So I was recently migrating one of my game systems from blueprints to C++ because I needed the added performance and a few low-level functions, and I ran into what seems to be a pretty common issue that I couldn’t find a documented solution for. If you’ve done this before, you’re probably already familiar with Core Redirectors, and in this case, specifically the StructRedirects array. The issue I encountered was mildly infuriating, especially after learning the cause. Let’s dig into it.

Core Redirectors

For those who are unfamiliar, core redirectors are an Unreal Engine feature that lets you remap the names of classes, structs, properties, and packages. For the most part, they are fairly straight forward to use, and Epic’s documentation covers the topic pretty well. I’ll include a few examples that are related to the problem I ran into, all of them come straight from my project so they are confirmed working examples.

If you haven’t used core redirectors or gotten into the nitty-gritty of unreal asset packages before, you may not be familiar with the name syntax. Here are the two most common formats you’ll be working with.

/Script/GameModule.ObjectName
/Game/Path/To/Thing/ObjectName.ObjectName

In many cases, it’s totally fine to drop the package path and just use the ObjectName directly. You’ll see this a lot in the engine config files and my examples below. If you’re dealing with ambiguous names including the full path doesn’t hurt though!

If you need to get the name of something in the editor the fastest way is to right click it and hit Copy Reference.

You’ll need to modify the result slightly, it gives you something like this:

/Script/Engine.Blueprint'/Game/LevelTiles/BP_LevelTileGenerator.BP_LevelTileGenerator'

What you need is the part between the quotes, you can drop the rest.

Class Redirectors

These are usually pretty straight forward, but can get a little weird when blueprints are involved. When dealing with blueprints you need to add _C to the name of the object, so BP_LevelTileData becomes BP_LevelTileData_C, and you need to override the class name to be a CoreUObject instead of BlueprintGeneratedClass.

+ClassRedirects=(OldName="/Script/Engine.ProjectileMovementComponent",NewName="/Script/StarwaveDefender.StarwaveProjectileMovementComponent")
+ClassRedirects=(OldName="BP_LevelTileData_C",NewName="/Script/StarwaveDefender.LevelTileData",OverrideClassName="/Script/CoreUObject.Class")

Property Redirectors

Very simple and straight forward. For the OldName you’ll want to use the full package path to make sure you don’t accidentally nuke some unsuspecting object somewhere. The NewName can just be the new name, no special handing required.

+PropertyRedirects=(OldName="/Script/StarwaveDefender.LevelTileData.AvailableInDemo",NewName="bAvailableInDemo")

Enum Redirectors

These can be a little annoying to write out. You need the usual OldName and NewName, but you also need to add a class name override and map the values one-by-one. In blueprint enums, the names you set aren’t used internally, they keep the NewEnumeratorXX as a name. Usually, you can just match the order of the enum on the blueprint side for this.

+EnumRedirects=(OldName="E_LevelTileConditionType",NewName="/Script/StarwaveDefender.ELevelTileConditionType",OverrideClassName="/Script/CoreUObject.Enum",ValueChanges=(("NewEnumerator0","ELevelTileConditionType::MinimumDistance"), ("NewEnumerator1","ELevelTileConditionType::Count"), ("NewEnumerator2","ELevelTileConditionType::MaximumDistance")))

Package Redirectors

I suspect most people will never need to use these, the only time I’ve needed one was when we started a game sequel project by copying the old game and renaming everything. If you need to rename an entire game module in your code these are what you need.

+PackageRedirects=(OldName="/Script/ZoneGraphBehaviors", NewName="/Script/ZoneGraphAnnotations")

Struct Redirectors

And finally, the simplest and yet most problematic one. For redirecting a C++ struct to another C++ struct you just need to set the old / new names and you’re done. With blueprints things get a little bit more involved as I’ll show in a moment. For now, here’s an example redirector:

+StructRedirects=(OldName="S_LevelTileCondition",NewName="/Script/StarwaveDefender.LevelTileCondition")

Looks pretty simple right? It is. That’s the problem, this will work perfectly by every measure the engine is checking.

The Problem

The problem is pretty straight forward, after adding the redirector the affected struct values lose all of their data despite the redirect claiming to be successful. Unfortunately, there aren’t too many great options for debugging issues with this and even with the most verbose logging possible enabled it will really only tell you if your redirector is applied or invalid. Even so, checking the logs is a good first step, if you suspect a core redirector issue start by running the editor with -FullDebugCoreRedirects set on the commandline and keep an eye on the output log.

In this case, the redirect was actually completely fine, and I ended up spending a few hours debugging the engine to figure out what on earth was causing this. I made my way down into the depths, before finally catching the issue with a breakpoint in Class.cpp. As it turns out, the reason this is happening is simply because the property names don’t match. It’s actually really similar to the Enum redirect in that regard, but resolving it is a bit more annoying.

The Solution

To solve this, we simply need a few property redirects. The problem with that is finding out what on earth you need to name them. The property names saved in the blueprint struct have other information attached to them, and that information needs to be included in your property redirect for it to work. Before we get into how you find the names though, I’ll show you what I mean.

+PropertyRedirects=(OldName="LevelTileCondition.ConditionType_2_CAA7B6194B1FB4C242B10E91F0602A95",NewName="ConditionType")
+PropertyRedirects=(OldName="LevelTileCondition.Value_5_C366FD27498EE62340636980557BCBE3",NewName="Value")
+PropertyRedirects=(OldName="LevelTileCondition.TileType_8_9FEF5E124C52E4D168681F9A18771C8C",NewName="TileType")