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warning reference will be nullptred bug


I have described the problem here: https://answers.unrealengine.com/questions/716269/warning-reference-will-be-nullptred.html

now I think it is a bug, so I am reporting it here, with a way to regenerate it. the problem is that some minor changes to base class may cause some pointers (of this class or its derived classes ) to be nullptred after build is done. this happens onley in the current opened level. other instances in other levels seems to be fine.

this happens when I get a warning like this one:

 LogHotReload: Re-instancing MyActor after hot-reload.
 LogProperty: Warning: Serialized Class /Engine/Transient.HOTRELOADED_MyActorDerived_2 for a property of Class /Script/test5.MyActor. Reference will be nullptred.
     Property = ObjectProperty /Script/test5.MyActor:ptr
     Item = HOTRELOADED_MyActorDerived_2 /Game/kk.kk:PersistentLevel.MyActorDerived_1  

Steps to Reproduce this bug:

  1. Launch the editor and create a new empty C++ project (Basic code).

  2. Add a new C++ class derived from Actor (call it MyActor).

  3. Modify MyActor.h to include the following in the class decleration:

             UPROPERTY(EditAnywhere, Category = ttr)
             class AMyActor* ptr;
             UPROPERTY(EditAnywhere, Category = ttr)
             bool bValue;
  4. Build the project in Visual Studio and wait for the UE4 editor to hot-reload the updated class.

  5. Back in the UE4 editor, create a new c++ class derived from MyActor (call it MyActorDerived).

  6. in UE4 Editor Create new Level (call it myLevel). open myLevel.

  7. Drag an instance of MyActor class into myLevel (it is automatically called MyActor1).

  8. Drag an instance of MyActorDerived class into myLevel (it is automatically called MyActorDerived1).

  9. Select MyActor1 (from world outliner), go to Ttr category (in the object properities) click on Ptr and from the list choose MyActorDerived1 . so now MyActor1::ptr pointing to MyActorDerived1

  10. Save all.

  11. Go to visual studio to MyActor.h. rename bValue to bValue2. (this minor change to MyActor will produce the bug !)

  12. Build the project in Visual Studio and wait for the UE4 editor to hot-reload the updated class.

  13. Back in the UE4 editor, select MyActor1 , you will notice that the pointer "ptr" nullptred and lost its data (of Step 9) .

  14. Open Output Log , you supposed to see the following warning:

    LogHotReload: Re-instancing MyActor after hot-reload. LogProperty: Warning: Serialized Class /Engine/Transient.HOTRELOADED_MyActorDerived_2 for a property of Class /Script/test5.MyActor. Reference will be nullptred. Property = ObjectProperty /Script/test5.MyActor:ptr Item = HOTRELOADED_MyActorDerived_2 /Game/myLevel.myLevel:PersistentLevel.MyActorDerived_1

this bug may cause big miss when level has many pointers , like our case. waiting for fix. Thank you.

Product Version: UE 4.17
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asked Oct 19 '17 at 12:12 PM in Bug Reports

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88 4 7 14

avatar image nanashi88 Oct 19 '17 at 12:49 PM

note: to keep reproducing it, go back to step 9. and in step 11 every time rename to something else.

avatar image perky Feb 01 '18 at 06:16 PM

I have reproduced this issue, the same thing happens with just UPROPERTY floats. Adding, removing or changing any UPROPERTY will reset all other UPROPERTY floats back to their default value. Interestingly, If I load a different map and then load the other map back, the values are back to the values I set before the hot-reload. (UE 4.18.3)

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3 answers: sort voted first


This has been a real pain for us so I did some debugging. I have found that most of the hot reload process seems to proceed as expected (or at least, as far as I can infer what is expected), however when we hit UEngine::CopyPropertiesForUnrelatedObjects, the class of the old object has lost references to any properties defined in the C++ super class.

i.e. lets assume we have a c++ class, "MyGameModeBase", and a blue print, "BP_MyGameMode". If we modify "MyGameModeBase" and click the cog to hot reload, the "BP_MyGameMode" class will be correctly detected, submitted for reinstancing, recompiled and eventually for each object of type "BP_MyGameMode", a new instance will be created and "CopyPropertiesForUnrelatedObjects" will be called. This, I assume is all correct.

