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During development, there are occasions when an existing class, property, function name, or similar code member needs to be renamed. If there are a high number of Assets affected by these changes, though, simply renaming the code member and recompiling your project will cause considerable data loss, because Unreal Engine will no longer recognize existing Assets. To address this issue, the Engine uses Core Redirects . Core Redirects should be configured in your project's DefaultEngine.ini file, or, in the case of a Plugin, the prefixed, self-named .ini file for that Plugin (for example, BasePaper2D.ini for the Engine's Paper2D Plugin, or Default<GamePluginName>.ini for a game Plugin). In either case, the Core Redirects will be placed in the "[CoreRedirects]" section. These Core Redirects will automatically remap obsolete data while loading Assets, thus preventing data loss resulting from the renaming process. For examples of Core Redirects currently in effect, check the BaseEngine.ini file.

Supported Core Redirect Types

The following Core Redirect formats are currently supported:

• ClassRedirects - Changes objects and properties using an obsolete (or removed) UCLASS to refer to a new UCLASS.

  Field	Type	Purpose
  OldName	String	Specifies the name of the obsolete (or removed) UCLASS.
  NewName	String	Specifies the name of the new UCLASS.
  MatchSubstring	Bool	(Optional) If present and set to true , this Core Redirect will apply to any class containing the value of OldName , rather than requiring an exact match.
  OverrideClassName	String	(Optional) Specifies a change to the underlying class of the UCLASS. This is generally used to change a Blueprint class to a native class ( /Script/CoreUObject.Class ).
  InstanceOnly	Bool	(Optional) If present and set to true , indicates that the original class still exists and can be referenced, but any existing instances of the old class (such as Actors or Components placed in Levels) should be remapped to the new class. This is especially useful when your project has a specialized version of a class that exists in the engine, but your Levels are full of instances of the original class, and you want to change them all to the project-specific version.
  ValueChanges	List of String pairs	(Optional) Renames instances of the old class that match the first String of a pair. The new name will be the second String of that pair.

  [CoreRedirects]
  +ClassRedirects=(OldName="Pawn",NewName="MyPawn",InstanceOnly=true)
  +ClassRedirects=(OldName="/Script/MyModule.MyOldClass",NewName="/Script/MyModule.MyNewClass")
  +ClassRedirects=(OldName="PointLightComponent",NewName="PointLightComponent",ValueChanges=(("PointLightComponent0","LightComponent0")))
  +ClassRedirects=(OldName="AnimNotify_PlayParticleEffect_C",NewName="/Script/Engine.AnimNotify_PlayParticleEffect",OverrideClassName="/Script/CoreUObject.Class")

• EnumRedirects - Remaps obsolete UENUM types and/or obsolete values within an enumerated type.

  Field	Type	Purpose
  OldName	String	Specifies the name of the obsolete UENUM (if NewName is specified), or the name of the existing UENUM (if only remapping values).
  NewName	String	(Optional) Specifies the name of the new UENUM, if remapping from an obsolete UENUM to a new one.
  MatchSubstring	Bool	(Optional) If present and set to true , this Core Redirect will apply to any enumerated type containing the value of OldName , rather than requiring an exact match.
  ValueChanges	List of String pairs	The first string in the pair is the old enumerated value, and the second string is the new value. If both values are in the same class, the old value must no longer exist in code.

  [CoreRedirects]
  +EnumRedirects=(OldName="ENumbers",NewName="ELetters",ValueChanges=(("NumberTwo","LetterB"),("NumberThree","LetterC")))

• FunctionRedirects - Remaps an obsolete UFUNCTION to a new one.

  Field	Type	Purpose
  OldName	String	Specifies the name of the obsolete (or removed) UFUNCTION. The function name is period-delimited so that the class name is included.
  NewName	String	Specifies the name of the new UFUNCTION. The function name can be period-delimited, which makes it possible to remap a function from one class to another class.
  MatchSubstring	Bool	(Optional) If present and set to true , this Core Redirect will apply to any function containing the value of OldName , rather than requiring an exact match.

  [CoreRedirects]
  +FunctionRedirects=(OldName="MyOldActor.OldFunction",NewName="MyNewActor.NewFunction")
  +FunctionRedirects=(OldName="MyActor.OldFunction",NewName="NewFunction")

• PackageRedirects - Remapping from one package to another, or suppressing warnings about references to a deleted package (references will be cleared or set to null).

