So, you've constructed some custom UCLASS intended for use inside UE4. But how do you instantiate them? Objects in UE4 are reference-counted and memory-managed, so you should not allocate them directly using the C++ keyword new. Instead, you'll have to use a function called ConstructObject to instantiate your UObject derivative. ConstructObject doesn't just take the C++ class of the object you are creating, it also requires a Blueprint class derivative of the C++ class (a UClass* reference). A UClass* reference is just a pointer to a Blueprint.
How do we instantiate an instance of a particular Blueprint from C++ code? C++ code does not, and should not, know concrete UCLASS names, since these names are created and edited in the UE4 Editor, which you can only access after compilation. We need a way to somehow hand back the Blueprint class name to instantiate to the C++ code.
The way we do this is by having the UE4 programmer select the UClass that the C++ code is to use from a simple drop-down menu listing all the Blueprints available (derived from a particular C++ class) inside the UE4 Editor. To do this, we simply have to provide a user-editable UPROPERTY with a TSubclassOf<C++ClassName>-typed variable. Alternatively, you can use FStringClassReference to achieve the same objective.
This makes selecting the UCLASS in the C++ code is exactly like selecting a texture to use. UCLASS should be considered as resources to the C++ code, and their names should never be hardcoded into the code base.
In your UE4 code, you're often going to need to refer to different UCLASS in the project. For example, say you need to know the UCLASS of the player object so that you can use SpawnObject in your code on it. Specifying a UCLASS from C++ code is extremely awkward, because the C++ code is not supposed to know about the concrete instances of the derived UCLASS that were created in the Blueprints editor at all. Just as we don't want to bake specific asset names into the C++ code, we don't want to hardcode derived Blueprints class names into the C++ code.
So, we use a C++ variable (for example, UClassOfPlayer), and select that from a Blueprints dialog in the UE4 editor. You can do so using a TSubclassOf member or an FStringClassReference member, as shown in the following screenshot:
- Navigate to the C++ class that you'd like to add the
UCLASSreference member to. For example, decking out a class derivative with theUCLASSof the player is fairly easy. - From inside a
UCLASS, use code of the following form to declare aUPROPERTYthat allows selection of aUClass(Blueprint class) that derives fromUObjectin the hierarchy:UCLASS() class CHAPTER2_API UUserProfile : public UObject { UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = Unit) TSubclassOf<UObject> UClassOfPlayer; // Displays any UClasses // deriving from UObject in a dropdown menu in Blueprints // Displays string names of UCLASSes that derive from // the GameMode C++ base class UPROPERTY( EditAnywhere, meta=(MetaClass="GameMode"), Category = Unit ) FStringClassReference UClassGameMode; }; - Blueprint the C++ class, and then open that Blueprint. Click on the drop-down menu beside your
UClassOfPlayermenu. - Select the appropriate
UClassOfPlayermember from the drop-down menu of the listedUClass.
The TSubclassOf< > member will allow you to specify a UClass name using a drop-down menu inside the UE4 editor when editing any Blueprints that have TSubclassOf< > members.