Cannot access private member on adding to TArray

Development Programming & Scripting
1

Hi,
The past few days i’ve been trying to get used to c++ in ue4 and I cant seem to find the reason for the following problem:
Im trying to add an element to a TArray, but I keep getting a compile error:

C2248    'UDFinedItem::UDFinedItem': cannot access private member declared in class 'UDFinedItem'	Gravity	D:\Program Files\Epic Games\UE_4.18\Engine\Source\Runtime\Core\Public\Containers\Array.h	1736 

`C2665	'UDFinedItem::operator new': none of the 2 overloads could convert all the argument types	Gravity	D:\Program Files\Epic Games\UE_4.18\Engine\Source\Runtime\Core\Public\Containers\Array.h	1736

Im new to c++ so sorry if i am missing something stupid. And thanks in advance for any help
The code involved is:

DFinedItem.h:

#pragma once

#include "CoreMinimal.h"
#include "UObject/NoExportTypes.h"
#include "DFinedItem.generated.h"

/**
 * 
 */
UCLASS()
class GRAVITY_API UDFinedItem : public UObject
{
	GENERATED_BODY()
		static TArray<UDFinedItem> Items;
	protected:
		void registerItem(UDFinedItem & itm);
		FString name;
		FString displayName;
		int id;
		bool stackable;
	public:
		UDFinedItem();
		~UDFinedItem();
		virtual int getId();
		virtual FString getName();
		virtual FString getDisplayName();
};

DFinedItem.cpp:

    #include "DFinedItem.h"
    
    static TArray<UDFinedItem> Items;// Init(10, 5);
    UDFinedItem::UDFinedItem()
    {
    	registerItem(*this);
    }
    
    UDFinedItem::~UDFinedItem()
    {
    }
    
    int UDFinedItem::getId() {
    	return id;
    }
    
    FString UDFinedItem::getName() {
    	return name;
    }
    
    FString UDFinedItem::getDisplayName() {
    	return displayName;
    }
    
    void UDFinedItem::registerItem(UDFinedItem & itm) {
    	Items.Add(itm);
    }
2

The problem is that Items is not only private but also static. This means that all instances of UDFinedItem share the same Items array. As a result of this you need to access it in a different way in registerItem(). Your call

Items.Add(itm);

is the same as

this->Items.Add(itm);

What is not working as Items does not belong to this instance but to the class. So what you need is:

UDFinedItem::Items.Add(itm);

What I can not judge about from the code on the first view is if it makes sense to have a static array of items. If you need this, you probably want to save (shared/weak) pointers to UDFinedItems in it, not deep copies of the objects.

3

Thanks for the fast answer… However, changing to
UDFinedItem::Items.Add(itm);
still gives the same compile error…
As for doing this at all - not quite sure, as im trying to readapt from java, where this would definitely be the correct way, so i dont really know whats the bet way here

4

You only need the static keyword if you need the Array and its elements to persist across multiple class instances

5

Not sure exactly what the problem is but one thing is certain: you can’t declare an array of objects like that. They need to be object pointers.

6

Yes, thats what i intended

7

So would this be closer?

// Fill out your copyright notice in the Description page of Project Settings.
#include "DFinedItem.h"

static TArray<UDFinedItem*> Items;
UDFinedItem::UDFinedItem()
{
	registerItem(this);
}

UDFinedItem::~UDFinedItem()
{
}

int UDFinedItem::getId() {
	return id;
}

FString UDFinedItem::getName() {
	return name;
}

FString UDFinedItem::getDisplayName() {
	return displayName;
}

void UDFinedItem::registerItem(UDFinedItem* itm) {
	UDFinedItem::Items.Add(itm);
}

But that gives an even stranger error:

Error	LNK2001	unresolved external symbol "private: static class TArray<class UDFinedItem *,class FDefaultAllocator> UDFinedItem::Items" (?Items@UDFinedItem@@0V?$TArray@PEAVUDFinedItem@@VFDefaultAllocator@@@@A)	Gravity	D:\Unreal Projects\Gravity\Intermediate\ProjectFiles\DFinedItem.cpp.obj	1
8

No that’s better. You declared the Items array in the .h file but you didn’t define it in the .cpp file correctly.

It should be TArray<UDFinedItem*> UDFinedItem::Items;

9

My first answer wasn’t showing so I posted again, this time in more detail. Now that the original answer is back, I deleted the second one and I am copying the text from that in to this comment so the information isn’t lost. Here’s that answer:

There could be more than one issue but the first one I see is this: Items needs to be an array of pointers.

static TArray<UDFinedItem*> Items;

Also, the register function should take in a pointer to the object.

This is because UE4 expects UCLASS instances to be dynamically allocated. You can work with pointers and references. In this case, I think you want to use pointers.

Pointers are the biggest difference between C++ and C#/Java so it makes sense you’d bump into this. Keep an eye out for next time.

10

Oh, ok, yeah, thanks A LOT! It works now! I am very much grateful, it’s nice to get actual help on a forum, especially so quickly, so again, thanks a lot!