  Field	Type	Purpose
  OldName	String	Specifies the name of the obsolete or deleted package.
  NewName	String	(Optional) If remapping is desired, this specifies the name of the package that replaces the obsolete or deleted package. If this is not present, Removed should be present and set to true .
  MatchSubstring	Bool	(Optional) If present and set to true , this Core Redirect will apply to any package containing the value of OldName , rather than requiring an exact match.
  Removed	Bool	(Optional) If present and set to true , the named package has been removed. References to any of the removed content will be set to null without generating warnings or errors. The NewName argument should not be present if this is the case.

  [CoreRedirects]
  +PackageRedirects=(OldName="OldPlugin",NewName="/NewPlugin/",MatchSubstring=true)
  +PackageRedirects=(OldName="/Game/DeletedContentPackage",Removed=true)

• PropertyRedirects - Remaps removed properties to new properties.

  Field	Type	Purpose
  OldName	String	The name of the removed property. This name is period-delimited so that the class name and any sub-variable names are included, For example, MyActor.MyStruct.MyProperty .
  NewName	String	The name of the new property. This name can be fully period-delimited like OldName , or it can be just the variable name if it exists in the same namespace as OldName .
  MatchSubstring	Bool	(Optional) If present and set to true , this Core Redirect will apply to any property containing the value of OldName , rather than requiring an exact match.

  [CoreRedirects]
  +PropertyRedirects=(OldName="MyOldActor.OldIntProperty",NewName="MyNewActor.NewIntProperty")
  +PropertyRedirects=(OldName="MyActor.OldFloatProperty",NewName="NewFloatProperty")

• StructRedirects - Changes properties using an obsolete (or removed) USTRUCT to refer to a new USTRUCT.

  Field	Type	Purpose
  OldName	String	Specifies the name of the obsolete (or removed) USTRUCT.
  NewName	String	Specifies the name of the new USTRUCT.
  MatchSubstring	Bool	(Optional) If present and set to true , this Core Redirect will apply to any struct containing the value of OldName , rather than requiring an exact match.

  [CoreRedirects]
  +StructRedirects=(OldName="MyStruct",NewName="MyNewStruct")

Name Flexibility and Specificity

Names used when describing classes, structs, properties, and functions can be written with varying degrees of specificity. Additionally, the Core Redirects system will use as much (or as little) information as you provide. The following table provides some examples of these levels of specificity.

Substring Matching

The MatchSubstring argument can be used in any Core Redirect type. If present and set to true , the OldName and NewName fields will be treated as substrings rather than requiring exact matches. This enables multiple matches with a single Core Redirect. In the following example, we will start with a struct and a class.

Original code and values:

    USTRUCT()
    struct FMyStruct
    {
        GENERATED_BODY()

        UPROPERTY(EditAnywhere, Category = "Documentation")
        int32 TestInt;

        UPROPERTY(EditAnywhere, Category = "Documentation")
        int32 TestIntFromStruct;
    };

    UCLASS()
    class REDIRECTORSTEST_API AMyActor : public AActor
    {
        GENERATED_BODY()

    public:
        UPROPERTY(EditAnywhere, Category = "Documentation")
        int32 TestInt;

        UPROPERTY(EditAnywhere, Category = "Documentation")
        int32 MainClassTestInt;

        UPROPERTY(EditAnywhere, Category = "Documentation")
        FMyStruct TestStruct;
    };

After creating and saving an AMyActor Asset with the values shown above, we can close the Editor and alter the the code in the .h file and the Core Redirects in the game's .ini file. We will change the code to read as follows, changing the names of our int32 properties:

    USTRUCT()
    struct FMyStruct
    {
        GENERATED_BODY()

        UPROPERTY(EditAnywhere, Category = "Documentation")
        int32 TestInteger;

        UPROPERTY(EditAnywhere, Category = "Documentation")
        int32 TestIntegerFromStruct;
    };

    UCLASS()
    class REDIRECTORSTEST_API AMyActor : public AActor
    {
        GENERATED_BODY()

    public:
        UPROPERTY(EditAnywhere, Category = "Documentation")
        int32 TestInteger;

        UPROPERTY(EditAnywhere, Category = "Documentation")
        int32 MainClassTestInteger;

        UPROPERTY(EditAnywhere, Category = "Documentation")
        FMyStruct TestStruct;
    };

With this change, we can examine the effects of a Core Redirect, and in particular the impact of MatchSubstring . Results follow: