C2784 UE5: Could not deduce template argument

Could Not Deduce Template Argument in UE5 – What It Means & How to Fix It

🧠 Why You’re Seeing This Error

The C2784 error means the compiler tried to use a template function or class, but couldn’t figure out what type(s) to fill in. Unlike C2782 (which happens when the type isn’t in the parameter list), this happens when the type is there, but the compiler still can’t make sense of it.

In UE5, this often occurs when:

You use Cast<>, TSubclassOf<>, or a custom template function
You pass something that doesn’t match the expected type
You expect the compiler to “just know” what type you’re working with — but it doesn’t

💥 Example Error Message

error C2784: 'T * Cast(UObject *)' : could not deduce template argument for 'T' from 'AActor *'

🛠️ Example Code

AActor* Actor = GetOwner();
AEnemy* Enemy = Cast(Actor); // ❌ C2784: can't deduce T from Actor

The compiler doesn’t know what type you’re trying to cast to.

✅ How to Fix C2784 in UE5 – Step-by-Step

✔️ 1. Always Specify the Template Type Explicitly

Fix:

AEnemy* Enemy = Cast(Actor); // ✅ Now the compiler knows T = AEnemy

Even if it seems obvious to you, C++ won’t guess what type you mean unless it can be inferred directly from the parameters.

✔️ 2. Make Sure You’re Passing a Compatible Type

If you pass a type that doesn’t match what the template expects (like passing a FVector to a Cast<> expecting a UObject*), this error will show up.

Wrong:

FVector Location;
AEnemy* Enemy = Cast(Location); // ❌ FVector is not a UObject*

Fix:

UObject* Object = GetSomeUObject();
AEnemy* Enemy = Cast(Object); // ✅ Compatible types

✔️ 3. Use the Right Template Function

Sometimes this error pops up when you’re calling a completely wrong function overload, and the compiler just can’t match the types at all.