However, if we allow the function to dump the properties of both the old and new objects, the old class will have no record of any of the properties of "MyGameModeBase", where the new class will. I believe as a result of this, the property values are not copied from the old instance to the new instance, and thus get reset to defaults.

It's also worth noting that any objects that are currently not loaded will not be reinstanced at this point (obviously!). This explains why it works if not hot reloading (or indeed, if you hot reload before loading the object). When the editor is reloaded, and you open the object, unreal seems to pick up on the fact that its type is out of date and triggers a reinstance. As before, this calls CopyPropertiesForUnrelatedObjects, however in this case both the old and new object classes have a correct list of properties.

I hope this accelerates the process of this being fixed. Whilst I will continue to dig, I find it amazing that this isn't considered extremely high priority. A reproducible bug that causes extensive loss of data as part of the core workflow in a content creation package. As a game developer, we wouldn't even consider shipping something with this kind of floor, let alone ignore it. And the fact that a coder with only a month of Unreal experience can get as far as the above in a few hours kind of suggests an experienced engine programmer should be able to work out the problem.


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answered May 11 '18 at 09:31 AM

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81 1 2

avatar image chriscummings May 11 '18 at 11:01 AM

Update, following it through the issue seems to be caused by the fact that the data is dodgy when UClass::Link is called. In my current build it gets called no less than 11 times for changing the 1 class, multiple times for each of SKEL_myblueprint, REINST_SKEL_myblueprint, myblueprint and REINST_myblueprint.

The REINST_ versions seem to be the old versions of the blue print classes. They derived from HOTRELOAD_ versions of my c++ class, which appear to be the old versions of the c++ class. However, these HOTRELOAD_ versions have a NULL pointer for their 'Children' linked list. As such, the link function assumes the old c++ class has no properties.

Throughout the process the linking also occurs for the new classes, and in these cases they do have a pointer for their 'Children', and thus the link function detects the super class's properties correctly.

I have also verified that the old HOTRELOAD_ class definitions are still invalid at the point at which the CopyPropertiesForUnrelatedObjects is called.

Thus I assume the issue is that the Children pointer is being unnecessarily cleared.

avatar image chriscummings May 11 '18 at 11:34 AM

Update again. So it appears the culprit code triggering the issue is this

             // Mark existing class as no longer constructed and collapse the Children list so that it gets rebuilt upon registration
             if (ClassToHotReload)
                 ClassToHotReload->ClassFlags &= ~CLASS_Constructed;
                 for (UField* Child = ClassToHotReload->Children; Child; )
                     UField* NextChild = Child->Next;
                     Child->Next = nullptr;
                     Child = NextChild;
                 ClassToHotReload->Children = nullptr;

in UObjectBase.cpp, UObjectCompiledInDefer. I am not sure what 'rebuilt upon registration' is referring to, so can't infer whether this is the correct behavior, and something later is failing, or if there is simply a bug in this bit of code. Perhaps it is supposed to be added to the "DeferredCompiledInRegistration" list?

It appears that commenting out the clearing of the child list (but leaving in the clear of the constructed flag) fixes the problem, at least in my minor test case. Whether it survives across more of the project remains to be seen.

Hopefully the unreal team could now take a look and work out what the proper fix is? :)



avatar image chriscummings May 11 '18 at 11:38 AM

fyi, here's my version of the working function in case anybody wants it.


avatar image arrpeatwo4 May 11 '18 at 02:36 PM

I haven't tested your fix, but I'm giving you an upvote for due diligence here. I'd become numb to this issue, by begrudgingly working around it every time, but your post really renewed my feelings for it.

I'm still utterly confounded that this is not a top priority issue for Epic. It's a data-loss issue, and an absolute impediment in the core workflow of Unreal, there's no other way to look at it. Yes, there's a workaround, but it's extremely onerous, and it corrupts the workflow.

