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C++ classes can be extended with Blueprints, allowing programmers to set up new gameplay classes in code that can be built upon and changed with Blueprints by level designers. There are a number of specifiers that change how a C++ class interacts with the Blueprint system, and some of them will be highlighted in this example.

Class Setup

For the first section of the class setup, a class called LightSwitchBoth is created using the C++ Class Wizard .

Most of the code setup in the LightSwitchBoth class is similar to the code in the C++ Only LightSwitch example . While you could extend the LightSwitchCodeOnly class with a Blueprint, the components, variables, and functions created in that class would not be accessible to Blueprint graphs. This example will go over the UPROPERTY() and UFUNCTION() specifiers that make LightSwitchBoth work as a template for the Blueprint that will derive from it.

You may find it helpful to first refer to the C++ Only LightSwitch example, to see how the header file and source file are set up to create the LightSwitchComponent, SphereComponent, DesiredIntensity variable, and OnOverlap function.

This header file is adapted from the C++ Only LightSwitch example to add the following functionality:

• The PointLightComponent and the SphereComponent are BlueprintReadOnly, and will show up in the category Switch Components in the My Blueprint tab.

• OnOverlapBegin and OnOverlapEnd are now BlueprintNativeEvents, and will show up in the category Switch Functions in the My Blueprint tab.

• DesiredIntensity is BlueprintReadWrite, and will show up in the category Switch Variables in the My Blueprint tab.

• DesiredIntensity is now EditAnywhere instead of VisibleAnywhere.

With the additional specifiers in the UPROPERTY() and UFUNCTION() macros, the header file for the LightSwitchBoth class looks like:

    // Copyright 1998-2018 Epic Games, Inc. All Rights Reserved.

    #pragma once

    #include "GameFramework/Actor.h"
    #include "LightSwitchBoth.generated.h"

    /**
     * 
     */
    UCLASS()
    class [PROJECTNAME]_API ALightSwitchBoth : public AActor
    {
        GENERATED_BODY()
        public:
        /** point light component */
        UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category="Switch Components")
        class UPointLightComponent* PointLight1;

        /** sphere component */
        UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category="Switch Components")
        class USphereComponent* Sphere1;

        ALightSwitchBoth();

        /** called when something enters the sphere component */
        UFUNCTION(BlueprintNativeEvent, Category="Switch Functions")
        void OnOverlapBegin(class UPrimitiveComponent* OverlappedComp, class AActor* OtherActor, class UPrimitiveComponent* OtherComp, int32 OtherBodyIndex, bool bFromSweep, const FHitResult& SweepResult);

        void OnOverlapBegin_Implementation(class UPrimitiveComponent* OverlappedComp, class AActor* OtherActor, class UPrimitiveComponent* OtherComp, int32 OtherBodyIndex, bool bFromSweep, const FHitResult& SweepResult);

        /** called when something leaves the sphere component */
        UFUNCTION(BlueprintNativeEvent, Category="Switch Functions")
        void OnOverlapEnd(class UPrimitiveComponent* OverlappedComp, class AActor* OtherActor, class UPrimitiveComponent* OtherComp, int32 OtherBodyIndex);

        void OnOverlapEnd_Implementation(class UPrimitiveComponent* OverlappedComp, class AActor* OtherActor, class UPrimitiveComponent* OtherComp, int32 OtherBodyIndex);

        /** Toggles the light component's visibility*/
        UFUNCTION()
        void ToggleLight();

        /** the desired intensity for the light */
        UPROPERTY(EditAnywhere, BlueprintReadWrite, Category="Switch Variables")
        float DesiredIntensity;

    };

In the source file for LightSwitchBoth, the constructor remains the same. However, a change needs to be made to the OnOverlapBegin and OnOverlapEnd functions. These functions are now a BlueprintNativeEvent. This means that in a Blueprint derived from this class, events can be placed that override OnOverlapBegin and OnOverlapEnd , and are executed when the function would normally be called. If one of those events does not exist, the C++ implementation of the function executes instead. To make this setup work, the C++ functions need to be renamed OnOverlapBegin_Implementation and OnOverlapEnd_Implementation , respectively. The Blueprint setup will be covered later in this example.
With the change to the OnOverlapBegin and OnOverlapEnd definitions, the source file for LightSwitchBoth looks like:

// Copyright 1998-2018 Epic Games, Inc. All Rights Reserved.

#include "BasicClasses.h"
#include "LightSwitchBoth.h"

ALightSwitchBoth::ALightSwitchBoth()
{

    DesiredIntensity = 3000.0f;

    PointLight1 = CreateDefaultSubobject<UPointLightComponent>(TEXT("PointLight1"));
    PointLight1->Intensity = DesiredIntensity;
    PointLight1->bVisible = true;
    RootComponent = PointLight1;

    Sphere1 = CreateDefaultSubobject<USphereComponent>(this, TEXT("Sphere1"));
    Sphere1->InitSphereRadius(250.0f);
    Sphere1->SetupAttachment(RootComponent);

