Instantiating Instance of Blueprint Class

Hello! Currently, I have a very simple class that I’m trying to work with. It’s called Weapon, it extends from just the generic Object, and it has a couple variables, and (this is important) a few methods that I need to call. However, the problem is, I can’t seem to instantiate a class in Blueprint scripting: there’s no “constructor” of any sort. It’s not an actor, so I can’t spawn it, and since I need it to use methods, a struct isn’t an option. Any ideas on how to do this? Or is there some little cheaty workaround that I can do?

Edit: Here’s an important part I forgot. I need to instantiate a class that inherits FROM weapon: I don’t need to instantiate weapon itself, just it’s subclasses.

I could always make it an actor, I guess, but that seems kinda cludgy.

I’m guessing that there’s no blueprint “pure” way to do this? I’d rather stay away from C++ if I can, but if that’s my only option, then sure. Also, is there any way I can make this a “standard” blueprint function, so that I can call it anywhere (for any class)?

Better way of saying this: can I make this completely actor/target independent, where I can just say “make an instance of this class” and it will just return that?

I have been hit by this same thing.

What I do in my project is:

  • create a c++ method that I can call in blueprints, that receives a Class as input parameter
  • implementation constructs an object
  • I call the method from blueprints and pass it a proper class.

You might have to be careful about the “outer” in order to avoid having an object have a reference path to a level, which caused me no small amounts of pain in 4.7. You might also have to store it in a UPROPERTY or add it to the Root Set so that it doesn’t get garbage collected.

Example in your header:

UFUNCTION(BlueprintCallable, Category = Character, meta = (DisplayName = "Add weapon with specific class"))
void AddWeapon(TSubclassOf<UWeapon> WeaponClass);

Example in your cpp:

void AYourCharacter::AddWeapon(TSubclassOf<UWeapon> UpgradeClass)
{
    //I assume MyWeapon is a UPROPERTY, you might want to do something different
	MyWeapon = ConstructObject<UWeapon>(WeaponClass, this);
}

In my blueprint class, the method looks like this (I add Upgrades instead of weapons):

37380-ejemplo-uobject.jpg

I have a general variant of this that is a static factory method, and creates objects with the class itself as outer. Works like a charm and objects get garbage collected properly (maybe it is an abuse of my part)

Actually, I think I’ve got it figured out: I’ll mark it as an answer once I get it working.

I don´t know of any “blueprint pure” way.

Creating actors works differently than my proposal: they have to be “spawned”, which can be done in blueprints:
Spawning actors

If you want to create UObjects anywhere, I would suggest introducing a C++ class with static methods to act as helpers. Then, you can just call the method anywhere in your blueprints. Such a method would look like:

// In the header
UFUNCTION(BlueprintCallable, Category = Character, meta = (DisplayName = "Add weapon with specific class"))
static UObject * CreateObject(TSubclassOf<UObject> ObjectClass);


// in the cpp:
UObject * FClassHelpers::CreateObject(TSubclassOf<UObject> ObjectClass, UObject *Outer)
{
  return ConstructObject<UObject>(ObjectClass, Outer);
}

But then again, depending on the lifetime of your objects, the implementation might need more bells and whistles.

I used a static method and it worked. Thanks!

Would you care to explain if it’s not too time consuming ? seems useful

Great!
(if you consider the answer solves the problem, please mark it)

:slight_smile:

Sure! (sorry for the delay…)

Basically, first, in the editor, I did an “Add Code to Project”, then selected (I think) Blueprint Function Library. Then, in the created code file, I added this code:

// CPPBlueprintUtilities.h
#pragma once

#include "Kismet/BlueprintFunctionLibrary.h"
#include "CPPBlueprintUtilities.generated.h"

/**
 * 
 */
UCLASS()
class GRAVITY_API UCPPBlueprintUtilities : public UBlueprintFunctionLibrary
{
	GENERATED_BODY()
	
public:
	UFUNCTION(BlueprintPure, meta = (HidePin = "WorldContextObject", DefaultToSelf = "WorldContextObject", FriendlyName = "New Object From Type", CompactNodeTitle = "New", Keywords = "new create instantiate type"), Category = Game)
	static UObject* NewObjectFromType(UObject* WorldContextObject, TSubclassOf<UObject> UC);
};

// CPPBlueprintUtilities.cpp
#include "Gravity.h"
#include "CPPBlueprintUtilities.h"

UObject* UCPPBlueprintUtilities::NewObjectFromType(UObject* WorldContextObject, TSubclassOf<UObject> UC)
{
	UWorld* World = GEngine->GetWorldFromContextObject(WorldContextObject);
	UObject* tempObject = StaticConstructObject(UC);

	return tempObject;
}

Then, you can just call the function in an event graph, and there you go!

And the nice part is, this works for any class, not for any specific one. You can instantiate literally any class with this! :smiley:

Update:

In UE 4.8, ConstructObject has been deprecated in favor of NewObject

In UE 4.8 I just made a static blueprint function arround NewObject().

Creator.h:

#include "Creator.generated.h"

UCLASS(Blueprintable)	// makes the object avaialble to UE. BlueprintType = makes it a variable type
class MYCPPPROJECT_API UCreator : public UObject
{
public:
	GENERATED_BODY()

	UFUNCTION(BlueprintCallable, Category = "Object")
	static void Create(UClass *ObjectClass, UObject* &CreatedObject);
};

Creator.cpp

#include "MyCppProject.h"
#include "Creator.h"

void	UCreator::Create(UClass *ObjectClass, UObject* &CreatedObject)
{
	//NewNode = new ObjectType();
	// Since 4.7/4.8 use:
	CreatedObject = NewObject<UObject>((UObject*)GetTransientPackage(), ObjectClass);
}

Which can be used in the blueprint as:

Ah, thank you, I’ll try that. Just curious, I’m guessing there’s no way to get rid of the exec pins on that? Pre-4.8 it didn’t need to have an execution pin, and I kinda liked that.

Actually, sorry, this DOES work. I wasn’t setting the outer object, because I assumed that it was set by default. I think I was misinterpreting the documentation.

Posting this of anyone else how googles this problem:

Unreal 4.9 Has an innate way to do this “Construct” | “Construct Object from Class” blue print node

1 Like

Yes, thanks for the update. The other answers were given with 4.8 in mind, and the blueprint node you mentioned didn’t exist back then.