I just really have to question what is going on, when an issue as severe as this one persists for multiple releases, and hotfixes with zero attention, and a coder with little experience in the engine, can roll up his sleeves and track down the issue in a couple of nights. Seriously mind-boggling.

I'm going to check your fix later tonight, and reply back here. Thanks again for taking the time to dig into this.

avatar image nanashi88 May 27 '18 at 10:22 AM

Hi all,

I haven't follow this issue for a while. today I logged-in to my e-mail then noticed that there are updates on this issue, after taking a look, I have noticed the following strange things:


  • after a long long time finaly someone (Chriscummings) decided dig into this issue. after little time he discovered the Bug source and posted a solution.

  • then arrpeatwo4 posted a blame on UE.


finaly UE staff posted a solution for this Bug. the bug source is the same one that discovered by Chriscummings. but still they claim : "The bug was fixed some time ago, but didn't make it into the 4.19 release. It will make it into the 4.20 release though."

then strange thing happened (I don't know exactly when but it is true to this date) : the issue of this bug (UE-52220) disappeared. why it is disappeard instead of it being tagged "Fixed" as normal ?! actually it was tagged as "Fixed" and here is a proof: https://web.archive.org/web/20180515203503/https://issues.unrealengine.com/ as u can see from this archived page at 15/5/2018, our issue "Hot reload causes placed actor to lose variable references" is latest issue that was tagged with "Fixed". then this was done recently, and for sure it is after 8/5/2018, proof: https://web.archive.org/web/20180508103241/https://issues.unrealengine.com/ then why did it disappeared? is it to hide the "Resolved" date?! which is a field inside every issue record!!


I received the following e-mail from epicgames: "We detected a series of unsuccessful login attempts for your Epic Games account." WOW, who was trying to hack my account ??! I hope it is not related to this topic :P now I wonder, is this post gonna be removed? I hope UE staff answer this suspictions instead, and if I was true, I hope they at least say Thank you for Chriscummings instead of this childrish behavior.

avatar image arrpeatwo4 May 28 '18 at 01:01 PM

nanashi88, here's my take on the situation. My assumption is that this bug was introduced as the result of a merge error in the engine source control. If you head over to the official Github for Unreal Engine, you can view the source code and individual commits for the entire engine, as well as for each version of the engine released.

The offending code that ChrisCummings tracked down first appears in version 4.17. I believe the issue was then filed and we got an extremely half-hearted response from Epic staffer Doug E. I'm willing to wager that an Epic engine programmer was able to track the issue down and resolve it on a bugfix branch fairly quickly, because these types of merge errors where old code, gets intermingled with new code in a way that causes it to still compile, but introduce a major bug are not terribly common, but do sometimes occur.

Now, if you look at Doug E.'s profile, you'll notice it's no longer active, and it is my assumption that he left the company and this issue was never transferred to another community manager, which led to it losing visibility and persisting for 3 full releases. I think after Chris's post we got lucky, and it bubbled up to Epic staff who were able to see what happened, and realized the bugfix branch was never merged into the release branch. This is probably what was meant in the response about having "fixed this issue a long time ago."

It was a major workflow error, but it does in fact happen with these massive software projects, so I'm kind of just glad it was resolved. As far as that account login thing happening, that's pretty scary, but there is this (https://fortnitehelp.epicgames.com/customer/en/portal/articles/2921261-account-security-bulletin). I've read a few posts about some accounts getting hacked, so keep your passwords secure.

Hope this helps,


avatar image nanashi88 May 28 '18 at 08:50 PM

That makes sense. so if this is the case, everybody, I am really sorry for the "undue fuss".

still wondering why did the issue record disappear?

avatar image AssemblerJohn May 28 '18 at 05:05 AM

Yea, weird that they deleted it.

However the fix from Chris was not the same with the fix they (UE guys) did, I've personally checked it and it is different.

