What It Means & How to Fix It

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🧠 Why You’re Seeing This Error

The C2338 error means the compiler hit a static_assert that evaluated to false, usually deep inside Unreal Engine’s macros or templates. Unlike runtime errors, this happens during compilation, and is used to catch invalid usage of engine features or templates before the program even builds.

In UE5, this is typically triggered when:

• You pass the wrong type to a templated function or macro
• You use a UPROPERTY() on an unsupported type
• You break constraints Unreal has baked into engine macros (like TIsDerivedFrom, TIsSame, etc.)
• You misuse things like TSubclassOf, TArray, Cast<>, or UCLASS() declarations

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💥 Example Error Message

error C2338: Static assertion failed: T must be derived from UObject

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🛠️ Example Code

TSubclassOf MyClassType; // ❌ C2338: int32 is not a UObject

This throws a compile-time error because TSubclassOf<T> only works with UObject-derived types.

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✅ How to Fix C2338 in UE5 – Step-by-Step

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✔️ 1. Read the Full Error Message Carefully

C2338 errors usually tell you exactly what went wrong in plain English:

T must be derived from UObject
Invalid type passed to TIsSame
TArray cannot contain raw pointers

Follow the message — it’s a static assert firing from a template, often to protect you from serious misuse.

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✔️ 2. Fix the Template or Macro Usage

Wrong:

TSubclassOf MyType; // ❌ Not a UObject

Fix:

TSubclassOf MyType; // ✅ AMyActor derives from UObject

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✔️ 3. UPROPERTY() Can’t Be Used on Non-Engine Types

Wrong:

UPROPERTY()
std::vector Numbers; // ❌ Not a UE type

Fix:

UPROPERTY()
TArray Numbers; // ✅ Valid UE type

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✔️ 4. If You’re Writing Your Own Templates, Use static_assert Properly

Fix:

cppCopyEdittemplate<typename T>
void DoSomething()
{
    static_assert(TIsDerivedFrom<T, UObject>::IsDerived, "T must be a UObject!");
}

This is the kind of check that UE5 does internally — and if you’re using a UE macro wrong, this is what fires and becomes a C2338.

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✅ Summary: How to Fix C2338 in UE5

Cause	Fix Example
Invalid type passed to a template	Use UObject-derived classes with TSubclassOf<>
UPROPERTY used with STL/unsupported type	Use UE types like TArray, FString, etc.
Misuse of engine macros or constraints	Follow the static assert message exactly
Wrong types with Cast or reflection	Stick to UObject-compatible types