Creating class instances in C++ is traditionally done using the keyword new. However, UE4 actually creates instances of its classes internally, and requires you to call special factory functions to produce copies of any UCLASS that you want to instantiate. You produce instances of the UE4 Blueprints classes, not the C++ class alone. When you create UObject-derived classes, you will need to instantiate them using special UE4 Engine functions.
The factory method allows UE4 to exercise some memory management on the object, controlling what happens to the object when it is deleted. This method allows UE4 to track all references to an object so that on object destruction, all references to the object can be easily unlinked. This ensures that no dangling pointers with references to invalidated memory exist in the program.
Instantiating UObject-derived classes that are not AActor class derivatives does not use UWorld::SpawnActor< >. Instead, we have to use special global functions named ConstructObject< >, or NewObject< >. Note that you should not use the bare C++ keyword new to allocate new instances of your UE4 UObject class derivatives.
You will need at least two pieces of information to properly instantiate your UCLASS instance:
- A C++ typed
UClassreference to the class type that you would like to instantiate (Blueprint class) - The original C++ base class from which the Blueprint class derives
- In a globally accessible object (like your
GameModeobject), add aTSubclassOf< YourC++ClassName > UPROPERTY()to specify and supply theUCLASSname to your C++ code. For example, we add the following two lines to ourGameModeobject:UPROPERTY( EditAnywhere, BlueprintReadWrite, Category = UClassNames ) TSubclassOf<UUserProfile> UPBlueprintClassName;
- Enter the UE4 editor, and select your
UClassname from the drop-down menu so that you can see what it does. Save, and exit the editor. - In your C++ code, find the section where you want to instantiate the
UCLASSinstance. - Instantiate the object using
ConstructObject< >with the following formula:ObjectType* object = ConstructObject< ObjectType >( UClassReference );
For example, using the UserProfile object that we specified in the last recipe, we would get code like this:
// Get the GameMode object, which has a reference to
// the UClass name that we should instantiate:
AChapter2GameMode *gm = Cast<AChapter2GameMode>( GetWorld()->GetAuthGameMode() );
if( gm )
{
UUserProfile* object = ConstructObject<UUserProfile>( gm->UPBlueprintClassName );
}Instantiating a UObject class using ConstructObject or NewObject is simple. NewObject and ConstructObject do nearly the same thing: instantiate an object of Blueprint class type, and return a C++ pointer of the correct type.
Unfortunately, NewObject has a nasty first parameter which requires you to pass GetTransientPackage() with each call. ConstructObject does not require this parameter with each call. In addition, ConstructObject provides you with more construction options.
Do not use the keyword new when constructing your UE4 UObject derivative! It will not be properly memory-managed.