But yes, the issue should not be deleted, I've just saw it is deleted now, it was 'fixed' some time ago.

avatar image nanashi88 May 28 '18 at 06:22 AM

Hi AssemblerJohn, I didn't say they have the same fix, what I have concentrated on, is the "bug source", because apparently Chris is the one who found the source of the problem, and even posted a solution before them. so why did they claim that they fixed it even before 4.19 ... ?! is it to save face ?!

avatar image AssemblerJohn May 28 '18 at 06:24 AM

Beats me... However yes, it is a big issue, and it's core part of the engine, there's no excuse to have this bug for 8 months with no fix.

avatar image chriscummings May 28 '18 at 02:18 PM

Just thought I'd say, I apologise if I've called undue fuss here :) assuming Epic's fix below is for the above bug, I'd also recommend taking it, not my code, as they almost certainly know what they're doing a whole lot more than me! Just glad we have a fix.

fyi, If anybody has had a chance to test their fix in 4.18 (is it compatible?) I'd love to know.

avatar image nanashi88 May 28 '18 at 08:59 PM

you don't need to apologise, you are the hero who brought this issue to life again. I am the one who did "undue fuss", so I am sorry.

I have accepted UE answer as you have recommended ^^

Thank you Chris for your hard work

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Hi all,

The bug was fixed some time ago, but didn't make it into the 4.19 release. It will make it into the 4.20 release though.

For a fix in the meantime, your UObjectCompiledInDefer() function should look like this:

 void UObjectCompiledInDefer(UClass *(*InRegister)(), UClass *(*InStaticClass)(), const TCHAR* Name, const TCHAR* PackageName, bool bDynamic, const TCHAR* DynamicPathName, void (*InInitSearchableValues)    (TMap<FName, FName>&))
     if (!bDynamic)
         // Either add all classes if not hot-reloading, or those which have changed
         TMap<FName, FFieldCompiledInInfo*>& DeferMap = GetDeferRegisterClassMap();
         if (!GIsHotReload || DeferMap.FindChecked(Name)->bHasChanged)
             FString NoPrefix(RemoveClassPrefix(Name));
             NotifyRegistrationEvent(PackageName, *NoPrefix, ENotifyRegistrationType::NRT_Class, ENotifyRegistrationPhase::NRP_Added, (UObject *(*)())(InRegister), false);
             NotifyRegistrationEvent(PackageName, *(FString(DEFAULT_OBJECT_PREFIX) + NoPrefix), ENotifyRegistrationType::NRT_ClassCDO, ENotifyRegistrationPhase::NRP_Added, (UObject *(*)())(InRegister), false);

             TArray<UClass *(*)()>& DeferredCompiledInRegistration = GetDeferredCompiledInRegistration();
         FDynamicClassStaticData ClassFunctions;
         ClassFunctions.ZConstructFn = InRegister;
         ClassFunctions.StaticClassFn = InStaticClass;
         if (InInitSearchableValues)
         GetDynamicClassMap().Add(FName(DynamicPathName), ClassFunctions);

         FString OriginalPackageName = DynamicPathName;

         NotifyRegistrationEvent(*OriginalPackageName, Name, ENotifyRegistrationType::NRT_Class, ENotifyRegistrationPhase::NRP_Added, (UObject *(*)())(InRegister), true);
         NotifyRegistrationEvent(*OriginalPackageName, *(FString(DEFAULT_OBJECT_PREFIX) + Name), ENotifyRegistrationType::NRT_ClassCDO, ENotifyRegistrationPhase::NRP_Added, (UObject *(*)())(InRegister), true);

Hope this helps,


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answered May 15 '18 at 03:34 PM

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Steve Robb STAFF
2.8k 58 31 101

avatar image arrpeatwo4 May 15 '18 at 03:49 PM

Thank you, Steve. I will try to implement this tonight, and appreciate you taking the time to respond here with some information as well as working fix.