    Sphere1->OnComponentBeginOverlap.AddDynamic(this, &ALightSwitchBoth::OnOverlapBegin);       // set up a notification for when this component overlaps something
    Sphere1->OnComponentEndOverlap.AddDynamic(this, &ALightSwitchBoth::OnOverlapEnd);       // set up a notification for when this component overlaps something
}

void ALightSwitchBoth::OnOverlapBegin_Implementation(UPrimitiveComponent* OverlappedComp, AActor* OtherActor, UPrimitiveComponent* OtherComp, int32 OtherBodyIndex, bool bFromSweep, const FHitResult& SweepResult)
{
    if (OtherActor && (OtherActor != this) && OtherComp)
    {
        ToggleLight();
    }
}

void ALightSwitchBoth::OnOverlapEnd_Implementation(UPrimitiveComponent* OverlappedComp, AActor* OtherActor, UPrimitiveComponent* OtherComp, int32 OtherBodyIndex)
{
    if (OtherActor && (OtherActor != this) && OtherComp)
    {
        ToggleLight();
    }
}

void ALightSwitchBoth::ToggleLight()
{
    PointLight1->ToggleVisibility();
}

If this is the first code class you have added to a blank project, you will need to close Unreal Editor, compile the project in Visual Studio or Xcode, and then open Unreal Editor and re-open the project to ensure that the game module is created and loaded properly. Also, it is important to make sure that the Build Configuration matches the version of the Unreal Editor executable you are using to open the project. Read more about build configurations and compiling projects in the Compiling Game Projects documentation.

If you are adding code to an existing C++ project, you can use the Hot Reload functionality to compile the new code within Unreal Editor.

After compiling this new class, it is possible to create a new Blueprint Class . In this case, LightSwitchBoth is chosen as the parent class for the Blueprint, which will be named LightSwitchBoth_BP .

The PointLightComponent and SphereComponent added in C++ also show in the Components tab in the Blueprint Editor . Their icons are dark blue to indicate that they are native components, inherited from the parent LightSwitchBoth class. Any new components added just to the LightSwitchBoth_BP Blueprint would have light blue icons instead. To learn more about adding and arranging components using the Components tab, see the Components tab documentation.

The Blueprint Editor 's Graph panel is the core of Blueprint editing. In the Graph panel, you can add new variables , functions , and macros in the My Blueprint tab. You also have access to all the graphs contained within the Blueprint Class. In graphs, nodes are wired together to create design-time and gameplay functionality that can be driven by class variables, gameplay events, and even an Actor's surroundings.

The My Blueprint tab in Graph panel displays the PointLightComponent and the SphereComponent added in C++ to the LightSwitchBoth class. This is because of the BlueprintReadOnly specifier. Nodes for these components can be added to a graph by clicking and dragging from their names in the My Blueprint tab into the graph. These nodes can then be connected to nodes that change variables like visibility or light color. The DesiredIntensity variable also appears in the My Blueprint tab. Because it is a variable and not a component, the BlueprintReadWrite specifier was able to be used. This means it is possible to create nodes to both get and set the value of DesiredIntensity in a Blueprint graph. See the My Blueprint documentation for general usage information.

There are two graphs used to set up the LightSwitchBoth_BP class behavior. The first is the Construction Script graph, which contains a specialized Construction Script

1. Without the Construction Script setup, a new LightSwitchBoth_BP Actor would just use the LightSwitchBoth constructor. However, the Construction Script executes when the Actor is moved within the level, as well as when DesiredIntensity is changed. Using the Construction Script means that Actor variables exposed to Blueprints can be changed easily and the effects of these changes can be seen quickly.

In the LightSwitchBoth_BP class, the Construction Script event is connected to a Set Intensity node, so that the brightness of Point Light 1 (the PointLightComponent) is set to the value of DesiredIntensity when the Actor is added to or moved within the level or DesiredIntensity changes.

The other graph set up in the LightSwitch_BPOnly class is the EventGraph . Execution in the EventGraph begins from events. In this case, the OnOverlap event executes whenever the C++ function OnOverlap would be called. In the LightSwitchBoth source file, delegates were setup so OnOverlap will execute when an Actor enters or leaves the SphereComponent:

    Sphere1->OnComponentBeginOverlap.AddDynamic(this, &ALightSwitchBoth::OnOverlapBegin);       // set up a notification for when this component overlaps something
    Sphere1->OnComponentEndOverlap.AddDynamic(this, &ALightSwitchBoth::OnOverlapEnd);       // set up a notification for when this component overlaps something

The OnOverlap event node is connected to a Set Light Color node. Whenever the event executes, it will cause the light color of the PointLightComponent to be set to a random color. This overrides the OnOverlap_Implementation function in the source file, which toggles the visibility of the PointLightComponent.

For more on events and working with graphs, see the Events , EventGraph , and Blueprint Class UI documentation.

The DesiredIntensity variable is set to EditAnywhere in the LightSwitchBoth header file, so it is visible in the Blueprint Editor 's Defaults and can be edited by clicking the Class Defaults button to show the Class Defaults in the Details Panel. This also means that the variable can be changed for each instance of the class, so each Actor can have its own DesiredIntensity. Because DesiredIntensity is also BlueprintReadWrite and used in the Construction Script, updating it also causes the Construction Script to execute again.

Blueprinted classes can be extended with other Blueprint Classes, either using the dropdown button next to the class in the Class Viewer to create a new Blueprint, or by Right-clicking on the Blueprint and selecting Create New Blueprint Based on This .

The Blueprint Class LightSwitchBoth_BP is in the Content Browser , and can be dragged into the level from there. It is also in the Class Viewer . For more information on placing Actors in a level using the Content Browser or Class Viewer, see the Placing Actors documentation.