollider = collider as SphereCollider;
if (sphereCollider != null)
{
		// ...
}

UE C++

C++ 
UPrimitiveComponent* Primitive = MyActor->GetComponentByClass(UPrimitiveComponent::StaticClass());
USphereComponent* SphereCollider = Cast<USphereComponent>(Primitive);
if (SphereCollider != nullptr)
{
		// ...
}

Blueprint

You can cast in Blueprint by using a Cast to node. For more detailed information, see Casting Quick Start Guide.

Destroying GameObject / Actor

Unity C#

C# 
Destroy(MyGameObject);

UE C++

C++ 
MyActor->Destroy();

Blueprint

Destroying GameObject / Actor (with 1-Second Delay)

Unity C#

C# 
Destroy(MyGameObject, 1);

UE C++

C++ 
MyActor->SetLifeSpan(1);

Blueprint

Disabling GameObjects / Actors

Unity C#

C# 
MyGameObject.SetActive(false);

UE C++

C++ 
// Hides visible components
MyActor->SetActorHiddenInGame(true);

// Disables collision components
MyActor->SetActorEnableCollision(false);

// Stops the Actor from ticking
MyActor->SetActorTickEnabled(false);

Blueprint

Accessing the GameObject / Actor from a Component

Unity C#

C++ 
GameObject ParentGO =
MyComponent.gameObject;

UE C++

C++ 
AActor* ParentActor =
MyComponent->GetOwner();

Blueprint

Accessing a Component from the GameObject / Actor

Unity

C# 
MyComponent MyComp = gameObject.GetComponent<MyComponent>();

UE C++

C++ 
UMyComponent* MyComp = MyActor->FindComponentByClass<UMyComponent>();

Blueprint

Finding GameObjects / Actors

Unity

C# 
// Find GameObject by name
GameObject MyGO = GameObject.Find("MyNamedGameObject");

// Find Objects by type
MyComponent[] Components = Object.FindObjectsOfType(typeof(MyComponent)) as MyComponent[];
foreach (MyComponent Component in Components)
{
		// ...
}

UE C++

C++ 
// Find UObjects by type
for (TObjectIterator<UMyObject> It; It; ++it)
{
	UMyObject* MyObject = *It;
	// ...
}

// Find Actor by name (also works on UObjects)
AActor* MyActor = FindObject<AActor>(nullptr, TEXT("MyNamedActor"));

Blueprint

Adding Tags to GameObjects / Actors

Unity

C# 
MyGameObject.tag = "MyTag";

UE C++

C++ 
// Actors can have multiple tags
MyActor.Tags.AddUnique(TEXT("MyTag"));

Blueprint

Adding Tags to MonoBehaviours / ActorComponents

Unity

C# 
// This changes the tag on the GameObject it is attached to
MyComponent.tag = "MyTag";

UE C++

C++ 
// Components have their own array of tags
MyComponent.ComponentTags.AddUnique(TEXT("MyTag"));

Comparing Tags on GameObjects / Actors and MonoBehaviours / ActorComponents

Unity

C# 
if (MyGameObject.CompareTag("MyTag"))
{
	// ...
}

// Checks the tag on the GameObject it is attached to
if (MyComponent.CompareTag("MyTag"))
{
	// ...
}

UE C++

C++ 
// Checks if an Actor has this tag
if (MyActor->ActorHasTag(FName(TEXT("MyTag"))))
{
	// ...
}

Blueprint

UE C++

C++ 
// Checks if an ActorComponent has this tag
if (MyComponent->ComponentHasTag(FName(TEXT("MyTag"))))
{
	// ...
}

Blueprint

Physics: RigidBody vs. Primitive Component

In Unity, to give a GameObject physics characteristics, you attach a RigidBody component.

In UE, any Primitive Component (UPrimitiveComponent in C++) can represent a physical object. Some common Primitive Components are:

  • Shape Components (USphereComponent, UCapsuleComponent, etc.)

  • Static Mesh Components

  • Skeletal Mesh Components

Unlike Unity, which separates the responsibilities of collision and visualizations into separate components, UE combines the concepts of "potentially physical" and "potentially visible" into a single Primitive Component. Any Component with geometry in the world that can be rendered or physically interacted with is a subclass of UPrimitiveComponent.

Collision Channels are the UE equivalent of layers in Unity. To learn more, refer to Collision Filtering.

RayCast vs. RayTrace

Unity

C# 
GameObject FindGOCameraIsLookingAt()
{
	Vector3 Start = Camera.main.transform.position;
	Vector3 Direction = Camera.main.transform.forward;
	float Distance = 100.0f;
	int LayerBitMask = 1 << LayerMask.NameToLayer("Pawn");

	RaycastHit Hit;
	bool bHit = Physics.Raycast(Start, Direction, out Hit, Distance, LayerBitMask);

UE C++

C++ 
APawn* AMyPlayerController::FindPawnCameraIsLookingAt()
{
	// You can use this to customize various properties about the trace
	FCollisionQueryParams Params;
	// Ignore the player's pawn
	Params.AddIgnoredActor(GetPawn());

	// The hit result gets populated by the line trace
	FHitResult Hit;

Blueprint

Click the image for full size.

Trigger Volumes

Unity

C# 
public class MyComponent : MonoBehaviour
{
	void Start()
	{
		collider.isTrigger = true;
	}
	void OnTriggerEnter(Collider Other)
	{
		// ...
	}

UE C++

C++ 
UCLASS()
class AMyActor : public AActor
{
	GENERATED_BODY()

	// My trigger component
	UPROPERTY()
	UPrimitiveComponent* Trigger;

	AMyActor()

Blueprint

To learn more about setting up collision responses, see Collision.

Kinematic Rigidbodies

Unity

C# 
public class MyComponent : MonoBehaviour
{
	void Start()
	{
		rigidbody.isKinematic = true;
		rigidbody.velocity = transform.forward * 10.0f;
	}
}

UE C++

C++ 
UCLASS()
class AMyActor : public AActor
{
	GENERATED_BODY()

	UPROPERTY()
	UPrimitiveComponent* PhysicalComp;

	AMyActor()
	{

Input Events

Unity

C# 
public class MyPlayerController : MonoBehaviour
{
	void Update()
	{
		if (Input.GetButtonDown("Fire"))
		{
			// ...
		}
		float Horiz = Input.GetAxis("Horizontal");
		float Vert = Input.GetAxis("Vertical");

UE C++

C++ 
UCLASS()
class AMyPlayerController : public APlayerController
{
	GENERATED_BODY()

	void SetupInputComponent()
	{
		Super::SetupInputComponent();

		InputComponent->BindAction("Fire", IE_Pressed, this, &AMyPlayerController::HandleFireInputEvent);

Blueprint

This is what your input properties in your Project Settings might look like:

Input settings in Unreal Engine

To learn more about how to set up input for your UE project, see Input.

Further Reading

For more information related to the concepts above, we recommend that you review the following sections:

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