Very much appreciated.

avatar image AssemblerJohn May 16 '18 at 06:50 AM

Thank you! Finally! It was quite a bit of a wait but it surely is worth it!

avatar image Steve Robb STAFF May 29 '18 at 01:19 PM

Hi all. Here is my take on the issue:

  • The bug was indeed fixed some time ago (just before Christmas last year), but instead as a fix to this similar issue: https://issues.unrealengine.com/issue/UE-53089

  • At that time, we hadn't released 4.19, but the code had already been branched for that release and so this fix missed the window for 4.19 changes.

  • The ticket only reared its head again recently during 4.20 regression testing. An unrelated hot reload issue was found, but it was marked as a regression of this issue, which it is not.

  • That was when I was made aware of this thread and everyone's troubles, and so I came to offer the above code as an official workaround for until 4.20 gets released.

  • I have no dealings with our public issue tracker, so I don't know why UE-52220 has disappeared. It may be due to a false positive on our 'sanitised for public' checker, which ensures we don't accidentally leak confidential information (for clarity: neither this bug nor the fix is in any way confidential).

  • UE-52220 is still visible in our internal tracker - no attempt is being made to rewrite history here. I will see about getting the issue made public again.

I'm an internal developer and am not actively looking at public-facing sites for these types of issues, so I only respond to them when I'm made aware. I'm still sorry that people have felt left in the dark on this. But I would advise anyone to raise their concerns with the support teams.


avatar image Steve Robb STAFF May 30 '18 at 12:47 AM

The disappearance of the issue was indeed a false positive caused by our sanitiser - it is publicly available again (and still marked unresolved because of the issue unrelated to this regression):



avatar image nanashi88 Jun 01 '18 at 05:21 PM

Thank you Steve for clarification. and sorry for the fuss.

avatar image Vahid Jun 02 '18 at 08:07 AM

Thank you so much for your message, do you have any idea when 4.20 will be available, this is very annoying problem.

avatar image dyanikoglu Jul 18 '18 at 01:51 PM

@Steve Robb

This solution still does not solve the problem mentioned by the OP.

After building engine source with this change, UE crashes while trying to reproduce the issue instead of resetting values.

This fix even requires a new version update like "4.18.3", but planned solution version is marked as "4.21". Are we going to wait 3 months more to fix that "IMPORTANT" and "BREAKING" bug?

avatar image Chuman Jun 13 '18 at 05:23 PM

Thank you for the answer, I'm just not sure where to put this, UObjectBase.cpp? Can someone let me know.

avatar image Steve Robb STAFF Jun 14 '18 at 11:35 AM

Yes, in UObjectBase.cpp, just replace your existing UObjectCompiledInDefer() function with the one above.


avatar image Chuman Jun 14 '18 at 05:44 PM

Thanks Steve

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Hey nanashi88-

Thank you for submitting a bug report. I have reproduced this issue and logged a report for it here https://issues.unrealengine.com/issue/UE-52220 . You can track the report's status as the issue is reviewed by our development staff. Please be aware that this issue may not be prioritized or fixed soon.


Doug E

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answered Nov 09 '17 at 08:26 PM

avatar image nanashi88 Nov 13 '17 at 04:26 AM

Hi Doug E, Thanks for response.

a small note about the bug report:

Result: MyActorPtr for MyActor1 is reset to none and RandomNum2 is reset to zero after the hot reload.

Expected: The variable values remains after the hot reload.

I think it is ok that RandomNum2 is reseted to zero , because you renamed it, then it is considered a new variable. then this expected (for me). but what is not expected that MyActorPtr is reseted while no body touch it !


avatar image nocivus Apr 02 '18 at 07:07 PM

Still waiting for this... :(

avatar image Huogo Apr 21 '18 at 03:31 AM

Something I discovered playing around with the problems associated with this bug. After triggering the Hot Reload, and the values are set to default. If you swap maps, then swap back to the map you were on, any variables whose names did not change will regain the proper value.

If the variable's name has changed then it still remains at default as expected, but, if you rename the variable back to what it was, hot reload, and to the map swap, it's value actually goes back to what it was previously (as long as you haven't made 2 hot reload edits).

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